32 
Proceedings of the Royal Irish Acnde/)/f/. 
mining the arrangement required for the basin of the instrument, I 
was led to that which I now submit, and by means of which the atmo- 
spheric pressure causes the column to oscillate about an axis of suspen- 
sion, precisely as the beam of a balance, and, moreover, to mark the 
amount of oscillation by means of an index attached to the column, 
precisely as the index needle of the beam balance does when oscillating. 
In other words — I attempt to weigh the atmospheric pressure by 
causing it to act at the extremity of a beam, suspended on an axis of 
revolution, and counterpoised at the other extremity by an invariable 
weight. It is, therefore, to all intents and purposes, a beam balance. 
I have endeavoured in the annexed drawing (Plate XI.), to show 
the method of arrangement which a first trial had led me to adopt for 
the better attainment of the proposed result ; and in the course of my 
note I shall have other modifications to submit, which I conceive 
would tend to insure more satisfactory results. But the drawing and 
the model, or first practical attempt exhibited, will prove, I hope, suffi- 
cient for a complete explanation of the idea. I shall first consider the 
model as representing the original plan. It is clearly to be understood 
that it was intended rather to test the fundamental principle of the 
arrangement in the simplest manner possible, than to represent the 
idea in a complete form ; and that, therefore, it in no way pretends to 
those conditions of correctness or delicacy which I believe may be 
attained by a more careful execution and better arrangement of the 
component parts. 
The model consists essentially of four parts — \h.e tube, partly of 
iron, and partly of glass ; the basin of reception for the mercury, made 
of wood, cubical exteriorly, and cylindrical interiorly ; the tube pene- 
trates into the basin, and should rest on the opposite part of the interior 
cylindrical surface ; the basin is not completely filled with mercury, 
but sufficiently to insure that the open end of the tube remains 
covered by the mercury within certain limits of oscillation of the 
column from the vertical. The facility of oscillating is secured by 
means of two bearings placed on the axis of the cylindrical cavity, and 
attached to the sides of the wooden basin. If the basin alone be con- 
sidered, it is evident that in any one position it may assume, by 
turning on the axis, it is in equilibrium, provided the centre of gravity 
of the system is below the axis of suspension. If now it be considered 
in connexion with the column of mercury, it is evident that this state 
of equilibrium will be maintained, provided the weight of the column 
be so counterpoised as to reduce the position of the centre of gravity 
of the combined parts to the position already indicated as regards the 
axis of revolution. 
]^ow, if it be imagined that the column thus counterpoised, and sus- 
pended on an axis of revolution, have its centre of gravity close to the 
axis, an increase of atmospheric pressure, by acting on the mercury in 
the basin, causes it to rise in the tube, and displaces, therefore, the 
centre of gravity of the system. The centre of gravity rising to the 
level of the axis of revolution, or above it, as the case may be, tends to 
