172 Transactions. — Miscellaneous. 



Tliese apparent paradoxes may be partly explained when we consider 

 the perfect elasticity of the air ; for when it strikes the point P it then, 

 after compression, by yhtue of its elasticity, diverges from P at all angles 

 along the plane ; so readily does it escape laterally that it appears to draw 

 a large volume of formerly quiescent air down too. 



Ex]]eriment, No. 4. — Reversing experiment No. 3, " action and re- 

 action being equal and contrary," it follows that when a plane A P B 

 (figure 2) oblique to the horizoij is carried with its upward and anterior 

 edge in a horizontal direction, the tendency of the incident current I P 

 is not to be reflected along the line P Q, but the air is retained closer to 

 the surface of the plane, which fact must very materially increase the 

 lateral pressure, and therefore greatly assist in buoying up the plane. 



Experiment, No. 5.- — In order to ascertain the lifting pressure exerted by 

 the inertia and elasticity of the air on a plane set at various angles and 

 travelling with a given velocity, the apparatus here exhibited was devised ; 

 it consists of a thin sheet of metal a foot square, so connected with a spring 

 balance that it can be set to any given angle. Action and reaction being 

 equal and contrary, it is clear that if this instrument be set in a current of 

 air the same effects are obtained as if the plane were moved at the same 

 velocity through still air. The instrument was placed in a strong wind, 

 and when the plane was first placed at a right angle to the current, with 

 the spring so arranged as to show the horizontal pressure, it registered an 

 average pressure of 2-7 lbs. on the square foot, indicating a velocity of the 

 air of 23 miles per hour. The instrument was then so arranged as to 

 measure the vertical pressure when the plane was set at various angles to 

 the horizon. The following table gives a summary of the average results 

 of a number of experiments therewith : — 



Angle to Hobizon. 



Lifting 



Peessuee in Lbs. 



5° 





1-13 



10° 





1-43 



20° 





1-65 



30° 





1-83 



40° 





2-00 



60° 





1-80 



By this we see that the lifting pressm-e of a plane one foot square travelling 

 through still air is more than half as great at an angle of 5° as it is at 40°, 

 while we know that the resistance to its forward or horizontal motion is 

 almost removed, for considerably less air has to be displaced. In fact, the 

 inertia of the air is utilized with small angles, for considerably greater 

 velocity can be imparted to the plane with the same expenditure of force. 



From the foregoing experiments it appears that the law of "resolution 

 of forces " as applied to solids is inapplicable in the case of gases. More- 



