but in order to take advantage of this property, the pendulum must be connected with machinery by which its motion is kept from languishing, and the number of its swings recorded. By inventing such machinery, Huyghens at once obtained a measure of time more accurate than the sun itself. Hence astronomers were soon led to obtain the right ascension of a star, not directly, by measuring any distance in the heavens, but indirectly, by observing the moment of its transit. This observation is now made with a degree of accuracy which might, at first sight, appear beyond the limits of human sense, being noted to a tenth of a second of time: but we may explain this, by remarking that though the number of the second at which the transit happens is given by the clock, and is reckoned according to the course of time, the subdivision of the second of time into smaller fractions is performed by the eye, by seeing the space described by the heavenly body in a whole second, and hence estimating a smaller time, according to the space which its description occupies.

But in order to make clocks so accurate as to justify this degree of precision, their construction was improved by various persons in succession. Picard soon found that Huyghens's clocks were affected in their going by temperature, for heat caused expansion of the metallic pendulum. This cause of error was remedied by combining different metals, as iron and copper, which expand in a different degree, in

such a way that their effects compensate each other. Graham afterwards used quicksilver for the same purpose. The escapement too, and other parts of the machinery, had the most refined mechanical skill and ingenuity of the best artists constantly bestowed upon them. The astronomer of the present day, constantly testing the going of such a clock by the motions of the fixed stars, has a scale of time as stable and as minutely exact as the scales on which he measures distance.

The construction of good watches, that is, portable or marine clocks, was important on another account, namely, because they might be used in determining the longitude of places. Hence the improvement of this little machine became an object of national interest, and was included in the reward of 20,000. which we have already noticed as offered by the English parliament for the discovery of the longitude. Harrison', originally a carpenter, turned his mind to this subject with success. After thirty years of labour, in which he was encouraged by many eminent persons, he produced, in 1758, a time-keeper, which was sent on a voyage to Jamaica for trial. After 161 days, the error of the watch was only one minute five seconds, and the artist received from the nation 5000l. At a later period, at the age of seventy-five years, after a life devoted to this object, having still further satisfied the commissioners, he received, in 1765, 10,000l., at the same time that 8 Ib. iv. 560.

Mont. iv. 554.

Euler and the heirs of Mayer received each 30007. for the lunar tables which they had constructed.

The two methods of finding the longitude, by chronometers and by lunar observations, have solved the problem for all practical purposes; but the latter could not have been employed at sea without the aid of that invaluable instrument, the sextant, in which the distance of two objects is observed, by bringing one to coincide apparently with the reflected image of the other. This instrument was invented by Hadley, in 1731. Though the problem of finding the longitude be, in fact, one of geography rather than astronomy, it is an application of astronomical science which has so materially affected the progress of our knowledge, that it deserves the notice we have bestowed upon it.

3. Telescopes. We have spoken of the application of the telescope to astronomical measurements, but not of the improvement of the telescope itself. If we endeavour to augment the optical power of this instrument, we run, according to the path we take, into various inconveniences;-distortion, confusion, want of light, or coloured images. Distortion and confusion are produced, if we increase the magnifying power, retaining the length, and the aperture of the object-glass. If we diminish the aperture we suffer from loss of light. What remains then is to increase the focal length. This was done to an extraordinary extent, in telescopes constructed in the beginning of the last century. Huyghens, in his

first attempts, made them 22 feet long; afterwards, Campani, by order of Louis the Fourteenth, made them of 86, 100, and 136 feet. Huyghens, by new exertions, made a telescope 210 feet long. Auzout and Hartsoecker are said to have gone much further, and to have succeeded in making an object-glass of 600 feet focus. But even such telescopes as those of Campani are almost unmanageable: in that of Huyghens, the object-glass was placed on a pole, and the observer was placed at the focus with an eye-glass.

The most serious objection to the increase of the aperture of object-glasses, was the coloration of the image produced, in consequence of the unequal refrangibility of differently coloured rays. Newton, who discovered the principle of this defect in lenses, had maintained that the evil was irremediable, and that a compound lens could no more refract without producing colour, than a single lens could. Euler and Klingenstierna doubted the exactness of Newton's proposition; and, in 1755, Dollond disproved it by experiment. This new light pointed out a method of making object-glasses which should give no colour-which should be achromatic. For this purpose Dollond fabricated various kinds of glass (flint and crown glass ;) and Clairaut and D'Alembert calculated formula. Dollond and his son' succeeded in constructing telescopes of three feet long (with a triple object-glass,) which produced an effect as great

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VOL. II.

Bailly, ii. 253.

5 Ib. iii. 118.

T

as those of forty-five feet on the ancient principles. At first it was conceived that these discoveries opened the way to a vast extension of the astronomer's power of vision; but it was found that the most material improvement was the compendious size of the new instruments; for, in increasing the dimensions, the optician was stopped by the impossibility of obtaining lenses of flint-glass of very large dimensions. And this branch of art remained long stationary; but, after a time, its epoch of advance again arrived. In the present century, Fraunhofer, in Germany, succeeded in forming lenses of flintglass of a magnitude till then unheard of; and this art descended to his successors, Utzschneider in that country, and Guinand in Paris. Achromatic objectglasses, of a foot in diameter, and twenty feet focal length, are now no longer impossible; although in such attempts the artist cannot reckon on certain

success.

Such telescopes might be expected to add something to our knowledge of the heavens, if they had not been anticipated by reflectors of an equal or greater scale. James Gregory had invented, and Newton had more efficaciously introduced, reflecting telescopes. But these were not used with any peculiar effect, till the elder Herschel made them his especial study. His skill and perseverance in grinding specula, and in contriving the best apparatus for their use, were rewarded by a number of curious and striking discoveries, among which, as we have