122 THE ANTARCTIC MANUAL. 
temperatures with different hydrometers. The specific gravities given 
are each the means of from nine to eleven separate observations 
on sea water and distilled water at the same time and at the same 
temperature, The greatest difference between any pair of values 
is 3:3 in the fifth decimal place, and the individuals of each pair 
depend on perfectly distinct sets of weighings, and are therefore quite 
independent. 
“Tt may safely be asserted that, working in this way, the specific 
gravity of a sea water or similar solution can be determined with a 
probable error of not more than + 1 in the fifth decimal place. Ina 
water whose specific gravity is 1:03000, 1 in the fifth decimal place 
represents a ae of the whole solid contents; so that by the careful 
use of the hydrometric method, the salinity to one part in 75,000 ot 
water, or differences of 1 grain per gallon, can be determined ; and it 
has proved itself of great use in general chemical practice, especially 
in cases of pollution of streams.” 
METEOROLOGICAL OBSERVATIONS AND INSTRUMENTS. 
Full instructions for installation of instruments, and for methods 
of observation and recording, will be supplied with the instruments. 
It is unnecessary to repeat them here. But there are one or two 
points, seldom adequately insisted on in meteorological instructions, 
which may here be recalled. 
Thermometers—With the thermometers of the Expedition there 
will be supplied information as regards the position of the point of 
melting ice and as regards the agreement of other parts of the scale 
with the standard; but it 1s safe to affirm that no information will 
be supplied with regard to the thermal mass of the thermometers, or 
the rapidity with which they respond to changes of the temperature 
of the medium in which they are placed. The Rate of Cooling, or its 
reciprocal, the Term of Cooling, of a thermometer is as important a 
constant as the position of the ice-point or the length of a degree. 
The following considerations will show that the want of knowledge 
of this constant and of its application may, and in point of fact does, 
frequently introduce error and confusion into meteorological results. 
When two unequal masses of the same substance have the same 
temperature and hang in the same medium, the temperature of 
which is constant and lower than their own, their temperature will 
fall at unequal rates. If, being at the same temperature, they are 
hung in a medium of constant but higher temperature, their tempera- 
