THE MECHANICAL EQUIVALENT OF HEAT. 
377 
means of tap M) and opening i^^and H —independently of the level of the mercury in the 
regulator. This contrivance, therefore, rendered it possible to “set” the instrument 
so as to produce any required change of temperature in the tank ; for it was found 
that the transference of about 312 millims. of the column HL into the regulator 
caused a difference of 1 ° C. in the temperature of the tank. It was thus possible to 
adjust the temperature to a very small fraction of a degree. 
The connecting tube from E to D was bent into a spring of sufficient length to 
allow of some play to the lower end, and thus it was possible to withdraw the end 
from D without moving the whole edifice. The glass work was supported on a 
vertical board, about 5 feet in height, and the various taps were placed within easy 
reach of the observer. All gas admitted at A was dried by passing through tubes 
containing quicklime, as it was found that the regulator worked in a much more satis¬ 
factory manner when the surface of the mercury was clean. After leaving B, the gas 
entered a tube about 2 feet long, containing a very large number of small pin-holes. 
The tube was connected by two cross pieces containing similar small holes, with two 
pilot lights fed by an independent supply, A change of 3-^0 of a degree in the tank 
visibly affected the brilliancy of the tiny gas jets, and a change of less than made 
all the difference between total eclipse and a complete series of bright blue jets. The 
supply of gas through the regulator was, however, sufficient when the mercury had 
receded far enough to maintain the jets at a length of nearly |-inch. 
Placed above this row of jets, but so far from the pilot lights as to be unaffected by 
them, was a flat silver tube, the section of whose bore was a rectangle of about 1 inch 
wide and inch deep, and, through this tube, the water passed on its way to the 
tank. The supply was regulated by a small “ constant level ” tank of the usual 
pattern, suspended from the ceiling by a string. Alterations in the level of this tank 
afforded a fine adjustment—the temperature depending on the rate of flow. The 
water was supplied direct from the main, and hence, even in the hottest summer 
weather, it was possible to maintain the tank at a temperature as low as 14° ; in 
winter, as low^ as 3° or 4° C. Any temperature above this minimum was, of course, 
obtainable, but we considered it unadvisable to raise it above 40° C. on account of the 
ebonite, &c. This regulator proved itself to be a most satisfactory instrument. 
Although, when set to give a new temperature, it v/as slow in its action (the capacity 
for heat of the tank and its contents being great) it was none the less sure, and, after 
about half-an-hour from the readjustment, the temperature would be found to have 
become steady. If set to give a higher temperature, the thermometer at E (Plate 3) 
would pass that temperature by about 3 ^ 5 - of a degree, and, after this preliminary 
oscillation, would become steady, although it is probable that this first was followed 
by other oscillations, which were too small to be apparent. It is possible that 
the temperature of the inner surface of the steel chamber was, to some extent, 
affected by the temperature of the calorimeter. The change must, however, have 
been very small, since it was insufficient to affect the thermometer at E. It must 
MDCCCXCIII.— A. 3 C 
