422 
ON THE MECHANICAL EQUIVALENT OF HEAT. 
mercury-gauge, and is maintained by occasional puminng with the syringe seen in connection. The 
nozzle on the barometer, to which the air-passage is connected, leads into the cast-iron bottle wliich 
forms the mercury-chamber, above the surface of the mercury. The level of this surface is observed 
through the circular windows, of which that which is in front is shown to the left of the axis of the 
barometer, above the nozzle. Immediately above this window is seen the cylindrical brass curtain, which 
screws on to the neck of the bottle, by which the light through the windows over the mercury can be 
eclipsed. Attached to this curtain, and co-axial with it, is the outer brass tube extending up to the gap, 
with a vertical scale attached reaching past the gap. Behind the vertical scale, and screwed into 
the tube on the lower curtain, is a tube screwed throughout its length, and having two parallel 
slots, as window's, some 5 inches long, through which the upper limb of the mercury may be observed. 
From the top of this windowed tube downward is screwed the cap, the lower limb of which forms a 
cylindrical curtain for eclipsing the light over the upper limb of the mercuiw (§ 48). 
