Prof. Draper’s Description of the Tithonometer. 231 
Put on the cap of the sentie! ; zd, connect the polar wires 
with pq, and raise the liquid 
Place the ‘cence 
rays properly. 
its sentient tube will receive the 
y aseconds watch, remove the cap 
begins to descend. At the end of the 
first minute, read off the division over which it is passing. Sup- 
pose itis 7. At the end of the second do the same, it should be 
14; at the end a third 21, &c. This may be done until the 
‘is reached, which is the terminus of the scale. 
ALO 
_ wires: hat 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 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 
