98 THE ANTARCTIC MANUAL. 
degree Centigrade was used, and the mercury stood at 100°-°18; that 
is the top of the mercury was exactly in line with the centre of the 
division on the stem marking 100°-18 C. It occupied this position 
when steam was being generated at its highest rate of 8°4 grammes 
per minute, and never varied by a fraction of the width of a division 
line until the rate of steam generation had been brought so low that 
it issued as an exhalation, and not a stream. The rate was then 2°5 
grammes per minute, and the top of the mercury fell to the lower 
edge of the line marking 100°°18 C., which may be taken to 
represent a temperature of 100°°179 C. 
In this experiment the rate of passage of steam was varied from 
2+5 to 8-4 grammes per minute, or over three-fold, and it produced 
no effect on the thermometer; therefore, the exit tube efficiently 
removed whatever amount of steam entered it, and offered no sensible 
resistance to it. No better evidence than this could be furnished 
of the perfect efficiency of the instrument for the purpose for which 
it was designed. 
With regard to the relative advantages of the thermometer 
and the barometer in hypsometric work, this thermometer may 
be taken as an example. It is divided into fiftieths of a Celsius 
degree, the length of the degree being 35 mm. The tension of 
saturated vapour, of 100° C., is 760 mm., and of 99° C., it is 
733°305 mm. giving a difference of 26°695 mm. pressure for a 
difference of 1° C. temperature. Thus, 26'7 mm. on the barometer 
are represented by 35 mm. on the thermometer. Converting the 
readings of the thermometer into the corresponding ones of the 
barometer, each division would correspond to about half a millimetre. 
At lower pressures the effect of change of pressure on the temperature 
of saturation is greater, and at higher pressures it is less. Thus, 
from Regnault’s experiments, we have the difference of pressure 
which causes a difference of 1° C. in the temperature of saturated 
steam at different parts of the scale, as follows: 
Taste IV.—Grivine THe Temperature (Celsius) or SaturaTep Steam T, at the 
Pressure P, and the Difference of Pressure D corresponding to 1° C. Difference 
of Temperature of Saturation. 
T P D T P D T P D 
°C: mm. mm. °C. mm. mm. °C. mm. mm. 
0 4°600 0-335 50 91°982 4°483 | 140 2,717°63 76°19 
10 9-165 0°591 60 148°791 | 6°776 | 160 4,651°62 | 117°26 
20 17°391 1-045 80 354°643 | 14°155 | 180 7,546°39 | 171°87 
30 31-548 1:766 | 100 760°000 | 26°695 } 200 | 11,688°96 | 241:50 
40 54°906 2°867 | 120 |1,491°280 | 46-730 | 220 | 17,390°36 | 327-07 
