Mr Forbes's description of a new Anemometer. 33 



Besides, it has the advantage of self-registration. We shall 

 first consider the theory of the deflection, then the method 

 employed to put it in practice. Briefly then, the principle of 

 the instrument is to ascertain the measure of the velocity of the 

 passing current^ by the deflection from the perpendicular of 

 small bodies mechanically let Jail at certain intervals, through 

 a certain space. 



The investigation is similar to that of the simplest case of 

 resistance introductory to the theory of projectiles, for it is the 

 same whether we consider the sphere (for we shall suppose the 

 falling bodies to be spherules) moved through the air by a 

 projectile force equal to the velocity of the wind, or the wind 

 itself rushing upon the sphere in a state of rest with that ve- 

 locity. In the former case the resistance would be considered 

 as producing an extinction of motion exactly equal to the pro- 

 duction of motion in the latter. That is, if A B, Fig. 8 

 Plate II. represent the velocity of the wind in any moment of 

 lime, the body being first exposed to it at A, and the resist- 

 ance of the current to its state of rest makes it move in the 

 same time through A C, it is the same as if we suppose the 

 air in a state of rest, and the body at A projected with a velo- 

 city A 6, when, the resistance being obviously the same as before, 

 the space moved through will only be A c, since 6 c = A C 



It is obvious that the sphere, when exposed to the resistance 

 of the air and the action of gravity united, will describe 

 some curve, as A a b c. But we have not, as in the case 

 of projectiles, to compute the resistance on the periphery of 

 the curve, which when at right angles to the wind is wholly 

 inconsiderable, but merely the effort of a constant resistance 

 along the ordinates a a, (3 b, &c. The question is therefore 

 resolved into the simple case of resistance in a rectilineal course 

 co-ordinate with that of gravity, identical, in fact, with Lib. ii. 

 Prop. V. of the Principia, and of which also a very extended 

 solution has been given by Professor Robison, in the elaborate 

 article Projectiles in the Encyclopcedia Britannica. A very 

 brief exposition of the same problem, borrowed partly from 

 both sources, and applied to the present case, may perhaps 

 be not unacceptable to some readers. 



The terminal velocity of a falling body in air is that final 

 limit in which the acceleration of gravity is balanced by the 



NEW SERIES. VOL. II, NO. I. JAN. 1830. C 



