64 
its end. This wheel is supported with its plane in a per- 
pendicular position by a knife edge of hardened steel which 
passes through its geometric centre and rests on agate 
planes. The tube of the thermometer is partly filled with 
mercury — preferably through half its curve — and for the 
reason given in my description of the simple sunshine 
recorder, to which I have alluded, a little sulphuric acid is 
introduced as well. If we now arrange it that the centre 
of gravity of the solid portion of the system here described 
shall be below the surface of the planes on which it turns 
(and the apparatus is provided with adjustments by means 
of which the point in question can be moved) it is clear 
that the arrangement may be made to swing in pendulous 
oscillations, notwithstanding the presence of the liquids it 
contains, for these remain substantially at rest whilst the 
tube which holds them does, in fact, slide over them (and 
with very little friction too) in swinging to and fro through 
arcs of the circles of which its parts are curves. Both bulbs 
of the thermometer are closed. It is obvious therefore that 
the tension of the air or gas which they contain will be 
uninfluenced by the barometric variations of the outer air, 
the temperature of which latter being experienced equally 
in each bulb will also leave the equilibrium of the apparatus 
undisturbed. When however one bulb is more heated than 
the other, the air contained therein will press more strongly 
on the heavy liquid piston in the tube and wheel the swing- 
ing portion of the system round until a fresh position of 
equilibrium is gained, and this will be (providing that the 
centre of gravity of the system has previously been made 
coincident with the point on which it turns) when the ten- 
sion of the gases is equal in both bulbs. In fact, in so far 
as now described, tlie instrument is a differential thermo- 
meter, and is that alone, differing in this from Leslie’s, that 
it is a solid and accessible portion of the thing which moves, 
and not the liquid it contains. When however one of the 
