272 THE PRINCIPLES OF SCIENCE. [CHAP. 



of nutation, have been determined within the tenth part 

 of a second of space. 1 



It would be a matter of great interest to trace out the 

 dependence of this progress upon the introduction of 

 new instruments. The astrolabe of Ptolemy, the tele- 

 scope of Galileo, the pendulum of Galileo and Huyghens, 

 the micrometer of Horrocks, and the telescopic sights and 

 micrometer of Gascoygne and Picard, llcemer's transit in- 

 strument, Newton's and Hadley's quadrant, Dollond's 

 achromatic lenses, Harrison's chronometer, and Eamsden's 

 dividing engine such were some of the principal addi- 

 tions to astronomical apparatus. The result is, that we 

 now take note of quantities, 300,000 or 400,000 times as 

 small as in the time of the Chaldseans. 



It would be interesting again to compare the scrupulous 

 accuracy of a modern trigonometrical survey with Erato- 

 sthenes' rude but ingenious guess at the difference of lati- 

 tude between Alexandria and Syene or with Norwood's 

 measurement of a degree of latitude in 1635. " Sometimes 

 I measured, sometimes I paced," said Norwood ; " and I 

 believe I am within a scantling of the truth." Such was 

 the germ of those elaborate geodesical measurements 

 which have made the dimensions of the globe known to 

 us within a few hundred yards. 



In other branches of science, the invention of an instru- 

 ment has usually marked, if it has not made, an epoch. 

 The science of heat might be said to commence with the 

 construction of the thermometer, and it has recently been 

 advanced by the introduction of the thermo-electric pile. 

 Chemistry has been created chiefly by the careful use of 

 the balance, which forms a unique instance of an instru- 

 ment remaining substantially in the form in which it was 

 first applied to scientific purposes by Archimedes. The 

 balance never has been and probably never can be im- 

 proved, except in details of construction. The torsion 

 balance, introduced by Coulomb towards th.p. end of last 

 century, has rapidly become essential in many branches 

 of investigation. In the hands of Cavendish and Baily, it 

 gave a determination of the earth's density ; applied in the 

 galvanometer, it gave a delicate measure of electrical 



1 Baily, British Association Catalogue of Stars, pp. 7, 23. 



