42 COSMOS. 



The perfection in the graduation of the arc would have failed 

 entirely, or to a considerable extent, in affording that great- 

 er precision of observation at vv^hich it aimed, if optical and 

 astronomical instruments had not been brought into accord, 

 and the correctness of vision made to correspond with that 

 of measurem 3nt. The micrometer-application of fine tlu'eads 

 stretched in the focus of the telescope, to wliich that instru- 

 ment owes its real and invaluable importance, was first de- 

 vised, six years afterward (1640), by the young and talented 

 Gascoigne.^ 



While, as I have already observed, telescopic vision, ob- 

 servation, and measurement extend only over a period of 

 about 240 years in the history of astronomical science, we 

 find, without including the epoch of the Chaldeans, Egyp- 

 tians, and Chinese, that more than nineteen centuries have 

 intervened between the age of Timochares and Aristillusf 

 and the discoveries of Galileo, during which period the posi- 

 tion and cou^Tse of the stars were observed by the eye alone, 

 unaided by instruments. AYhen we consider the numerous 

 disturbances which, during this prolonged period, checked the 

 advance of cwilization, and the extension of the sphere of 

 ideas among *,he nations inhabiting the basin of the Medi- 

 terranean, \y** are astonished that Hipparchus and Ptolemy 

 should have been so well acquainted with the precession of 

 the equinoxes., the complicated naovements of the planets, the 

 two principal inequalities of the moon, and the position of the 

 stars ; that Copernicus should have had so great a knowledge 

 of the true Fystem of the universe ; and that Tycho Brahe 

 should have been so familiar with the methods of practical 

 astronomy b-?fore the discovery of the telescope. Long tubes, 



272. Mfniu, ia his work, Scientia Longitudinum, which appeared in 

 1G34, v/lritea .t^ follows: Applicatlo tuhi optlci ad alliidadam pro dellis 

 fixis p-^ompte et accurate mensurandis a me excogitata est. Picard had 

 not, up to the year 1667, employed any telescope on the mural circle: 

 and Hevelius, when Halley visited him at Dantzic in 1679, and admired 

 tha precision of his measurement of altitudes, was observing through 

 Improved slits or openings. (Baily's Catal. of Sfars, p. 38.) 



* The unfortunate Gascoigue, whose merits remained so long unac- 

 knowledged, lost his life, when scarcely twenty-three years of age, at 

 the battle of Marston Moor, fought by Cromwell against the Royalists. 

 See Derham, in the Philos. Transact., vol. xxx., for 1717-1719, p. 603 

 -610. To him belongs the merit of a discovery which was long ascribed 

 to Picard and Auzout, and which has given an impulse previously un- 

 known to practical astronomy, the principal object of which is to de- 

 termine poii4ions in the vault of heaven. 



t Cosmos, vol. ii., p. 177, ITS. 



