positions of which were required for the reduction of the observations made by the late Mr. James Ferguson during the years that he was so industriously and effectively observing with the equatorial. As a whole, therefore, the catalogue is a very miscellaneous one. The first edition contained 10,658 stars, with a number of cases, however, where the star had been observed only in one element, and included objects observed during the years 1845 to 1871. The new edition contains the results of observations to 1877, and includes 11,103 stars; the mean places are for the beginning of the year 1860, but it is to be borne in mind that they do not include any effect of proper motion from the mean date of observation, which is always given, to that general epoch. The annual precessions are annexed, without secular variation. Like other publications of the U.S. National Observatory, the new Washington catalogue appears to have been most liberally circulated amongst astronomers. NEW COMETS.-On August 21 a telescopic comet was discovered at Pola by Herr Palisa; its position at 10h. 26m. M.T. was in right ascension 150° 35′ and declination + 49° 6'; daily motion in right ascension 1° 34′ increasing, and in declination 3 minutes diminishing; it was small but bright. A second comet was discovered on August 24 at the Imperial Observatory, Strassburg, by Dr. Ernst Hartwig; it was then very faint and about 1 minutes in diameter. The following elements and ephemeris have been calculated by Dr. Hartwig, from the Strassburg observations on August 24 and 28, and one at Leipsic on August 26 :— Perihelion passage August 26'4661 M.T. at Berlin. Longitude of perihelion Inclination... ascending node 309 56.31 71 55'0 M.Eq. 1879'0. It will be found that these elements have no resemblance to those of any comet at present in our catalogues. The following positions are for Berlin midnight : THE ... ... ... 13 55 44 14 3 20 14 9 38 14 15 3 14 19 47 TO ASTRONOMERS ... ... ... Sun. 9'9973 0.0036 00116 0'0212 0'0320 0'0439 0'0567 HE United States Naval Observatory will gratefully receive for its Library separate copies or reprints of memoirs published in the Transactions of learned societies or in journals. The volumes of Transactions are regularly received, but often many months after the reprints of particular papers, which are, therefore, especially valued. It is also requested that all communications of this nature, and all correspondence relating to them, may be addressed to The Library, U.S. Naval Observatory, Washington, U.S.A. Agents of the Smithsonian Institution abroad will receive large parcels for transmission. Smaller ones will be received more quickly if they are sent by post. As far as possible the publications of the Observatory will be distributed to all working astronomers. JOHN RODGERS, Rear Admiral, U.S. N., Superintendent Naval Observatory, Washington, D.C., August 18 GEOGRAPHICAL NOTES THE Permanent Commission of the International Geodetic Association, presided over by General Ibañez, has 66 decided to meet on the 16th inst. at Geneva, on the invitation which has been addressed to it by the Government of the little republic. The first official sitting is to be at 2 o'clock, on the 16th, at the Hotel de Ville of Geneva, in the room known as the Alabama." In the evening Prof. E. Plantamour will hold a reception. The official sittings will continue daily at 10 A.M., in the same room, to the end of the week, interrupted on the 18th by a procession of steamers on the lake, which will occupy the whole of the day. On the evening of the 17th the Commissioners are invited to dine by the Council of the State of Geneva, and on the afternoon of the 19th there will be a reception at Sécheron by Prof. Plantamour. The programme of the session comprises: The reports of the Permanent Commission and the Central Bureau, the report of the Commission appointed at Hamburg in 1877 to consider the proposals of Lieut.-Col. Adan; the choice of the place of meeting of the sixth conference, and the appointment of special reporters to record the proceedings (1) As to determinations of latitude, longitude, and azimuth; (2) Triangulations and calculations of compensation of the networks; (3) Levelling operations and result of mareographic operations; (4) Measurements of the intensity of gravitation; (5) Publications relating to the measurement of degrees in Europe. THE eminent African traveller, Dr. Junker, intends to start for Africa during this month. His first object is to reach Monbutta, which is to form the basis for his further investigations. THE Daily News Lisbon correspondent telegraphs on September 2, that official news has arrived which states that on July 24 the explorers Ivens and Capello were in the district called Duque de Braganza. At the last session of the Geographical Society it was stated that the explorers were unable to continue their journey through want of The president promised to ask the Government to send them assitance. It will be remembered that Ivens and Capello started with Major Pinto. means. WE find in the Isvestia of the Russian Geographical Society an interesting note by M. Potanin on the eastern Altai Mountains. Until 1869 these highlands were quite unknown, and even after the recent explorations of MM. Matusovsky and Sosnovsky it was considered that the Altai range did not go east of the meridian of Kobdo, where geographers, according to the map of Klaproth, supposed the existence of low hills which connected the Altai with the Tian Shan. Now M. Potanin proves that the Altai range goes further east, at least to the meridian of Lake Orok-nor, and that it is separated on its whole length by the Gobi steppe from the Tian Shan mountains. The altitude of the mountain passes in the parts visited by M. Potanin reaches as much as 8,000 feet. The eastern parts of the Altai mountains are rather dry, and forests in this part of the range are rather scarce. THE same number of the Isvestia of the Russian Geo graphical Society contains an interesting note on the levelling accomplished during the last three years by the Russian General Staff on several lines of railways in Western Russia. The results are very satisfactory, as the probable error of this topographical levelling (with level and rule) does not exceed ± 2 inches on a distance of 100 versts (67 miles), i.e., less than half the probable error of the best geodetical levellings. These levellings have brought to light a very interesting circumstance, namely, that the average level of the sea at Cronstadt is 13233 inches higher than at Dünamünde; the distance between the two towns being 240 miles. The Prussian levellings prove that the level of the Baltic is 20 inches higher at Kiel than at Memel. CAPT. HOWGATE writes to us that he is preparing to send an expedition to the North Pole next year, independently of the course to be taken by Congress next session. A ship fitted by him will start for Lady Franklin Bay, even if Congress leaves him unassisted. NOTES THE Central Meteorological Office of Italy (the Collegio Romano) has just issued the third part of a most useful series, forming one volume of 282 pp., imperial 8vo. (Imprimerie Héritiers Botta, Rome, 1879), which will be of great service to meteorologists generally. They contain a translation, in French, of all the Reports (in extenso or abridged) prepared upon the different questions comprised in the Programme of the Second International Meteorological Congress held at Rome in April of this year, together with many other papers communicated to the Congress. The work has been undertaken with the view of presenting to meteorologists, not only the whole of the questions which have been discussed by the Congress, but also the ensemble of the experiments and documents which have formed, so to speak, the basis of each discussion, and which represent, at the same time, the opinion of the distinguished men from the whole of Europe upon the most important points of international meteorology. The translation has been carried out under the able superintendence of Prof. Guido Gras i, director of the Roman Central Office, and we congratulate that office upon the careful translation of the reports from the various languages and upon their speedy issue in one convenient volume. OUR readers will be pleased to learn that Prof. Huxley's Introductory Primer to Macmillan's series of Science Primers will probably be published during the autumn; a considerable portion of it is already in type. THE inauguration of Arago's statue will have the éclat of a national fête. The Municipal Council of Paris, of which Arago was an active member during Louis Philippe's reign, will send a deputation. The Bureau des Longitudes, the Observatory of Paris, and the Academy of Sciences, institutions which for years owed their lustre to the great Arago, are sending special representatives delegated for the purpose. M. Etienne Arago, the younger brother of the departed astronomer, a dramatic author, and M. Emmanuel Arago, his son, an influential member of the Senate, will be present at the ceremony, and will deliver addresses. As will be seen from our British Association Reports, the Zoological Station at Naples has undertaken the publication of a new Zoological Record, in which equal attention will be paid to all departments of zoology. A large staff of zoologists of various nationalities will act as recorders, under the editorship of Prof. J. V. Carus, of Leipzig; and the first volume, dealing with the literature of the current year will appear in 1880. All those engaged in zoological work on any group of the animal kingdom are invited to send a copy of their papers to Prof. J. V. Carus, Leipzig, Querstrasse, 30; and to write on the address "for the Jahresbericht." Papers so sent will be distributed by Prof. Carus amongst the recorders, and after being abstracted for the Record, will be deposited in the library of the Zoological Station at Naples. THE St. Petersburg Society of Naturalists has undertaken the publication of a complete Ornithology of Northern Russia. All who possess any data on that subject, or collections of birds, are requested to communicate them to "the St. Petersburg Society of Naturalists, at the University of St. Petersburg." M. DOKUCHAIEFF, who was sent by the St. Petersburg Society of Naturalists for the exploration of the river and lacustrine quaternary deposits on the banks of the Oka, has dis covered at the confluence of this river with the Frubesh, an immense quantity of stone implements. The dunes on the banks of the Oka in the neighbourhood of Kasimov town have also But the spot yielded a good many remains of prehistoric man. MR. CROOKES' admirable set of instruments for exhibiting the properties of radiant matter will be lectured upon at the Sorbonne at the beginning of next October, at the inauguration of the Autumn term of the Academy of Sciences. ON October 6 next, a new Polytechnic Institution will be inaugurated at Hanover. The new building has recently been completed, and no cost has been spared to render it worthy to rank amongst the most complete and extensive buildings of the kind. Deputations from all the other polytechnic high school of Germany will participate in the inauguration-festivities. THE death is announced of Dr. Otto Funke, Professor of Physiology at the University of Freiburg im Breisgau (Baden). Dr. Funke was an eminent physiologist, and lived at Leipzig for many years previous to his call to Freiburg. He died on August 16, at the age of fifty-one years. THE Congress of German Viticulturists is now meeting at Coblenz, and is discussing a number of viticultural questions of question. At the same place a meeting of Rhenish agriculturists importance, including, of course, the much-ventilated phylloxera will take place between September 7 and 10, accompanied by an agricultural exhibition. THE Zoological Section of the Westphalian Provincial Society for Arts and Sciences had an interesting exhibition at the Zoological Gardens of Münster from August 17 to August 24 last. It consisted exclusively of invertebrate animals, illustrations of their habits and specimens of their products. The exhibition comprised insects (bees, beetles, butterflies, flies, grasshoppers, &c.), centipedes, spiders, crustaceans, annelids, molluscs (cephalopoda, gasteropoda, conchifera), echinoderms (holothuriæ, echinoidea, asteroidea), cœlenterata (medusæ), polyps, sponges, and infusoria. Most of the animals were represented in living as well as preserved specimens. DURING the second week in August the German Anthropological Society met at Strasburg, under the presidency of Prof. Fraas. 164 members were present. Amongst the numerous interesting papers read we notice the following:-On the prehistoric map of Southern Germany and Eastern France, by Herr von Troeltsch; Professors Oehlenschlaeger (Munich) and Wagner (Karlsruhe) spoke on the same subject; Prof. Schaaffhausen (Bonn) lectured on skull measurements; Dr. Much (Vienna), on prehistoric traces of copper-mining; Prof. Klopfleisch (Jena), on his own excavations in Thuringia; a specially interesting papers was that by Herr Fischer (Freiburg), on the method of determining the age of stone weapons and uter.sils. Other papers were read by Dr. Gross (Naefels), on the pile-dwellings in the Biel Lake; Dr. Krause (Hamburg), on artificial alterations of the skulls of the natives of the New Hebrides; Dr. Mehlis (Tückheim), on the excavations at Limburg; Dr. Hook, on the stone age in Egypt. The next meeting of the Society will take place at Berlin, under the presidency of Prof. Virchow. THE International Society for the prevention of the pollution of rivers, the soil, and the atmosphere, will hold its third meeting at Baden-Baden on the 16th and 17th inst. AT Rome a new Society for furthering the introduction of cremation was formed on August 12 last. Many eminent medical men are members. NEWS from the village of Havnen, Iceland, states that a violent volcanic eruption was observed at the end of May in the vicinity of the so-called Geisfugle shears, in the south-west of the island. It is curious that about the same time the eruption of Mount Etna took place. THE first volume of a remarkable botanical work entitled "Versuch einer Entwickelungsgeschichte der Pflanzenwelt, insbesondere der Florengebiete seit der Tertiärperiode," by Dr. Ad. Engler, of Kiel, will be published next month by W. Engelmann, of Leipzig. THE Rev. S. J. Whitmee informs us that he has received letters from the Society Islands assuring him there was no such devastation of the islands of Raiatea and Borabora (Porapora) by earthquake as was reported by Capt. Evers, and which is mentioned in the article "On Volcanic Phenomena during 1878,” in NATURE, vol. xx. p. 378. No reference, he reminds us, is made in the article to the pumice and torn-up trees, carried apparently from the island of Birara, south-east to and beyond the Ellia Atolls, which he reported to us some months ago. A CORRESPONDENT informs us that the observer of the Scottish Meteorological Society in Ireland reports fine weather there in June and July, with little or no rain, by which the pastures have suffered severely; but the fishing at most places is good, in direct contrast to what has prevailed in the British Isles. THE City and Guilds Institute having granted 400/. per annum for purposes of technical education at University College, London, have resolved that the grant be appropriated in maintaining the chair of Chemical Technology, and that of Engi: neering and Mechanical Technology. The Professor of Chemical Technology, Dr. Charles Graham, has announced "Technical Education" as the subject of his public lecture at the College on October I. AN experiment before the Parisian press was tried on August 28, in the large room of the St. Lazare Railway Station, by a company started for establishing a Central Hall of Telephony under the Edison patent. The experiments were found quite satisfactory for musical instruments, but not so for the ordinary voice. The Company has received from the Government authority to inaugurate its operations, and a sum of 27. per month is required for the use of a wire with the right of interchanging communications with any person having a wire directed to the Central Hall. A SPECIAL excursion tour for members of the French Parliament has been organised to Algiers. The members will start in this month, and devote their vacation to the study of the land on behalf of which they are to legislate. The Municipal Council of Algiers has voted a sum of 2007. for the reception of their legislators. WE take the following statements from a preliminary communication made to the Vienna Academy of Sciences by Herr G. L. Ciamician, with reference to the further results of his spectroscopic investigations :-"If the spectra of the metals of the alkaline earths are produced by the spark of an induction apparatus (with inserted Leyden jar) passing between the metals as electrodes in a hydrogen tube, then spectra are obtained which show the homology of the spectral lines in a most beauti. ful manner. The spectrum of magnesium, however, cannot be compared to the spectra thus obtained because it does not contain the less refrangible lines. If, however, the Leyden jar be removed, or if a weaker battery and a smaller induction coil be employed, all red and yellow lines in the spectra of calcium and strontium will disappear and spectra are obtained which are extremely similar to that of magnesium. If the less refrangible part of the spectrum of the group of alkaline earth-metals, which therefore is only visible at a high temperature (corresponding to a high electric tension), be compared to the less refrangible half of the complete oxygen spectrum, the remarkable similarity of these two spectrum-halves will at once strike the observer. The inference to be drawn from these facts would seem to be that the spectrum of the group of alkaline earth-metals is composed of the magnesium spectrum and of the less refrangible half of the oxygen spectrum. A NUMBER of interesting observations made during a recent cruise of the French frigate La Magicienne, to various parts of the Pacific, chiefly formed the subject of a recent paper by Admiral Serres to the French Academy. Among other points attention had been drawn while at San Francisco to the swift tall-masted clipper ships which convey wheat to the European market. The modern practice of increasing the high sails at the expense of the lower is justified by science. During the voyage of La Magicienne, a Robinson anemometer was observed daily at an altitude of 8 metres, and twice every day the same instrument was observed at 36 metres. With very rare exceptions the velocity of the wind was always found much greater in the latter case than in the former. The average ratio deduced from thousands of observations was about 12 to 10. One can thus see the reason of seeking motive force in the upper regions. PURSUING his researches on the scintillation of stars, M. Montigny shows, in a recent note to the Belgian Academy, that the following conclusion may be formulated :-When, in those observations where the colours characterising the phenomenon are distinctly separated, the blue tint predominates or is found in excess, rain may be expected, if it have not already come. is great probability that the rain will be the more persistent and plentiful the more marked the predominance of blue. M. Montigny recalls the observations of P. Secchi, M. Janssen, and Prof. Piazzi Smyth, according to which the telluric lines of the solar spectrum increase in number and intensity in circumstances where the solar rays encounter a larger quantity of aqueous vapour in the atmosphere, either as the sun nears the horizon, or as the humidity of the air increases. Prof. Smyth bases predictions of rain on certain telluric bands in the spectrum, which he calls rain-bands. M. Montigny thinks there is no doubt that similar phenomena of absorption are produced in the case of certain rays emanating from the stars, where these traverse more or less moist layers of our atmosphere. There AN apparatus called the "telephone syren" has been recently described to the Schleswig-Holstein Society of Natural Sciences by Herr Karsten. On a circular disk 10 ctm. in diameter are fixed radially twenty-four small magnetic bars. This disk is rapidly rotated before a Bell telephone deprived of the iron plate. Where the same poles of the magnets are all directed outwards, one hears a certain tone; if the poles alternate, the lower octave is heard. If the succession of poles at the border of the disk be (say) N N S, there are heard three tones: one corresponding to the interval N N, one an octave lower corresponding to NS N, and a third combination-tone of three times the time of vibration of the highest, corresponding to the return, each time, of the first N. The vibration numbers are thus as 3:21. Similar experiments may be made with the combination NNNS, where tones are obtained with the relation 4:2: I. THE additions to the Zoological Society's Gardens during the past week include a Vervet Monkey (Cercopithecus lalandii) from South Africa, presented by Mr. W. T. Millar; a Rose Hill Parrakeet (Platycercus eximius) from New South Wales, presented by Mr. Arthur Stirling; a Common Kestrel (Tinnunculus alaudarius), British, presented by Mr. R. Moon; a Chequered Elaps (Elaps lemniscatus) from South America, presented by Dr. A. Stradling; an Annulated Snake (Leptodira annulata) from Colon, presented by Mr. R. F. Davis; three Horned Lizards (Phrynosoma cornutum) from Texas, presented by Mr. Ernest E. Sabel; a Sulphur-breasted Toucan (Ramphastos carinatus) three Black-necked Stilt Plovers (Himantopus nigricollis), two Cayenne Lapwings (Vanellus cayennensis) from South America, a Slow Loris (Nycticebus tardigradus) from Malacca, a Radiated Tortoise (Testudo radiata) from Madagascar, two Electric Silurus (Malapterurus beninensis) from West Africa, purchased; a Squirrel-like Phalanger (Belideus sciurea), born in the Gardens. ON RADIANT MATTER1 II. Radiant Matter exerts strong Mechanical Action where it Strikes WE have seen, from the sharpness of the molecular shadows, that radiant matter is arrested by solid matter placed in its path. for this electrical radiometer is a little beyond that at which the dark space round the negative pole extends to the sides of the glass bulb. When the pressure is only a few millims, of mercury, on passing the induction current a halo of velvety violet light forms on the metallic side of the vanes, the mica side remaining dark. As the pressure diminishes, a dark space is seen to separate the violet halo from the metal. At a pressure of half a millim. this dark space extends to the glass, and rotation comOn continuing the exhaustion the dark space further widens out and appears to flatten itself against the glass, when the rotation becomes very rapid. mences. Here is another piece of apparatus (Fig. 13) which illustrates If this solid body is easily moved the impact of the molecules will reveal itself in strong mechanical action. Mr. Gimingham has constructed for me an ingenious piece of apparatus which when placed in the electric lantern will render this mechanical action visible to all present. It consists of a highly-exhausted glass tube (Fig. 11), having a little glass railway running along it from one end to the other. The axle of a small wheel revolves on the rails, the spokes of the wheel carrying wide mica paddles. At each end of the tube, and rather above the centre, is an aluminium pole, so that whichever pole is made negative the stream of radiant matter darts from it along the tube, and striking the upper vanes of the little paddle-wheel, causes it to turn round and travel along the railway. By reversing the poles I can arrest the wheel and send it the reverse way, and if I gently incline the tube the force of impact is observed to be sufficient even to drive the wheel up-hill. This experiment therefore shows that the molecular stream The the mechanical force of the radiant matter from the negative pole. A stem (a) carries a needle-point in which revolves a light mica fly (bb). The fly consists of four square vanes of thin clear mica, supported on light aluminium arms, and in the centre is a small glass cap which rests on the needle-point. vanes are inclined at an angle of 45° to the horizontal plane. Below the fly is a ring of fine platinum wire (cc), the ends of which pass through the glass at dd. An aluminium terminal (e) is sealed in at the top of the tube, and the whole is exhausted to à very high point. By means of the electric lantern I project an image of the vanes on the screen. Wires from the induction coil are attached, FIG. II. from the negative pole is able to move any light object in front of it. The molecules being driven violently from the pole there should be a recoil of the pole from the molecules, and by arranging an apparatus so as to have the negative pole movable and the body receiving the impact of the radiant matter fixed, this recoil can be rendered sensible. In appearance the apparatus (Fig. 12) is not unlike an ordinary radiometer with aluminium disks for vanes, each disk coated on one side with a film of mica. The fly is supported by a hard steel instead of glass cup, and the needle-point on which it works is connected by means of a wire with a platinum terminal sealed into the glass. At the top of the radiometer bulb a second terminal is sealed in. The radiometer therefore can be connected with an induction-coil, the movable fly being made the negative pole. For these mechanical effects the exhaustion need not be so high as when phosphorescence is produced. The best pressure A lecture delivered to the British Association for the Advancement of Science, at Sheffield, Friday, August 22, 1879, by William Crookes, F. R.S. Continued from p. 423. FIG. 14. so that the platinum ring is made the negative pole, the aluminium wire (e) being positive. Instantly, owing to the projection of radiant matter from the platinum ring, the vanes rotate with extreme velocity. Thus far the apparatus has shown nothing more than the previous experiments have prepared us to expect; but observe what now happens. I disconnect the induction-cou altogether, and connect the two ends of the platinum wire with a small galvanic battery; this makes the ring ce red-hot, and under this influence you see that the vanes spin as fast as they did when the induction-coil was at work. in these high vacua is not only excited by the negative pole of an induction-coil, but a hot wire will set it in motion with force sufficient to drive round the sloping vanes. Here, then, is another most important fact. Radiant matter Radiant Matter is deflected by a Magnet I now pass to another property of radiant matter. This long glass tube (Fig. 14), is very highly exhausted; it has a negative pole at one end (a) and a long phosphorescent screen (b, c) down the centre of the tube. In front of the negative pole is a plate of mica (b, d) with a hole (e) in it, and the result is, when I turn on the current, a line of phosphorescent light (e, f) is projected along the whole length of the tube. I now place beneath the tube a powerful horse-shoe magnet: observe how the line of light (e, g) becomes curved under the magnetic influence waving about like a flexible wand as I move the magnet to and fro. This action of the magnet is very curious, and if carefully followed up will elucidate other properties of radiant matter. Here (Fig. 15) is an exactly similar tube, but having at one end a small potash tube, which if heated will slightly injure the vacuum. I turn on the induction current, and you see the ray of radiant matter tracing its trajectory in a curved line along the screen, under the influence of the horse-shoe magnet beneath. Observe the shape of the curve. The molecules shot from the negative pole may be likened to a discharge of iron bullets from a mitrailleuse, and the magnet beneath will represent the earth curving the trajectory of the shot by gravitation. Here on this luminous screen you see the curved trajectory of the shot accurately traced. Now suppose the deflecting force to remain constant, the curve traced by the projectile varies with the velocity. If I put more powder in the gun the velocity will be greater and the trajectory flatter, and if I interpose a denser resisting medium between the gun and the target, I diminish the velocity of the shot, and thereby cause it to move in a greater curve and come to the ground sooner. I cannot well increase before you the velocity of my stream of radiant molecules by putting more powder in my battery, but I will try and make them suffer greater resistance in their flight from one end of the tube to the other. I heat the caustic potash with a spirit-lamp and so throw in a trace more gas. Instantly the stream of radiant matter responds. Its velocity is impeded, the magnetism has longer time on which to act on the individual molecules, the trajectory gets more and more curved, until, instead of shooting nearly to the end of the tube, my molecular bullets fall to the bottom before they have got more than half-way. It is of great interest to ascertain whether the law governing the magnetic deflection of the trajectory of radiant matter is the same as has been found to hold good at a lower vacuum. The experiments I have just shown you were with a very high vacuum. Here is a tube with a low vacuum (Fig. 16). When I turn on the induction spark, it passes as a narrow line of violet light N pole. Behind this screen is a mica wheel (e, f) with a series of vanes, making a sort of paddle-wheel. So arranged, the mole cular rays from the pole a b will be cut off from the wheel, and will not produce any movement. I now put a magnet, g, over the tube, so as to deflect the stream over or under the obstacle c, d, and the result will be rapid motion in one or the other direction, according to the way the magnet is turned. I throw the image of the apparatus on the screen. The spiral lines painted on the wheel show which way it turns. I arrange the magnet to draw the molecular stream so as to beat against the upper vanes, and the wheel revolves rapidly as if it were an overshot water-wheel. I turn the magnet so as to drive the radiant joining the two poles. Underneath I have a powerful electromagnet. I make contact with the magnet, and the line of light dips in the centre towards the magnet. I reverse the poles, and the line is driven up to the top of the tube. Notice the difference between the two phenomena. Here the action is temporary. The dip takes place under the magnetic influence; the line of discharge then rises and pursues its path to the positive pole. In the high exhaustion, however, after the stream of radiant matter had dipped to the magnet, it did not recover itself, but continued its path in the altered direction. By means of this little wheel, skilfully constructed by Mr. Gimingham, I am able to show the magnetic deflection in the electric lantern. The apparatus is shown in this diagram (Fig. 17). The negative pole (a, b) is in the form of a very shallow cup. In front of the cup is a mica screen (c, d), wide enough to intercept the radiant matter coming from the negative FIG. 17. matter underneath; the wheel slackens speed, stops, and then begins to rotate the other way, like an under-shot water-wheel. This can be repeated as often as I reverse the position of the magnet. I have mentioned that the molecules of the radiant matter discharged from the negative pole are negatively electrified. It is probable that their velocity is owing to the mutual repulsion between the similarly electrified pole and the molecules. In less high vacua, such as you saw a few minutes ago (Fig. 16), the discharge passes from one pole to another, carrying an electric current, as if it were a flexible wire. Now it is of great interest |