O'Reilly-— the Barometer. 
35 
ment of mercury becomes rapidly great, the angle of inclination 
increasing the level in the basin is proportionally altered, while a nice 
adjustment of the counterpoise can hardly under these circumstances 
be attained ; the extreme angular displacement must be avoided, and 
therefore, the total angle of oscillation is so far reduced. On the con- 
trary, by adopting a conical form for the portion of column above 
0™, 80, such a modification of the displacement of mercury as may be 
considered advisable can be attained, regularity of motion be insured, 
the volume of mercury in the basin reduced, and its level more per- 
sistently maintained ; while, finally, an extreme position of the column 
will not present serious inconvenience, and the maximum angle of 
oscillation will be insured. 
The points of suspension might be steel screw points, working on 
agate cups. 
The counterpoise proper might be made long proportionally, so as 
to reduce its weight. 
The arrangement of the index and counterpoise symmetrically with 
the axis of the tube, as in the drawing (PI. XL), would be the best, since 
space would thus be allowed for an orifice in the basin on the axis 
of the column, by means of which the mercury could be introduced or 
removed with facility. 
The diameter of the basin is such that the open extremity of the 
open tube remains covered for an extreme position by at least 4 centi- 
metres deep of mercury, while the length of the basin is supposed to 
be such as will reduce the total variation of level of mercury, to about 
5 centimetres ; of course, if desirable, a greater length may be adopted, 
so as to reduce this variation still more. 
It is unnecessary to remark that such an instrument presents the 
great inconvenience of requiring considerable space, and would require, 
equally as a balance, to be closed in from dust, air currents, &c., so as 
to insure precision and delicacy. But, admitting these inconveniences, 
and the defects inherent to a rough idea, there remains the advantage 
of a direct system of indication without the intervention of any trans- 
mission whatever, as also that of allowing the scale of indications to 
be considerably extended, by a simple increase in length of the index, 
without in any way increasing the weight of the instrument, and 
therefore either altering its delicacy or diminishing its sensibility of 
movement. Such an instrument could only be adapted for public 
buildings where barometric observations have a direct practical value, 
such as those connected with the service of ports, and where small 
variations of pressure may have great significance as regards weather 
forecasts. Eor such uses, there is a direct interest to render the 
observation of the Barometer at once distinct and marked, and the 
indications easily legible for the ordinary eyesight. 
In connexion with this question, might I not suggest that the at- 
mospheric pressure in pounds to the square inch, or in kilogrammes 
per centimetre square, be shown on the scale of the Barometer, in- 
stead of a vertical height of mercury in inches or millimetres. Of 
