[ 1 73 ] 
Here it may reafonably be afked, that fince our 
geometrical demon fixation is general, and proves, that 
one angle of obliquity is as good as another ; why in 
our experiments it appears, that there is a certain 
angle which is to be preferred to all the reft ? It is to 
be obferved, that if the breadth of the fail I N is 
given, the greater the angle K I N, and the lefs will 
be the bafe I K : that is, the fedtion of wind inter- 
fered, will be lefs : on the other hand, the more 
acute the angle KIN, the lefs will be the perpendi- 
cular KN : that is, the impulfe of the wind, in the 
diredtion I K being lefs, and the velocity of the fail 
greater ; the refiftance of the medium will be greater 
alfo. Hence therefore, as there is a diminution of the 
fedtion of the wind intercepted on one hand, and an 
increafe of refiftance on the other, there is fome angle, 
where the difad vantage arifing from thefe caufes up- 
on the whole is the leaft of all ; but as the difadvan- 
tage arifing from refiftance is more of a phyfical than 
geometrical confideration, the true angle will beft be 
afiigned by experiment. 
Scholium. 
In trying the experiments contained in Tab. III. 
and IV. the different fpecific gravity of the air, which 
is undoubtedly different at different times, will caufe 
a difference in the load, proportional to the difference 
of its fpecific gravity, tho’ its velocity remains the 
fame ; and a variation of fpecific gravity may arife 
not only from a variation of the weight of the whole 
column, but alfo by the difference of heat of the air 
concerned in the experiment, and pofiibly of other 
caufes 3 yet the irregularities that might arife from a 
dif- 
