246 Sundry Improvements in Apparatus or Manipulation. 
Hydro-Pneumatic Cistern. 
Fig. 1. In Silliman’s Journal, Vol. xiv, p- 200, will be found an 
engraving and description of a pneumatic cistern, which I employed 
in the experimental illustrations of my lectures for more than ten 
years ; and which I should probably continue to use now, had not 
the command of water from the public works, put it into my power 
to dispense with the mechanism for keeping the water at a proper 
level. As I am now situated, any deficit of water is easily supplied 
from the pipes known here as the hydrant pipes, by which the city 
is supplied with water ; and any excess is carried off by a waste pipe. 
Many chemists designate as a pneumatic trough or tub, apparatus 
for the purposes to which that in question is applied. Neither of 
these names is, in my opinion, as applicable to the apparatus which 
1 have hitherto used, as that of cistern, to which I resorted; and 
although the last term be less suitable to the apparatus which I am 
about to describe, yet 1 beg leave to adhere to it for want of a better 
appellation. 
A A, a water tight platform, surrounded by a woodenrim, RRRRA, 
rising above it about an inch anda half. B,C, D, three wells or 
cavities, each in the form of a hollow parallelopiped, with all of 
which the cavity bounded by the rim communicates, so that when 
supplied with water to the level of the waste pipe, this liquid fills 
the wells, and covers the platform to the depth of about three fourths 
ofanjinch,-> 
E, F, G, shelves, which severally move in grooves over the wells, 
so that they may be placed in the most convenient position. Under 
H is a waste pipe. AtIis ahydrant pipe. K,a pipe for emptying 
the wells and casks, with all of which it may be made to communi- 
cate by cocks, when requisite. N,O, casks which act as gas hold- 
ers, each having a communication with the cistern at Q or q, for 
letting in water from that source; the orifices being controlled by 
valves. By means of a pipe proceeding from its vertex, each gas 
holder communicates with a pipe or cock, at S ors. 
To these gallows screws, flexible leaden pipes may be attached, 
for transferring gas either from one of the holders to a bell glass, or 
from a bell glass to one of the holders. When a communication is 
established between the cavities, either of these offices may be per- 
formed, accordingly as the pressure within the holder is made greater 
or less than that of the atmosphere. It will be greater when the 
