232 Prof. Draper's Description of the Tithonometer. 
over. It is scarcely — to op that the sentient tube must 
be uniformly and perfectly ¢ Asa general rule also, the 
first observation may be cast e, for reasons which I will give 
hereafter. Further, it is to be remarked, as it is an essential prin- 
ciple that during the different changes of volume of the gas its 
exposed surface must never vary in extent, the liquid is not to 
be suffered to rise above the blac 1 portion at d. If the 
measures of the different parts be such a ve been here given, 
this cannot take place, for the liquid will’ fall below the fiftieth 
division before its other extremity rises above d. Nig 
The same original volume of gas in @ d will last for a long — 
time, as we keep as een it as often as the fiftieth division 
is reached. i‘ 
The experimenter cannot help remarking, that on suddenly ei ex- 
posing the sentient tube toa bright light, the liquid for an instant 
rises on the scale, and on dropping the cap in an instant falls. 
This important phenomenon, which is strikingly seen under the 
action of an electric spark, I shall consider hereafter. : 
In conclusion, as to comparing the tithonometric indication at | 
different times, if the gases have the same constitution, the ob- 4 
servations will compare ; and if they have not, the value can from F 
time to time be ascertained by exposure to a lamp of constant in- q 
tensity. 'T'o this method I commonly resort. _ 
From the space occupied in this description the reader might 
be disposed to infer that the tithonometer is a very complicated — 
instrument and difficult to use. He would form, however, an — 
erroneous opinion. ‘The preliminary adjustment can be made in — 
five minutes, and with it an extensive series of measures obtained. | 
These long details have been entered into that the theory of the — 
instrument may be known, and optical artists construct it with- — 
out difficulty.. Though surprisingly sensitive to the action of the 
indigo ray, it is as manageable by a careful ites cee asa 
common differential thermometer. 
University of New York, Sept. 26, 1843. 
