THE EXACT MEASUREMENT OF PHENOMENA. 315 



In the present day the average error of a single observa- 

 tion is probably reduced to the half or quarter of what it 

 was in Bradley 's time; and further extreme accuracy is 

 attained by the multiplication of observations, and their 

 skilful combination according to the theory of error. 



Some of the more important constants, for instance that 

 of nutation, have been determined within the tenth part 

 of a second of space . 



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

 dependence of this vast progress upon the introduction of 

 new instruments. The astrolabe of Plotemy, the tele- 

 scope of Galileo, the pendulum of Galileo and Huygens, 

 the micrometer of Horrocks, and the telescopic sights and 

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

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

 achromatic lenses, Harrison's chronometer, and Ramsden'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 Chaldaaans. 



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 



c Baily, 'British Association Catalogue of Stars,' pp. 7, 23. 



