132 THE AMERICAN MONTHLY [July, 
It is obvious, that if the bath be filled with water below the temperature 
desired, the mercury will not rise and touch the wire B, thus making con- 
nection with the other wire, until the bath reaches that temperature, and that, 
as soon as the temperature 
falls below this point, the 
mercury will fall with it, 
and away from the wire B; 
also that by raising,or lower- 
ing, this wire the connection 
can be made to take place 
at any desired higher or 
lower temperature. This 
regulating thermometer 
will be found to be suf- 
ficiently delicate to keep 
the temperature to within 
two degrees. It can be 
made, by simply blowing a 
bulb on a glass tube and 
filling the bulb and tube 
with mercury alone. 
Fitting over the top of the 
lamp chimney is a chimney 
(D, fig. 2), elevenvinghes 
long and 23 inches square, 
having at its top a hot-air 
chamber E, into which the 
water bath fits. This cham- 
ber has noles around the 
sides, near the bottom, for 
the escape of hot air. The 
chimney D has at one side a 
branch chimney F, twelve inches long, opening into it at an angle of 45°. 
In the opening between the chimneys is hung a valve G, turning on a hinge 
H, and moved by a lever I, on the outside. This valve should be very light 
and must turn easily onthe hinge which is made by hanging the valve, fast- 
ened on a wire passed through holes on the sides of the chimney ; to this wire 
is attached on the outside the lever, which is to be weighted on the end with 
a small bullet so as nearly to balance the valve, which must just fall of its own 
weight. 
At K is a shelf, 24 inches wide, extending into the chimney D. This 
shelf leaves an opening in the chimney 4 inch wide, which is sufficient for 
the passage upward of the hot air, and which can be readily closed, or opened 
by the valve, without too far swinging. At L is an electro-magnet, 
which is connected with one pole of the battery, and with the regulating ther- 
mometer by means of the wire B. The magnet used by me is a small one, 
wound to 20 ohms resistance, and costing 80 cents. Also the regulating 
thermometer is connected with the other pole of the battery by means of the 
wire A. Both these wires are of course insulated, and can be purchased cov- 
ered, together with the battery and magnet, of any electrical supply house. 
The action of the apparatus is sufficiently plain. The lamp being lighted 
the temperature, of the water bath, will rise, and the mercury, in the regu- 
lating thermometer M, with it, until it touches the wire B, thus closing the 
circuit, and magnetizing the electro-magnet, which will attract the lever I, 
pulling it down, and raising the valve G, so closing the opening in the chim- 
a. 
