232 Prof. Draper's Description of the Tithonometer. 



over. It is scarcely necessary to say that the sentient tube must 

 be uniformly and perfectly clean. As a general rule also, the 

 first observation may be cast aside, 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 blackened portion at d. If the 

 measures of the different parts be such as have been here given, 

 this cannot take place, for the liquid will fall below the fiftieth 

 division before its other extremity rises above d. 



The same original volume of gas in a d will last for a long 

 time, as we keep replenishing it as often as the fiftieth division 

 is reached. 



The experimenter cannot help remarking, that on suddenly ex- 

 posing the sentient tube to a 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- 

 servations will compare ; and if they have not, the value can from 

 time to time be ascertained by exposure to a lamp of constant in- 

 tensity. To 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 experimenter as a 

 common differential thermometer. 



University of New York, Sept. 26, 1843. 



