EXAMPLES OF PREDICTED DISCOVERIES. 229 



the difficulties, the labour, grew upon him as he ad- 

 vanced ; but he fought his way through them, assisted by 

 Lalande, and by a female calculator, Madame Lepaute. 

 He predicted that the comet would reach its perihelion 

 April 13, 1759, but claimed the licence of a month for the 

 inevitable inaccuracies of a calculation which, in addition 

 to all other sources of error, was made in haste, that it 

 might appear as a prediction. The comet justified his 

 calculations and his caution together, for it arrived at its 

 perihelion on March 1 3.' l 



D'Alibard and others, in the year 1752, tested Franklin's 

 conjecture, made in 1750, of the analogy of electricity and 

 lightning, by erecting at Marli a pointed rod of iron, 40 feet 

 high, and discovered that when a thunder-cloud passed over 

 the place, the rod was capable of yielding electric sparks. 



'Fresnel proved, by a most profound mathematical 

 calculation, a priori, that the extraordinary ray' (of 

 polarised, light) 'must be wanting in glass and other 

 uncrystallised substances, and that it must necessarily 

 exist in carbonate of lime, quartz, and other bodies 

 having one optic axis, but that, in the numerous class of 

 substances which possess two optic axes, both rays must 

 undergo extraordinary refraction, and consequently that 

 both must deviate from the original plane, and these results 

 have been perfectly confirmed by subsequent experiments.' 2 



c M. Melloni, observing that the maximum point of 

 heat is transferred farther and farther towards the red end 

 of the spectrum, according as the substance of the prism 

 is more and more permeable to heat, inferred that a prism 

 of rock-salt, which possesses a greater power of transmit- 

 ting the calorific rays than any known body, ought to 



1 Whewell, History of Inductive Sciences, 3rd edit. vol. ii. p. 182. 



2 Mrs. Somerville, Connection of tlie Physical Sciences, 2nd edit, 

 p. 218. 



