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great exactitude. Extremely small additions of fluid will be 
measured, as in the instance with the solid already mentioned, 
and, therefore, its utility as a rain gauge. I also find that, by 
plunging one end of an open tube, bent at right angles, into 
the water in the cistern, allowing the wind to act upon the 
other end,—the surface of the water in the rest of the cistern, 
and the float, being protected from the influence of the wind,— 
the float will ascend in exact proportion to the force of the 
wind, depressing the water in the tube. I have one of these 
instruments so sensitive that mere breathing or speaking in 
front of the open end of the tube will act on the float. It has 
also occurred to us, that a solid cylinder of iron placed in a 
properly formed cistern of a barometer, would keep the level 
of the mercury in the cistern constant during the ascent and 
descent of the mercury in the tube, and would save much 
trouble in determining the true difference of height of a co- 
lumn in the barometer, which could always be read off at once 
on the scale without any allowance for difference of level; the 
surface level of the cistern being maintained to the 1-2500th 
of an inch by the action of the float. 
Since I became acquainted with the properties of the ro- 
tating float, my friend, Mr. Richard E. Donovan, who first in- 
troduced it to my notice, has informed me that he has recently 
heard that a similar float had been proposed many years ago 
as a method for maintaining the oil level inalamp. However, 
if that be true, the valuable properties it possesses as a hydro- 
static balance could not have been investigated, otherwise it 
would not have been forgotten. I must here acknowledge 
the obligations I am under to Mr. Donovan, for the part he 
took in carrying on the experiments and the calculations ne- 
cessary for this paper. 
The President presented to the Academy, on the part of 
the present Earl of Charleville, a portrait of his grandfather, 
