1888.] J. T. Bottomley on a Practical Air Thermometer. 89 
blowing at the junction, as well as in tlie adjusting of the quantity 
of liquid in the bend, the endeavour is made to keep the volume of 
the air-space between the bulb and the point p as small as possible, 
consideration being given to the capillarity of the tube d. Either 
mercury or sulphuric acid may be used in the volume gauge. I 
prefer sulphuric acid on account of its smaller density. The great- 
ness of the density of mercury, and the uncertainty of its capillary 
action, make its use very liable to produce serious errors in reading. 
But, on the other hand, in the case of sulphuric acid, the wetting of 
the tubes, which constitutes its advantageous quality so far as capil- 
larity is concerned and gives regularity of capillary action which 
mercury never possesses, renders watchfulness necessary to keep the 
acid well clear of the fine tube c. If once the acid is allowed to 
enter that tube, it tends to make its way along it towards the bulb. 
The manometric tube is simply a U tube capable of giving a 
difference of levels of from 100 to 150 centimetres of mercury, and 
wide enough to make capillarity very small and difference of 
capillarities in the two tubes negligible. With a tube giving 
a difference of levels of 150 centimetres, a temperature of about 
550° C. may be reached, starting with air at normal density 
at common temperatures. The difference of levels may be read 
by means of a kathetometer, or, what is preferable, the tubes 
themselves may be graduated to millimetres. The tubes which I 
use are graduated from a zero line which is at the middle of the 
long branch of the U (see fig. 1). The longer tube is numbered 
upward, and the shorter downwards from the zero line, and the 
mercury is filled in so as to stand at the zero in both branches when 
there is no difference of pressure, and thus the sum of the readings 
of the two tubes is equal to the difference of pressures when any 
difference of pressure exists. 
The pressure apparatus consists of a simple pressure syringe 
which forces air into a small air-bag of india-rubber fortified with 
canvas. The air-bag is placed between two boards, which are con- 
nected by a leather hinge, and pressed together by means of a nut 
which works on a wooden screw. The air-bag is also connected by 
means of a T tube with the three-way stop-cock t ' ; and, by means 
of this stop-cock, presses both on the liquid in the volume gauge, and 
on the shorter column of the manometer. The india-rubber tubes 
