[ 3^4 ] 
dupois. w may reprefent a fquare yard, the furface of 
a fail, 8cc. 
If the velocity and denlity of the wind in any parti- 
cular cafe were accurately determined, this inflrument, 
which gives its force or momentum^ would enable us to 
afcertain the velocity in every other cafe, the denlity be- 
ing known. For it appears from experiments, made by 
Mr. JAMES FERGUSON, F. R. s. Oil the w'hirling-table, 
that its force is as the fquare of its velocity. But as the 
denlity, which is one of the data requilite for determin- 
ing the velocity by this inftrument, was not taken into 
conlideration in thefe experiments, all that we can do at 
prefent is to fuggeft the idea. 
It may not, perhaps, be improper to take notice, that 
evaporation will have fome elfe6l in diminilliing the al- 
titude of the column of water ; though its influence, for 
the moll part, will be very inconliderable. The more 
frequently, therefore, the inftrument is examined, it will 
be fo much the better. If it be expofed to the a6lion of 
the wind, whilft it happens to fnow, it will be necelfary 
to look at it frequently, leaft the fnow lliould choak up 
the mouth of the wind-gage. 
Extract of a letter from Dr. lind to Col. roy. Dated 
Edinburgh, May 26, 1775. 
The wind-gage ought to be fome what longer than that 
I lately fent Sir JOHN pringle. For we had a gale here on 
the 9th current, which fupported a column of water 
of 
