the Refinance of the Air . 
angle as before. I now found, that more weight was requir 
to produce the fame velocity, though the quantity of fur face 
was the fame as in the preceding experiment. The weight 
neceflary to put the machine alone in motion, with the velocity 
above mentioned, was two pounds and a half. When it car- 
ried the parallelogram with one of its fhorteftfides downwards, 
it required four pounds and a half additional weight ; and 
when the parallelogram was reverfed, another half pound was 
barely fufficient to give it the fame velocity. 
The difference, therefore, occafoned by placing the fame pa- 
rallelogram with its longer or fhorter fides inclined from the 
direction of its motion was equal to one- tenth of the greateft 
reliftance. 
It has been obferved, that in thefe two experiments the 
mean velocity of the plane was not the fame, as its extremity 
extended farther from the center of the machine in one than in 
the other. This is ftridtly true ; but the fize of the parallelo- 
gram bore fo fmall a proportion to the length of the radius to 
which it was fallen ed, that the error arifing from this circum- 
ftance is fcarcely perceptible, and the advantage being in favour 
of that which required the leaf weight, I did not think it 
neceflary to bring it into account. 
Having formed a general idea of the reafon of the difference 
in thefe experiments, it occurred to me, that there would be a 
greater difproportion between the refiftance of fome other 
figures, which Mr. robins had not tried; and having put a 
rhomboid, in the form of a lozenge, nine inches long, and 
four broad, in the place of the parallelogram, the difference 
was increafed from one-tenth to one-feventh of the weight 
employed to give them the required velocity. 
T 2 
I \i rfiuing 
