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ing the best oils turn yellow and tarnish the beauty of white and the light tints which, in wax remain unchanged during any length of time. Cloths prepared for oil-painting arc heavy from the quantity of white-lead spread on them, especially when they are prepared as at present without size; besides which the oil gradually rots the linnen. On the other hand when the linnen is sized, although less weight of paint is employed and the linnen does not rot, yet the picture is liable to crack and even to fall off in scales, especially when affected by alternate moisture and heat. The waxed canvas is light, protected from the contact of air, consequently cannot rot and is cxcccdingly pliable, with which the painting forms one homogencous mass, never liable to crack nor scale off. But it is in the execution of the painting that some of the greatest advantages occur; and from the quickness with which the spirits of turpentine evaporate and the advantage afforded by heating or burning in the picture (by which the appearance called sinking in is entirely done away) the artist is enabled to take several sittings or lay on several coats of colour the same day or successive days, and finally to give a superior degree of finish in delicacy, transparency and truth of colouring or shading with the least sacrifice of the time of the sitter or model. Pots always full of every colour, viz. all the tints of flesh, linnen, drapery, background, &c. (which do not injure by keeping) are ready for painting at any momentan advantage which the portrait-painter knows how to appreciate.
Since my early departure for America will deprive me of the advantages of the ensuing exposition at the saloon of the Louvre, I can do no more than thus to give a hasty view of the subject to the Institue. If they find it worthy of their approbation it will go far to repay me the labour it has cost, before I shall be perniitted to make the mode of procedure known.
Paris, September 3d. 1810.
THE USEFUL ARTS.
In the course of a late number of the Port Folio we have published certain notices concerning the new process of printing on stone. While our work was yet in the press we reccived from: Mr. Nicholson, the truly laudable cditor of the journal of Natural Philosophy, the following useful hints on the subject allucied to and addressed to him by one of his correspondents. These merit the attention of the lovers and the cultivators of the typographical art.
In your last number, you inserted from the, “ Annales de Chimie” an account of the method of printing from stone. It is certainly an ingenious, and most probably a uscful art; though I believe very little known in England. There are some circumstances respecting it with which some of your readers may like to be acquainted, that Mr. Deserres has passed by. A close texture, as he observes, is advantageous, and indeed necessary to its giving a clear impression.
I make the ink according to his direction (which was considered so great a secret) but prefer to it coloured turpentine, copal, or lac varnish. Muriatic acid is cheaper than nitric, and has the advantage of not acting upon the resin or wax, which forms the base of the varnish used.
After purchasing some picces of marble, I was much vexed to find that both the muriatic and nitric acid left some of the veins untouched, and only partially dissolved others; this must be attended to in selecting the blocks. I find some picces of the limestone from Clifton near Bristol lake take a tolerable polish,
. and dissolve readily.
But the easiest and cheapest way for those who wish to have a card, ciphers, &c. is Chauvron's on stone or even on lead. A little piece may be executed in a quarter of an hour; and if wetting is not sufficient to prevent the ink from adhering to the block, it will bear sponging, and yet leave enough of the Ink upon the figures.
The inscrtion of these hints in your valuable journal will oblige your constant reader.
SCIENCE. FOR THE PORT FOLIO.
The English nation, with a sort of pertinacity, not very honourable to the philosophical character, and not quite compatible with that candour and fairness for which Britons are often so nobly distinguished, still persist in denominating a certain nautical instrument Hadley's Quadrant, though, as it is well known to a majority of your readers, the invention is challenged by an American, named Thomas Godfrey. Taking this for granted, still, Mr. Oldschool, as I am constantly of opinion, with the liberal Romans, that the suum cuique should always be impartially awarded, I take the liberty of associating Mr. Ramsden with Mr. Godfrey. He, who makes decided improvements upon the invention of another is entitled to a just consideration in the scientific world. We learn from professor Piazzi of Palermo, who, in a letter to M. De La Lande, translated from the Journal des sçavans, for the month of November 1788, has communicated a very well written biography of an ingenious mechanist, that, at an early period, young Ramsden conceived a strong desire of devoting himself to science; that the mathematics, in particular, engaged his attention, and that when he was in his apprenticeship, as an engraver, mathematical instruments were often brought to him to be engraven. These, the more he examined, the more was he sensible of their defects. A secret instinct, the impulse of genius, made him desirous of constructing better ones. He, therefore, resolved upon the attempt. He soon acquired the use of the file, and made himself acquainted with the method of turning brass, and of grinding glasses. In the year 1763, he constructed instruments for Sisson, Dollond, Nairne, Adams, and other mathematical instrument makers. He then established a shop, on his own account, in the Haymarket, about the year 1768, from which he removed to Ficcadilly in 1775. Having formed a design of examining all the astronomical instruments, he resolved to correct those, which being founded on good principlce, were defective only in ile construction,
Hadley's scxtant which is so much employed in the British navy appeared to Mr. Ramsden one of the most useful, but it was then very imperfect; the essential parts were not of sufficient strength; the centre was subject to too much friction, and the index could be moved several minutes without any change being produced in the position of the mirror; the divisions, in general, were very coarse; and Mr. Ransden found that the abbé de la Caille was right, when he estimated at five minutes the error which might take place in the observed distances of the moon and stars, and which might occasion in the longitude an error of fifty nautical leagues. Mr. Ramsden, therefore, changed the construction in regard to the centre, and made these instruinents so correct as to give never more than half a minute of uncertainty. At present, he warrants sextants of fiftecn'inches radius to within six seconds. Since the time when he first improved these instruments, he has constructed nearly a thousand; and several of them having been carried to India and America, the error has been found to correspond with what he determined it to be, before their departure. He has since made sextants from fifteen inches to an inch and a half radius, and, in the latter, the minutes can be clearly distinguished; but, in general, he prefers those of ten inches, as being more easily managed, and susceptible of the same exactness.
There is so much of justice and liberality in this narrative that I conceive it to be a duly to give it publicity in your journal.
Meanwhile, wishing you all imaginable success in your learned and in your lounging moods, I am your constant friend,
FOR THE PORT FOLLIÓ.
Remarks on an Essay on the Study of Natural Philosophy.
I beg leave to make some observations on an essay, on the study of natural philosophy, which appeared in the number of the Port Folio, for November last.
The author informs us, that the science of hydrostatics teaches the properties of nonelastic fluids.
This may be productive of erroneous impressions. Physical properties, are either chemical, or mechanical. The science abovementioned, only treats of those which claim the latter epithet. It is not confined to nonelastic fluids, but treats of the equilibrium of fluids in general, whether elastic, or nonelastic, so far as is not disturbed by elasticity.
He erroneously asserts, that the discovery of specific gravities is the principal end of hydrostatic science. The process by which this discovery is attained, is only one of the applications of this department of our knowledge, which comprises the theory of every hydrostatical machine, and furnishes some fundamental data, to hydraulics, and pneumatics.
It is unnecessary to prove this remark, even to the author; as he has admitted the truth of it, in a previous paragraph.
“ Hydrostatics and hydraulics are usually combined in a course of philosophical study; for as the first treats of the properties appertaining to a certain description of fluids, so the latter teaches the method of applying those qualities to the working of useful machines."
In defining pneumatics, the author falls into the same error as in the definition upon which I have already animadverted, in omitting the term mechanical before properties. The word nature is also improperly used, instead of the phrase mechanical action.
To develop the nature of elastic fluids is the object of pneumatic chemistry.