120 THE ANTARCTIC MANUAL. 
a similar variation would take place in the unit to which they are 
referred, we effect a reduction to their value, taking the density of 
distilled water at 4° as unity. This is obtained by multiplying the 
figures obtained as above for the specific gravity by the weight of 
1 cc. of distilled water at the temperature of observation. This 
is one of the physical constants which have been determined with 
the greatest care and presumably with the greatest precision, and 
therefore, if reductions are to be admissible at all, this one can be 
made without fear of error. The specific gravity multiplied by 
this weight of unit volume of distilled water of the same temperature 
gives the density of the sample water at that temperature, that is, it 
gives the weight of 1 cc. of it in grammes. It may also be correctly 
described as the specific gravity of the water at the temperature, 
that of distilled water of 4° C. being unity. 
“The density at temperature of observation can now at once be 
reduced to its value at whatever is chosen as common temperature, 
and at the temperature which it had 7 sitw. Naturally, these 
reduced values are affected by whatever uncertainty attaches to the 
tables used. For purposes of control, it is well, in an expedition, to 
preserve very carefully considerable samples of typical waters, and, 
as opportunity offers, to determine their specific gravity against dis- 
tilled water at different temperatures. There is no reason to suppose 
that the precision of these determinations would stand in any way 
behind that of the observations on which the tables are founded, and 
as they would have been made on the actual waters which are under 
consideration, they should have the preference. 
“Tt will be obvious from these remarks how necessary it is, in an 
expedition, to have a supply of perfectly pure distilled water, to 
make parallel observations under the conditions on board. Also, as 
before recommended, typical samples of the waters collected should 
be kept for careful determination of their density with the same 
hydrometer at different temperatures, and especially at or near the 
temperature taken as the common temperature of reduction, 
‘For observing on board ship, I find that the method used in the 
Challenger, of placing the cylinder on a swinging table, gives better 
results than any other. In the P’rineesse Alice, a ship of not more 
than 600 tons, the motion at sea is always considerable, but in ordi- 
nary circumstances the maximum amplitude of the motion of the 
floating hydrometer was not over 3 mm. The vibration period of 
the hydrometer always interferes with that of the ship and swinging 
table, producing moments of rest, and my experience is that a mode- 
ate rate of motion ig an advantage, The individual readings of a 
