30*4 PHILOSOPHICAL TRANSACTIONS. |_ANNO J 783. 



and the advantage being in favour of that which required the least weight, I did 

 not think it necessary to bring it into account. 



Having formed a general idea of the reason of the difference in these experi- 

 ments, it occurred to me, that there would be a greater disproportion between 

 the resistance of some other figures, which Mr. Robins had not tried; and hav- 

 ing put a rhomboid, in the form of a lozenge, Q inches long, and 4 broad, in 

 the place of the parallelogram, the difference was increased from -^ to \ of the 

 weight employed to give them the required velocity. 



Pursuing the same reasoning that led me to the last experiment, it occurred, 

 that even against figures of exactly the same shape, the resistance of the air, 

 when the dimensions of the figures were enlarged, would not be increased in the 

 same proportion as the size of the planes, but in a much higher ratio ; and that, 

 by bending the planes as a sail, the resistance would be still further increased, 

 though the section of air, that would be intercepted by the planes, must by 

 these means be considerably lessened. The result far surpassed my expectations. 

 A square ot tin, containing 16 square inches, placed perpendicularly, was re- 

 sisted as 2i. A square, containing 64 inches, or 4 times the former quantity, 

 instead of meeting with a resistance as 10, or 4 times the former resistance, re- 

 quired no less than 14 lb. to give it the same velocity. 



I now placed the parallelogram of 9 inches long on the arms of the machine, 

 with its shorter sides parallel to the horizon, bending it to such an arch that its 

 chord measured 8 inches, and inclining it to an angle of 45 degrees. And 

 though the section of air that it intercepted was by these means diminished -i-, 

 yet the resistance was increased from 5 to 5-±-. And when the parallelogram was 

 bent yet more, and its chord contracted almost to 7 inches, the resistance was 

 increased to 5-f-. I mention these numbers in gross to avoid confusion ; but in 

 the table at the end of this paper, the measures and weights are set down 

 exactly. 



Dr. Hook, whose name must be respected by every experimental philosopher, 

 was aware, that though he thought he could demonstrate that flat sails were pre- 

 ferable to such as were curved and hollowed by the wind, yet until proper ex- 

 periments had been tried, nothing could be positively determined. He says, 

 somewhere in his posthumous works, " That he was surprised at the obstinacy 

 of seamen, in continuing, after what appeared the clearest demonstration to the 

 contrary, to prefer bellying or bunting sails to such as were hauled taught ; but 

 that he would, at some future time, add the test of experiment to mathematical 

 investigation." He reasoned on a supposition, that the air in motion followed 

 the same laws as light ; and that it was reflected from surfaces with the angle of 

 reflection equal to the angle of incidence, which is not the case, as it never 

 makes an angle with the plane, but is always reflected in curves. Mons. Parent, 



