Hare’s Eudiometers, &c. 313 
employed to confine it, and the heat of the hand may ren- 
der the result inaccurate. There is no simple mode of 
causing the surface of the gas in a measure glass to form a 
plane corresponding with the brim of the measure glass 
containing it. The transfer of small portions of gas without 
loss, especially from large bells into small tubes is very diffi- 
cult. Hence there is trouble, delay and waste. 
I shall proceed to describe some instruments which I have 
lately invented, and which appear to be free from the: dis- 
advantages above described. ‘They are all essentially de- 
pendent on one principle for their superiority.* 
A recurved glass tube is furnished with a sliding wire of 
iron or copper, graduated into two hundred parts. The 
process of making wire by drawing it through a hole, ren- 
ders its circumferences of necessity every where equal and 
homologous. Consequently equal lengths will contain 
equal bulks. 
The wire slides through a cork soaked in bees-wax and 
oil, and compressed by a screw, so that neither air nor water 
can pass by it. 
The length of the longer leg is fifteen inches, that of the 
shorter one six inches. The bore of the tube is from ;4 to 
-£, an inch in diameter, but converges towards the termina- 
tion of the shorter leg to an orifice about large enough to ad- 
mit a brass pin. Over this a screw is sometimes affixed, so 
as to close it when necessary. 
The tube being filled with water or mercury, and the 
wire pushed into it as far as it can go, on drawing this out 
again any desired distance, an equivalent bulk of air must 
enter the capillary orifice if open. By forcing the rod back 
again into the tube, the air must be proportionably excluded. 
Thus the movements of the sliding wire are accompanied 
by a corresponding ingress or egress of air, and to know 
how many divisions of the former have been pushed into 
the tube, or withdrawn from it, is the same as to know how 
much air has been drawn in or expelled. 
If, instead of allowing the orifice to be in the open air, it 
be introduced within a bell glass, holding gas over the pneu- 
matic apparatus, on pulling out the wire, there will bea 
corresponding entrance of gas into the instrument; and it » 
must be evident that if the point of the gas. measures be 
* See the plate at the end of the velume. 
p 
