ON AERIAL NAVtGATION. 



Hfr 



Ian this point. The instrument was similar to that used 

 by Mr. Robins, but the surface used was larger, being an 

 exact square foot, moving round upon an arm about live 

 feet long, and turned by weights over a pulley. The time 

 was measured by a stop watch, and the distance travelled 

 over in each experiment was 600 feet. I shall for the pre- 

 sent only give the result cf many carefully repeated expe- 

 riments, which is, that a velocity of 11'538 feet per second 

 generated a resistance of 4 ounces; and that a velocity of 

 17 - 16 feet per second gave 8 ounces resistance. This deli- 

 cate instrument would have been strained by the additional 

 weight necessary to have tried the velocity generating a 

 pressure of one pound per square foot ^ but if the resist- 

 ance be taken to vary as the square of the velocity, the for- 

 mer will give the velocity necessary for this purpose at I3'l 

 feet, the latter 24*28 per second. I shall therefore take 

 23*0 feet as somewhat approaching the truth. 



Having ascertained this point, had our tables of angular Our tables of 



resistance been complete, the size of the surface necessary an £ n,I,r 1 **** 



. ance unpentct, 



for any given weight would easily have been determined. 



Theory, which gives the resistance of a surface opposed to 

 the same current in different angles, to be as the squares of 

 the sine of the angle of incidence, is of no use in this case; 

 as it appears from the experiments of the French Academy, 

 that in acute angles, the resistance varies much more nearly 

 in the direct ratio of the sines, than as the squares of the 

 sines of the angles of incidence. The flight of birds will Concave wine 

 prove to an attentive observer, that, with a concave wing «f a bad. 

 apparently parallel to the horizontal path of the bird, the 

 same support, and of course resistance, is obtained. And 

 hence I am inclined to suspect, that, under extremely acute 

 angles, with concave surfaces, the resistance is nearly simi- 

 lar in them all. I conceive the operation may be of a dif- 

 ferent nature from what takes place in larger angles, and 

 may partake more of the principle of pressure exhibited iu 

 tha instrument known by the name of the hydrostatic para- 

 dox, a slender filament of the current is constantly received 

 under the anterior edge of the surface, and directed up- 

 ward into the cavity, by the filament above it, in being 

 obliged to mount along the convexity of the surface, having 



created 



