of Edinburgh, Session 1879-80. 533 
surface in the shorter vertical column. It is satisfactory, however, 
that t ie instrument is really not needed for temperatures above 
+ 10 C., because for such the water steam-pressure thermometer, 
in the first of the three diagrams of 
-30 
-25 
-20 
- 15 ‘ 
-10 
- 5 ' 
10 ° 
10 ° 
15 ° 
20 ° 
- 25 ° 
- 30 ° 
Fig. 1. 
my former communication, has ample sensibility 
for most practical purposes. Hence, instead of 
the range up to 25° C. in the instrument already 
realised, and the great length of tube (295 centi- 
metres for the long vertical branch) which it 
requires, I propose in future to let +10° be the 
superior limit of the temperatures to be measured 
by an ordinary sulphurous acid steam thermo- 
meter. For this, the long vertical branch need 
not be more than 175 centimetres; thus the 
instrument is much more easily made, and when 
made, is much less cumbrous. 
(2.) The upper end of the long branch, being 
open to begin with, is to be securely cemented 
to a small and very perfectly air-tight iron stop- 
cock L, communicating with an iron pipe, bent 
at right angles, as shown in the drawing (fig. 2). 
This iron pipe is, in the first place, to be put into 
communication temporarily by an india-rubber 
junction with the generator, and with an air-pump, 
by means of a metal branch tube, with two stop- 
cocks R and S, as shown in the drawing. 
(3.) To begin, close R and open S and L; and 
exhaust moderately (down to half an inch of 
mercury will suffice). Warm the whole length of 
the bent tube moderately by a spirit lamp, or spirit 
lamps, to dry the inner surface sufficiently. Then, 
still maintaining the exhaustion by the air-pump, 
apply a freezing mixture to the bulb and shorter 
vertical tube, and all of the long vertical tube except a convenient 
length of a foot or two next its upper end, as shown in the 
drawing. Before joining the generator to Q, let enough of sul- 
phurous acid gas be passed out through P to clear out fairly well 
the air from the generator and the purifying sulphuric acid wash- 
