Chemical and Physical Notes 105 



Directions for Hanging a Barometer on Board Ship. The 

 barometer is to be hung in the inhabited part of the ship, in 

 a saloon or cabin, not on deck, but 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 thermometer 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 mercury in the open limb as datum 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 1 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 baro- 

 meter 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 



1 Such slight differences as are due to the greater distance of the centre of 

 the earth from the centre of gravity of the atmospheric column than from that of 

 the mercurial column are here neglected. 



