152 THE ANTARCTIC MANUAL. 
Directions for Hanging a Barometer on Board Ship.-—The baro- 
meter is to be hung in the inhabited part of the ship, in a saloon or 
cabin, not on deck, bet with a substantial deck above it, and out 
of the radius of either sidelights or skylights. In such conditions 
the temperature of the barometer and that of its attached thermo- 
meter may agree. 
The Effect of Change of the Force of Gravity on the Pressure of the 
Atmosphere and on the Height of the Barometer.—If we consider 
a siphon-barometer, and take the plane of the surface of the mer- 
cury in the open limb as datwm level, all that is above it in one 
limb exercises the same pressure as all that is above it in the 
other. But all that is above it in the outer or open limb is the 
column of the atmosphere, and all that is above it in the inner or 
closed limb is the column of mercury: therefore, the weight of the 
column of mercury is the same as the weight of the column of air. 
The weight of a body is the product of its mass into the force of 
gravity at the place. The force of gravity is the same * in both limbs 
of the barometer, therefore the mass of the column of air is the same 
as the mass of the column of mercury. If, other things remaining 
the same, we reduce the density of the earth to one-half, there will 
still be equilibrium between the columns of air and of mercury in 
the barometer. The height of the barometer will be unaltered, yet 
the pressure of each of these columns on its base will now be only 
one-half of what it was before, because the pressure of the column 
depends on its weight, and the weight is proportional to the force of 
gravity. In both cases we shall have correctly the height of the 
barometer, and it will have remained unaltered, but with equal heights 
of the barometer the pressure of the air will, in the latter case, be 
one-half of what it was in the former. 
The force of gravity is not the same at all points of the earth’s 
surface. Hence arises the necessity for the gravitation correction, in 
order, from the observed height of the barometer, to ascertain the 
true pressure of the atmosphere expressed in stardard units, that is, 
in millimetres of mercury having a temperature of 0° C., and subject 
to the attractive force of gravity which it would experience at the 
sea-level in latitude 45°. This force generates in one second a 
velocity of 980°6 centimetres per second. 
If we consider the layer of air of 1 mm. in thickness which is 
contiguous to the surface of the mercury in the outer limb of the 
* Such slight differences as are due to the greater distance of the centre of the 
earth from the centre of gravity of the almospheric column than from that of the 
mercurial column are here neglected. 
