154 THE ANTARCTIC MANUAL. 
lower the less air there is in the column above it, or the greater 
the height of the locality above the level of the sea. If we know 
the law regulating the relation between pressure of the atmosphere 
and elevation above sea level, we have in the boiling-point of water 
a means of determining the elevation, because the tension of the 
vapour of the boiling liquid is equal to the tension of the air on 
its surface, and the tension of the air at the bottom of a column of 
it is equal to the pressure of the column, and the relation between 
air pressure and altitude is known. 
If the quantity of air in the column resting on the boiling water 
remain constant, and the force of gravity, or the attractive force of 
the earth ona particle at the surface of the water be altered, then the 
pressure of the column of air on the surface of the water and the 
tension of the air at the bottom of the column will be altered 
correspondingly. 
If the force of gravity has been increased, the pressure of the air 
and the tension of the air at the bottom of the column will be 
increased ; and if the temperature of the water remain the same the 
tension of its vapour will be less than the tension of the air in con- 
tact with it, and it will cease to boil. If further heat be supplied to 
it, its temperature will rise to that at which the vapour tension of the 
water is equal to that of the increased tension of the air at its surface, 
and the water will again boil. 
If the force of gravity has been reduced, the pressure of the atmo- 
sphere and the tension of the air in contact with the boiling water 
will be reduced. The vapour tension of the water will then be 
higher than that of the air in contact with it, the water will be super- 
heated and will boil for a time without any further supply of heat, 
the super-heat in the water being sufficient. When this is exhausted, 
the temperature of the water will be reduced to that at which its 
vapour tension is the same as the reduced tension of the air in con- 
tact with it, and with a further supply of heat it will continue to boil 
at this temperature. 
During these changes of the force of gravity and consequent 
changes of the boiling-point of water, the height of the barometer 
has remained without change, because the changes of force of gravity 
act equally on both limbs of the siphon. 
In the same way a balance which is loaded with a litre of water 
on the one pan and a kilogramme of platinum on the other remains in 
equilibrium, whatever changes may be made in the force of gravity. 
If, however, the litre of water were weighed on a spring balance its 
weight would vary in the same proportion as the force of gravity. 
