Proceedings of Societies . 
323 
1879.] 
three nitrogen lines, with three well-defined bands and one doubt- 
ful band. The nitrogen lines correspond to the spedlrum of air, 
and the bands appear to Dr. Schuster to agree with the spedtrum 
of the light round the negative pole of the spark in a tube con- 
taining oxygen with adulteration of carbonic oxide. 
Prof. Ayrton exhibited an Exisothermal Model of a Cooling 
Globe. If we imagine a globe initially heated throughout to a 
uniform temperature (as was probably the earth), and then kept 
in a space having a constant temperature, but much lower than 
that to which the globe was heated, then the temperature at 
every point of the ball will fall, but at very different rates ; the 
parts, for example, near the surface cooling comparatively rapidly, 
while those near the centre will cool very slowly. A surface 
could, therefore, be constructed, such that the x of any point on 
this surface represented its distance from the centre of the globe, 
they the time t from the commencement of the cooling, and the 
z the temperature of that point at that moment. The nature of 
this surface would depend on the size of the globe, on the spe- 
cific heat, conductivity, and surface emissivity of the material. 
The “ Experiments on the Heat Conductivity in Stone ” of Profs. 
Ayrton and Perry, described in the “ Philosophical Magazine” 
for April, 1878, enables them to determine these constants accu- 
rately for a trachyte sphere, and by using these data they have 
been enabled subsequently to construct such a surface, called by 
them an “ exisothermal ” one for a trachyte globe of 8000 miles 
in diameter, and which gives graphically the temperature of 
every single point of the earth from the moment when it was at 
the temperature of molten trachyte down to eight hundred 
thousand million years afterwards, — that is, until long after the 
present era. 
March 8. — Prof. Ayrton brought forward a new theory of ter- 
restrial magnetism, originated by himself and Prof. Perry, of 
the Imperial Engineering College, Japan. (See p. 287.) 
Mr. F. D. Brown described his apparatus for maintaining 
constant temperatures and pressures. A constant temperature 
can be obtained if the pressure can be kept constant. The 
vessel in which the constant pressure is desired communicates 
with an air-pump by a pipe in which a movable tap or valve is 
placed. By opening or closing this tap the pressure is regulated. 
This is effected by an eledlric clutch arrangement. A mercury 
anemometer sends a positive or negative current from a battery 
through the clutch according as the pressure is too high or low, 
and this current adluates the clutch to close or open the valve. 
The clutch consists of an axle driven by a turbine to get power 
to work the valve, and the current, by means of eledtro-mag- 
netism, connedls the tap to the axle, which then opens or closes 
it as the case may be. In this way a pressure varying no more 
than one-fifth millimetre each way can be obtained. 
