36 
Proceedings of the Royal Irish Academy. 
course, this height and the pressure are convertible terms, but for 
the sake of precision as also for the more distinct instruction of 
the public, the double indication would perhaps be preferable. As 
moreover, the Barometer is intended to measure a weight, there 
would be a certain propriety in distinctly showing the nature and 
amount of weight measured by it. 
As the cylindrical form of tube presents very great advantages, by 
reason of its regularity and the facility with which it can be obtained, 
there would appear to be a certain advantage in employing it for the 
barometric column. In this case it is evident that a counterpoise of 
fixed weight cannot insure regularity of movement and a due control 
of the extent of oscillation. To remedy this inconvenience, I would 
propose for the cylindrical tube that the weight of the counterpoise be 
rendered variable, proportionally to the increasing volume of mercury 
in the column requiring counterpoise. This may, I believe, be effected 
very simply, and with any degree of precision, by suspending in the 
plane of oscillation of the counterpoise, and immediately over it, a chain 
formed of transverse bars, whose length, and therefore weight, can be 
varied according to any arbitrary graduation, while the degree of curve or 
deflexion fi'om the straight line would be equally under command. If 
now it be supposed that the counterpoise in rising come in contact with 
this chain, it will be loaded, progressively with its rise, according to any 
desii^ed law of increase of weight ; so that the increases of weight on the 
one side of the axis can be perfectly counterpoised on the other, or so 
counterpoised as always to insure action of a constant deflecting weight, 
whatever the degree of inclination from the vertical. In order, more- 
over, to increase the extent of the indication on the graduated scale, it 
would be possible to establish two columns in juxtaposition — that is, 
having their axes of suspension on the same line, but arranged, as 
regards their counterpoises, so as to oscillate on opposite sides of the 
vertical. The indicators would thus open out like the legs of a pair 
of compasses ; and the extent of arc lying between their extremities 
would represent the double of the real indicators, giving a 7nean read- 
ing, and therefore doubling the accuracy of the instrument. 
IX. — Additioi^ to the I^ote on two Steeams flowing feom the same 
SoTJECE IN Opposite Dieections.^- By Peofessoe Hennessy, F.E. S., 
Vice-Peesident. 
[Read March 16, 1870.] 
In describing the flow of two different streams from one source, in my 
Note published in Part iii., vol. x. (Ser. i.) of the Proceedings," I an- 
ticipated that the phenomenon which I had observed during very dry 
weather would become clearly manifested after heavy rains. This an- 
ticipation has been recently verified by Mr. E. A. Gray, C. E., County 
* Vol. X., p. 335. 
