Prof. Drapes Description of the Tiihonometer. 231 



Put on the cap of the sentient tube ad, connect the polar wires 

 with p q, and raise the liquid to zero. 



Place the tithonometer so that its sentient tube will receive the 

 rays properly. 



At a given instant, marked by a seconds watch, remove the cap 

 A D, and the liquid at once begins to descend. At the end of the 

 first minute, read off the division over which it is passing. Sup- 

 pose it is 7. At the end of the second do the same, it should be 

 14; at the end of the third 21, &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 muriatic 

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

 tion ; 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 hap- 

 pens that observations cannot be had during a continuous descent, 

 as when changes have to be made in parts of apparatus or arrange- 

 ments. We have then to resort to interrupted 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 position. 

 At a given instant remove its cap, and the liquid descends. 

 When the time marked by a seconds watch has elapsed, drop the 

 cap on the sentient tube. The liquid simultaneously pauses in 

 its descent, but does not entirely stop, for a little uncondensed 

 muriatic acid still exists, which is slowly disappearing in the sen- 

 tient tube. Now incline the instrument for a moment on one 

 side, so that the liquid may run up to the cord a, 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 observa- 

 tion is this, that in the latter we pause to wash out the muriatic 

 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 



