100 Description of the Steam Pyrometer. 
The advantage of this arrangement is the saving of time, and the 
only inconvenience, that the lamp must be relighted after each ex- 
periment, as it will be extinguished by closing P and preventing the 
access of air from below to K. It must obviously not be kept burn- 
ing under the generator during the experiment. 
Instead of employing weights as at W, w, to reproduce the counter- 
poise, or show the equivalent weight of steam produced, I have gradu- 
ated one end or base of the counterpoise C, by radiant lines, and caus- 
ed to be removed a segment of about 60° along the screw D, through 
its whole length, so as to present a vertical plane Surface, on which 
to form a scale; the graduations of this scale are, of course, regula- 
ted by the distance apart of the threads of the screw; the weight 
of the counterpoise is such that one revolution on the thread produces 
a difference of one hundredth of a pound at the boiler end of the 
beam. ‘The periphery of C is then graduated into one hundred equal 
divisions, (indicated by the figures 0, 1, 2, 3, &c.) so that, as a com- 
plete revolution of the counterpoise, towards the end of the rod, 
marks an increase of one hundredth of a pound in the weight of 
water put into A, so a corresponding movement in the reverse direc- 
tion, compensates for the same amount of loss by evaporation; and 
a movement through one of the centigrade divisions only, or one 
hundredth of a revolution, as marked on the end of the cylinder, of 
course indicates one hundredth of the above amount, namely one 
ten thousandth of a pound. If greater exactness were required, it 
might be obtained either by making the threads at a less distance 
apart, or by diminishing the counterpoise C, and substituting for a 
part of its weight a fixed weight M. 
I have found the apparatus sensible to the fourteen thousandth of a 
pound or half of a grain, when fully charged for use, that is, when the 
boiler contained at least sixteen ounces of water. 
The arrangement above indicated may be varied to suit the differ- 
ent purposes to which the instrument may be applied, and the stand- 
ards will be different according to the temperatures to which they 
must be exposed. Ifthe substance, of which the temperature is to 
be ascertained, can with safety be plunged beneath the surface of — 
boiling water, without causing either chemical change or variation of 
specific gravity, this direct action of the substance is doubtless to be 
preferred to the intervention of any second substance as a standard. 
The quantity, or number of therms* of heat present, in a given weight 
of the substance in question, will then be known, and if we know or 
* See Ch. Dupin. Méchanique, tome 3. p. 353, et seqq. 
