662 .PHILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



wind, will be pressed on by a force equal to the weight of a column of water, 

 having its base equal to the surface that is opposed, and its height equal to the 

 altitude of the column of water sustained by the wind in the wind gage. Hence 

 the force of the wind on any body, where the surface opposed to it is known, 

 may be easily found ; and a ready comparison may be made between the strength 

 of one gale of wind and that of another, by knowing the heights of the columns 

 of water, which the different winds were capable of sustaining. The heights of 

 the columns in each leg will be equal, provided the legs are of equal bores ; 

 but unequal, if their bores are unequal. For suppose the legs equal, and the 

 column of water the wind sustains to be 3 inches, the water in the leg, which 

 the wind blows into, will be depressed 1-l inch below O, and raised just as much 

 above it in the other leg. But if the bore of the leg which the wind blows 

 into, be double that of the other, the water in that leg will be depressed only 



1 inch, while it is raised twice as much, or 2 inches, in the other ; and vice 

 versa, if the same wind blows into the smaller leg, it will depress the water in it 



2 inches, while it raises it only 1 inch in the other. The force of the wind may 

 be likewise measured with this instrument, by filling it till the water runs out 

 of the hole g. For if we then hold it up to the wind as before, a quantity of 

 water will be blown out ; and, if both legs of the instrument are of the same 

 bore, the height of the column sustained, will be equal to double the column 

 of water in either leg, or the sum of what is wanting in both legs. But if the 

 legs are of unequal bores, neither of these will give the true height of the 

 column of water which the wind sustained. But the true height may be 

 obtained by the following formulae. 



Suppose that after a gale of wind, which had blown the water in one of the 

 tubes from A to b, fig. 6, forcing it at the same time through the other tube out 

 at E, the surface of the water should be found standing at some level dg, and 

 it were required to know what was the height of the column ef or ab, which 

 the wind sustained. In order to obtain which, it is only necessary to find the 

 height of the columns db or gp, which are constantly equal to each other : for 

 either of these added to one of the equal columns ad, eg, will give the true 

 height of the column of water which the wind sustained. 



Case 1. Let the diameters ac, eh, of the tubes be respectively represented by 

 c, d; and let a = ad or eg, and a? =. db or gf. Then it is evident that the 

 column DB, is to the column eg, as c^x to d'^a. But these columns are equal. 

 Therefore c'x ■=. dra; and consequently a? = — r. — Example. If the diameters 

 AC, eh, be respectively 10 and 1, and ad or eg = 3.96 inches, x will be = 

 .0396 of an inch. For d^a — \ X 8.96 = 3.96, which divided by c^ = 100, , 

 gives x = .0396. 



Case 2. But if at any instant of time, while the wind was blowing, it was 



