coe 
PHYSICAL ano LITERARY. 63. 
52. Many ingenious men have beftow- 
ed infinite thought and labour on the 
more complex and aftonifhing phenomena 
of nature, without arriving at any certain 
or definite difcoveries ; fuch as earth: 
quakes, thunder, and other meteors 3 
magnetifm, electricity, vegetation, fermen- 
tation, and other chemical operations: 
-and the fubtihty of thofe matters will pro- 
bably continue to elude the fearch of 
Jateft pofterity. But, in the fimpler, ftea- 
dier, and more regular fubjects, fuch as 
light and colours, which are capable of 
accurate. menfuration and mathematical 
reafoning, a fagacious and induftrions 
obferver can hardly fail of making fome 
progrefs ; efpecially:in a branch of the in- 
quiry which is already puthed to’a confi- 
| derable 
4 
- {pace FE will be occupied by a penumbra gradually dark- 
er from Eto F: Let now GH continue fixed, and CD 
move parallel to the plane EF; and, as foon as it 
paffes the line LF, it is evident, that the thadow QF will : 
feem to {well outwards, and when CD reaches ME, fo as 
to cover with its fhadow the {pace RE, QF by its exten- 
fion will cover FE. This is found to uuold true likeways 
by experiment. 
