414; Professor Draper's Description of the Tithonometer. 



passing. Suppose it is 7. At the end of the second do the 

 same, it should be 14; at the end of the third 21, &c. &c. 

 This may be done until the fiftieth division is reached, which 

 is the terminus of the scale. 



Recharge the tube by a momentary application of the polar 

 wires : but it is convenient first to remove any excess of mu- 

 riatic acid gas in the sentient tube by allowing it time for con- 

 densation ; or, if that be inadmissible, by inclining a little on 

 one side, so as to give an extensive liquid contact. 



Of the Method of interrupted observation. — It frequently 

 happens that observations cannot be had during a continuous 

 descent, as when changes have to be made in parts of appa- 

 ratus or arrangements. We have then to resort to inter- 

 rupted observations. 



This method requires that the gas and liquid should be 

 well adjusted, so that no change can arise in volume when 

 extensive contact is made by inclination. 



The tithonometer being charged, place it in a proper posi- 

 tion. At a given instant remove its cap, and the liquid de- 

 scends. When the time marked by a seconds watch has 

 elapsed, drop the cap on the sentient tube. The liquid si- 

 multaneously pauses in its descent, but does not entirely stop, 

 for a little uncondensed muriatic acid still exists, which is 

 slowly disappearing in the sentient tube. Now, incline the 

 instrument for a moment on one side, so that the liquid may 

 run up to the cord «, but not so much as to let any gas 

 escape. Restore it to its position and read off on the scale. 

 It is then ready for a second trial. 



The difference between continuous and interrupted obser- 

 vation is this, that in the latter we pause to wash out the mu- 

 riatic acid, and though this is effected by the simplest of all 

 possible methods, continuous observations are always to be 

 preferred when they can be obtained. 



I have extended this paper to so great a length that many 

 points on which remarks might have been made must be 

 passed 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 principle that during different changes of vo- 

 lume 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. 



