ON THE MECHANICAL EQUIVALENT OF HEAT. 
345 
The glass stem of the barometer is drawn down into a neck towards the lower end, 
and this is bent through 180° so as to bring the month upwards, and thus admit of 
its introduction into the mercury in the bottle without letting in air. This bend 
has to be passed through the stuffing-box, then the tube is secured by screwing the 
gland on to the beeswax stopping. A brass guard tube is then screwed into the 
neck, to support the glass tube, to a height of 24 inches from the mercury in the 
vessel. 
For reading the height of the lower limb, a cylindrical brass curtain, with a conical 
contraction on the top, the aperture in which is threaded internally at twenty threads 
to an inch to correspond to the screw on the outside of the neck of the bottle, is 
screwed on to this neck, the lip or bottom of the curtain being truly turned so 
that, when screwed down to the level of the mercury, it cuts off the light through 
the windows from a white sheet behind. 
To the top of the brass casting, which forms the curtain, a brass cylindrical tube is 
rigidly attached coaxial with the curtain which fits over the brass guard round the 
barometer tube, this extends to a height of 26 inches from the lower lip, the internal 
diameter for the last inch being a little smaller and internally screwed at twenty 
threads to an inch. Into this is screwed a brass tube, externally screwed throughout 
its length, about 6 inches long, with j^cU-allel opposite slots inch wide extending to 
within an inch at either end, to form windows through which to see the light over 
the upper limb of the mercury. And on to the upper portion of this tube there is 
screwed a long cap, capable of screwing down to the bottom of the slot. The lower 
lip of this cap forms the curtain which cuts off the light when the lip is level with 
the upper limb of the mercury. 
By this arrangement the variation of the distance between the lips of the lower 
and the upper curtains depends only on the change in their relative angular positions. 
For, since the slotted tube has a uniform thread, it can be turned, screwing into the 
lower curtain and out of the upper, both of which remain unmoved. Thus the 
position of the windows may be fixed, while the curtains are moved. So that for 
reading the distances it is only necessary to measure the relative angle. 
This angle is measured by dividing the circumference of the cap just above the lip 
into five equal divisions, from 0 to 5, and these again into ten, then a turn through 
one of the smaller divisions means an alteration in the distance of one-fiftieth of oiie- 
twentieth of an inch. As this angle is measured relatively to the lower curtain, a 
vertical brass scale, divided to tenths and twentieths of an inch, is fixed externally 
to the top of the extension of the lower curtain, extending vertically just outside the 
graduated limb of the upper curtain, and thus serves for reading the angular distance 
of the index mark on the limb of the upper curtain on any particular thread and 
the number of threads from the index on the scale. 
VOL. CXC.-—A. 
2 Y 
