HISTORY OF PHYSICAL ASTRONOMY.

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tliese operations which have reflected so much honour on France . Theirclose was illustrated by the exertions of Lacailie, one of the most distin-guished astronomers of the eighteenth century * * * § .

In connexion with the Eoyal Observatory of Paris may be mentionedLouville’s application of the micrometer to divided instruments. Thiswas effected by directing the instrument approximatively to the object,and then adjusting it, so that the plumb-line fell exactly upon the nearestpoint of division. The moveable thread of the micrometer was thenbrought into exact coincidence with the object, and its distance from thefixed thread, as determined by the revolutions of the screw, gave thefraction of an angle which was to be added to or subtracted from thealtitude indicated by the point of division over which the plumb-linepassed. Louville first described this mode of applying the micrometerin the “Memoirs of the Academy of Sciences” for 1714j-. A sector,furnished with a micrometer of this kind, was shortly afterwards em-ployed by the astronomers engaged in verifying the arc of the meridianthat had been measured in France in the seventeenth century. It wouldappear that a similar application of his instrument did not escape theattention of Gascoigne, the original inventor of the micrometer. Thus,Crabtree, writing to him in a letter dated December 6, 1641, expresseshis astonishment that he should be enabled not only to project thediameter of Jupiter into prodigious measures, but also to take distances,altitudes, inclinations, and azimuths without the limb of an instrument,and each to any required number of parts j.

The difficulty of making accurate observations from the deck of avessel, arising from the unsteady position of the observer, was found tooperate as an insuperable obstacle to the success of any method for find-ing the longitude at sea. This difficulty was entirely removed by theinvention of the reflecting quadrant. The germ of this invaluableinstrument is to be found in Sprat’s “ History of the Koyal Society.”Thus, in an account of the inventions of members of the Society , healludes to “a new instrument for taking angles by reflection, by whichmeans the eye at the same time sees the two objects, both as touchingin the same point, though distant almost to a semicircle, which is ofgreat use for making exact observations at sea.”§ The author of thisinstrument was Hooke, in whose “Posthumous Works” there is to befound a description of it ||; but it does not seem that he attempted tofollow out his ingenious idea by a further improvement of the instrument.Newton was the first person who devised tho construction of the re-

* Picard commenced his operations for the measurement of an arc of the meridian in1669. The arc of the meridian in Lapland was measured by Maupertius and his com-panions in 1735-6. The ellipticity of the earth’s figure was finally established beyondall doubt by the verification of the French arc of the meridian in 1741. The operationsconnected with the measurement of the arc in Peru were completed a few years after-wards.

+ Mem. Acad, des Sciences, 1714, p. 73.

$ Phil. Trans., 1717, p. 609. In 1669 Hooke determined the zenith distances ofy Draconis, by the aid of a micrometrical contrivance similar to that of Louville’s, inas-much as the intersection of cross wires of the telescope was not brought into exact coin-cidence with the star; but in this case the deviations of the star from the optical axis oftho telescope were measured by means of a graduated scale.

§ History of the Royal Society, p. 246, 4to. Lond., 1667.

|| Hooke’s “ Posthumous Works,” p. 503, folio, Lond., 1705.