664 PHILOSOPHICAL TRANSACTIONS. [aNNO 1775. 



drop or spherule; and this effect the wind very soon produces itself. There 

 ought always to be a cover on the top of the tube, out of which the water is 

 expelled by the wind; but it should be made very thin. For if there be no such 

 cover, and the mouth of the kneed tube be stopped, after the instrument is 

 quite full of water, in order to prevent the wind from having any influence in 

 raising it, you will find, on exposing it to a strong gale, that in a very short 

 time it will blow out perhaps half an inch of water. Whence it appears, that a 

 very considerable error would arise from using the wind-gage in this state. 

 The use of the small tube of communication ab (fig. 4) is to check the undula- 

 tion of the water, so that its height may be read off from the scale with ease 

 and certainty. But it is particularly designed to prevent the water from being 

 thrown up to a much greater or less altitude, than the true height of the column, 

 which the wind is able at that time to sustain, from its receiving a sudden im- 

 pulse, while it is vibrating either in its ascent or descent. For water in the legs 

 of a siphon is capable of being put into a vibrating motion like a pendulum ; 

 and therefore, if acted on when in the ascent, the height which it ascends to will 

 come out greater than the truth, and less, if acted on in the descent. 



The height of the column of water sustained in the wind-gage being given, 

 the force of the wind on a foot square is easily had by the following table, and 

 consequently on any known surface. 



TABLE I. It may be sometimes neces- 



Height of the wa- Force of the wind on the Common desig- Sary tO employ Other fluids be- 



ler in the wind- foot square in avoirdu- nation of such . , , , • t i • ,- , i 



gage. pois pounds. a wind. sidcs Water, particularly if the 



12 inches 62.5 decree of cold be below freez- 



11 ..57.293 ■ r ., 



10 52.083") , ., , . ing; for then we must use a 



n .,-„_,> most Violent humcane. n j ^i. ^ u <. r 



9 40.875 J fluid that will not freeze in 



8 ...41.057. .very great ditto. ^u j r u • uu 



7 36.548. . great hurricane. the degree of cold in which 



6 31 .75 . . hurricane. vve expose the instrument, 



5 26.041. .very great storm. ^.i • ^i • j i 



4 20.833.. great ditto. Otherwise the wind can have 



3 15.625.. storm. no influence on it, and the 



2 10.416. . very high wind. ,. /■ . • .i ^ i_ mi 



1 5.208. . high wind. "quor freezing m the tube will 



oi 2.604. . brisk gale. break it. Dr. L. therefore 



Ms 521. . fresh breeze. ,• r v ■ .u 



oS 260. . pleasant wind. mentions a few liquors in the 



0^ 130. .a gentle wind. following table that will an- 



swer the purpose, as also subjoins a general method of reducing them all to one 

 common measure. But of all the fluids he was acquainted with, when the effects 

 of frost are to be feared, he knew none better adapted to the purpose than a sa- 

 turated solution of sea-salt; since it does not freeze till the thermometer falls to 

 O degrees, and is a fluid constantly of the same specific gravity. Spirit of wine, 

 independent of its being more variable in respect of specific gravity by the influ- 



