MECHANICS. 38 
regulation of the clock. Newton, however, first announced the proposition, 
that the same pendulum, in different places on the earth’s surface, must 
make different oscillations. The astronomer, Richer, who journeyed to 
Cayenne in 1672, verified this observation, as the difference of the rate of a 
clock at Paris and Cayenne required a shortening of the pendulum by 14 
line. By means of accurate experiments it was afterwards found, that for 
the different latitudes of St. Thomas (0° 24’ 41”) and Spitzbergen (79° 49 
58’), the length of the pendulum varied from 39.021 and 39.215 Paris 
inches (more accurately in English inches, and reduced to the level of the 
sea, 39.02074 and 39.21469). 
Even if the highest mountains and the deepest seas produce no change in 
the general form of the earth, by reason of their small size compared with 
the earth’s radius, yet the rotation of the earth on its axis must theoretically 
cause a heaping up of its mass at the equator, and a flattening at the poles, 
so that the earth, instead of being a sphere, must be really an oblate 
spheroid. Measurements of degrees of the meridian have determined the 
amount of this oblateness. [f, for example, Dunkirk and Formentera lie 
nearly on the same meridian, and their distance from trigonometrical mea- 
surement amounts to 1374438.72 metres (the angular distance being 12° 
22’ 14’’), it becomes easy to determine the length of one degree of the 
meridian. If now the earth were a sphere, all degrees of the meridian 
would be equal. Measurements of degrees in different latitudes, however, 
have shown that this is not the case, but that the length of a degree of the 
terrestrial meridian continually decreases from the poles to the equator ; the 
radius of the equator accordingly amounts to 6,376,984 metres, and that of 
the poles, 6,356,324; a difference of 20,660 metres. The mean radius of 
the earth corresponds to that of latitude 45°, and amounts to 6,366,745 
metres. The length of the pendulum is in strict relation to these measure- 
ments, for the seconds’ pendulum is shorter, the nearer the place of obser- 
vation to the equator, so that the seconds’ pendulum of Paris would make 
126 oscillations less in a day, at the equator. Hence it follows that the 
intensity of gravity diminishes with the distance from the centre of the 
earth, and experiments with the pendulum, carried on at different heights 
above the level of the sea, confirm this statement. 
Considering that the centrifugal force increases towards the equator, and 
that nearer the equator the distance from the centre is greater, it becomes 
possible, knowing the length of a seconds’ pendulum at Paris. to determine 
that for any other place on the earth’s surface; here, however, the greater 
or less density of the earth’s crust comes into account; as it is found that 
there are always slight discrepancies between the calculated and actual 
pendulum lengths—differences which may sometimes amount to four or five 
oscillations inaday. To this belongs the deviation experienced by the plum- 
met in the vicinity of mountains. Bouguer was the first who was struck 
with the idea of finding in mountains a proof of the universal attraction of 
matter. His investigations in the slopes of Chimborazo, combined with 
astronomical measurements, showed a deviation of the plummet of seven to 
eight seconds. Maskelyne found the deviation at the foot of Shehallien in 
TCONOGRAPHIC ENCYCLOP£ZDIA.—VOL. I. 14 209 
