VOL, LXV.] PHILOSOPHICAL TRANSACTIONS. 663 



observed, that when the water stood at e, the top of the tube out of which it is 

 forced, it was depressed in the other tube to some given level bf, the altitude at 

 which it would have stood in each, had it immediately subsided, may be found 

 in the following manner. Let i = ab or ep. Then it is evident, that the co- 

 lumn DB is equal to the difference of the columns ef, gf. But the diff'erence 

 of these columns is as cPb — d'x. Therefore c^x = d^b — d^x; and conse- 

 quently,a;=^-;^,. ' HB tb.. 



For the cases when the wind blows in at the narrow leg of the instrument. 

 Let AB = EF = /), EG or AD = a, GF = DB = r, and the diameters eh, ca, 

 respectively = d, c, as before. Then it is evident that the column ad, is to the 

 column GP, as ac^ to d'x. But these columns are equal. Therefore d^x = ac''; 

 and consequently x = -^. This answers to case 1. It is also evident, that the 



column AD is equal to the difference of the columns ab, db. But the difference 

 of these columns is as bc^ — c^x. Therefore d^x = bc^ — c^x. Whence we get 

 X = ., , . This corresponds to case 2. 



As there is always a calculation to be made for every experiment when the legs 

 of the instrument are of unequal bores. Dr. L. recommends it to the makers of 

 these instruments to choose tubes that are equal, or at least nearly so, that the 

 error may become next to nothing, it being a thing very easy to be done. In 

 this manner we can readily determine the greatest force, with which the wind 

 has blown during the time the instrument has been exposed to its action. But 

 as it may be safely left alone, by screwing its spindle into the proper stand, or 

 into the top of a post, and as the wind never fails to turn the mouth of it towards 

 itself, it is not necessary for the observer to continue always by it ; for it may be 

 allowed to stand all night, exposed to the wind, without any inconvenience, 

 though it should even happen to rain very heavily. However, recourse can only 

 be had to this method of using the instrument on shore; for at sea it must al- 

 ways be held up in a perpendicular position in the hand, whether it be used when 

 only half full of water, or when quite full ; which last will be frequently found 

 to be the only practicable method of ascertaining the force of the wind during 

 the night, when it blows so hard that it is impossible to keep any lights on deck. 



A person filling the wind-gage, in a calm place, with water, in order to de- 

 termine the force of the wind, in the way just described, will be apt to imagine, 

 that it cannot give the measurement correct; for he will find such a repulsion 

 to arise from the edges of the hole g, as to sustain a column of water in the 

 kneed or bent tube, perhaps half an inch above the level; but by either blowing 

 across the round hole, or moving his finger over it, he will soon bring the water 

 in the kneed tube to stand at the same level with it, by taking off^ gradually the 

 convex surface of the water, which projects out at the hole in the form of a 



