PHYSICAL AND LITERARY. f^^ 



52. Many ingenious men have beftowed 

 infinite thought and labour on the more 

 complex and aftonifliing ■ph(2nomena of 

 Nature, without arriving at any certain or 

 definite difcoveries j fuch as ■ earthquakes, 

 thunder and other meteors, magnetifin, 

 eledricity, vegetation, fermentation and 

 other chemical operations : and the fubtilty 

 of thofe matters will pfobably continue to 

 elude the fearch of latell pofterity. But, in 

 the fimpler, fteadier, and more regular fub^ 

 jeds, fuch as light and colours, which arc 

 capable of accurate menfuration and mathe- 

 matical reafoningj a fagacious and induftri- 

 ous obferver can hardly fail of making fome 

 progrefs ; efpecially in a branch of the in- 

 quiry which is already pvifhed to a confider- 

 able length. Difcoveries of this kind are 

 capable of a particular fort of proof which 

 is very beautiful and convincing, from the 

 exad; coincidence of the computed effed:s ' 

 Vol. IL H with 



fhaded from the rays, and the fpace FE will be occupied 

 by a penumbra gradually darker from E to F : Let now GH 

 continue fixed, and CD move parallel to the plane EF ; 

 and, as foon as it is pafles the line LF, it is evident, that the 

 Jhadow QF will feem to fwell outwards, and when CD 

 teaches ME fo as to cover with its fhadow the fpace RE, 

 QF by its extenfion will cover FE. This is found to hold 

 true likeways by experiment, 



