DISCOVERY BY MEANS OF COMPARISON AND CLASSIFICATION. 577 



a. By simple comparison of facts or phenomena. 

 The simple comparison of facts has enabled us to discover 

 an immense amount of new knowledge. The effects of 

 precession on the apparent places of the fixed stars were 

 discovered by Hipparchus, in the year 128 before Christ, 

 by comparison of his own observations with those of 

 Timocharis, made 155 years previously. Comparison of the 

 heavens with star maps and catalogues largely conduced 

 to the discovery of the minor planets or asteroids, by 

 enabling a new or movable star to be more easily re- 

 cognised ; it was in a great measure by means of such 

 comparison that Astrea was found ; the other asteroids 

 also were largely discovered by comparing the heavens 

 with Bode's ' Berlin Catalogue.' It was by making, classi- 

 fying, and comparing a long series of observations during 

 a number of years, of the spots on the sun, that Schwabe 

 discovered regular periods of increase and decrease of the 

 number of spots. 



Graham, a philosophical instrument maker of London, 

 and also astronomer, by making as many as 1,000 very 

 careful observations of the movements of a freely-suspended 

 magnetic-needle, and comparing them, discovered, in 1722, 

 the diurnal magnetic variation. Canton also, by means of 

 4,000 such observations and similar comparison, discovered, 

 about the year 1756, the yearly variation, and also that 

 the daily variation was greater in summer than in winter. 

 It was by means of comparison of the effects of electrified 

 bodies upon each other, and upon neutral ones, that 

 Du Fay, between the years 1733 and 1737, discovered the 

 existence of two forms of electricity, and that similarly 

 electrified bodies mutually repel, and dissimilarly electrified 

 ones attract each other, and that bodies excited by either 

 form of electricity attract neutral ones. It was by com- 

 paring the rates of chronometers in the presence and 



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