CHEMICAL AND PHYSICAL NOTES, 159 
of variation of distance has been exactly compensated by variation 
of the effective attracting mass. In the example taken we find the 
force of gravity in lat. 10° greater than it should be, and in lat. 70° 
less than it should be. 
As the attraction of the whole earth on a particle at its surface 
is the sum of the attractions of all the particles that make up its 
mass; and as the attractive force of each particle of the earth’s mass 
on the particle at its surface is inversely proportional to the square 
of its distance from that particle, it is evident that the particles in 
the vicinity of the attracted body will exercise, mass for mass, a 
much greater attraction on the body than the particles that are more 
remote, for instance, at the opposite end of the diameter of the 
sphere. Therefore, the occurrence of rocks of relatively high density 
near the surface in a locality may easily cause an exaggerated forse 
of gravity in the locality, and the occurrence of rocks of relatively 
low density may cause an analogous deficiency of local gravitational 
attraction. 
Looking to the great variety of mineral substances which we meet 
with in the surface crust of the earth, and to the differences of their 
densities, it is not surprising that recent observations with the 
pendulum show that local peculiarities of the force of gravity are the 
rule and not the exception. The differences in neighbouring localities 
are not usually great, but the pendulum is an instrument of almost 
infinite delicacy, so that with patience the gravitational map of a 
district can be correctly constructed, no matter how slight the 
differences may be. 
It will be evident from the examination of Table XX., that by the 
combined use of the barometer and the hypsometer we have in our 
hands the means of determining local deviations of the force of 
gravity from its normal value. 
Rule for detecting the deviation of the local force of gravity from 
the normal by simultaneous observations of the standard height of the 
barometer and of the boiling-point of water. 
Determine the height of the barometer at 0° C. Apply the 
gravitation correction for distance of the locality from the centre of 
the earth, so as to reduce it to its equivalent height when the mercury 
is exposed to the attraction of the standard force of gravity, as at the 
sea-level in latitude 45°. Determine the boiling point of water at 
the same time and place. Refer to the tables connecting this boiling 
point of water with standard barometric pressure, and from them take 
out the boiling point corresponding to the observed standard baro- 
metric pressure. If the tabular and the observed boiling points are 
