ATTRACTION, 183 



Fig. 2. 



" Fig. 2 affords a view of the lower side 

 of the sliding shelf, in the wood of which 

 it will be seen that there are two excava- 

 tions, converging into two holes, one of 

 which is seen at A, fig. 1. This shelf is 

 loaded with an ingot of lead at L, to prevent it from floating in the 

 water of the cistern." 



" Besides the chests abovementioned, there are two others, C C, 

 near the bottom of the cistern, but not so close as to prevent the wa- 

 ter from passing freely into and out of them." 



Referring to Dr. Hare's Compendium for the remainder of the 

 description, I will add only, that the inverted kettle by a treadle be- 

 low, and by the aid of a peculiar internal construction, is made to 

 throw in air through the lower arched tubes, into the cells under 

 C C, which are intended for regulating the height of the water ; while 

 it is allowed to escape through the upper arched tubes at their com- 

 mon orifice at/. The cells under A A, are for receiving any gas 

 not absorbable by water, and it is easily drawn off at the cocks at e e, 

 into vessels standing in the shelves A A. 



The student will not suppose that, strictly, any thing more is ne- 

 cessary for a pneumatic cistern, than a water vessel with a fixed shelf 

 or shelves as at A A, and a sliding shelf as at B, and even the latter 

 may be dispensed with by making holes through one of the fixed 

 shelves, and introducing an inverted funnel. 



GAZOMETERS. 



Gazometers are important in many chemical experiments. In 

 contriving the pneumatic cistern mentioned above, it was one object 

 to furnish gazometers in the cistern itself, where most of the gases 

 are prepared ; and there was, for many purposes, great utility in the 

 contrivance ; but the gases being always under pressure, were of 

 course liable to escape at any leak. 



There is so much convenience, however, in occupying with air 

 cells, this otherwise useless space, that I should still recommend this 

 mode of construction, of the pneumatic cistern, so far as the cells are 

 concerned ; without attempting any thing farther, except the neces- 

 sary appendages to draw off the gases. 



On the' whole, I have found the most useful species of gazometer 

 to be the following, which, it will be perceived, is only a modifica- 

 tion of the form generally used. 



1. The containing air vessel is made of tinned iron, or the thin- 

 nest sheet copper, painted and varnished : the form is cylindrical, as at 



