142 Captain Sabine on the 



out ; but if its substance had admitted of compression, the outer 

 portion would have been the most rare, and the mass would have 

 increased gradually in density to the centre. 



But the form of the earth is affected by another circumstance. 

 When a body is made to revolve around a fixed axis, its several 

 particles describe circles, whose planes are perpendicular to, and 

 centres are in the axis. In this way all the particles, except 

 those situated in the axis itself, become affected by a centrifugal 

 force, that, did no other power oppose its action, would cause 

 them to fly off, in tangents to the curves in which they revolve : 

 this force is in each particle proportioned to the radius of the 

 circle it describes. In solid bodies the attraction of aggregation 

 is, generally speaking, sufficient to prevent any disintegration, as 

 a consequence of the action of the centrifugal force ; and in the 

 larger masses of matter, whether solid or fluid, the attraction of 

 gravitation produces the same effect. Our earth is a body that 

 is in a state of rapid rotatory motion, performing a complete re- 

 volution around its axis in the space of a sidereal day. Each 

 point upon its surface is therefore acted upon by a centrifugal 

 force. This is greatest at the Equator, and becomes zero at the 

 Poles ; and although the attraction of gravitation is far more 

 than sufficient to render this centrifugal force of no effect, in 

 throwing off any portion of the matter of which the earth, or its 

 surrounding atmosphere, is composed, it is yet rendered manifest 

 by a diminution in the intensity of the force of gravity. This 

 diminution of the intensity of gravitation will affect the rate at 

 which heavy bodies fall to the surface of the earth, and the time 

 of the oscillations of pendulums. It was first observed by Richer, 

 a French astronomer, who visited Cayenne in 1672, for the pur- 

 pose of making astronomical observations : he was furnished with 

 a clock that marked mean solar time in the latitude of Paris ; and to 

 his surprise he found that at Cayenne, in Lat. 5° N. its rate had 

 become 2' 28" per day too slow. As this was a far greater 

 change than could be accounted for by any alteration in the 

 length of the pendulum, caused by difference of temperature, no 

 explanation remained, except that furnished by the opposition of 

 the centrifugal force to the attractive power of the earth. The 

 centrifugal force, as has already been stated, is proportionate, at 

 any point of the earth's surface, to the radius of the circle de- 

 scribed by that point in its diurnal revolution, or to the cosine of 

 the latitude : but this is not the measure of the diminution it 

 causes in the intensity of gravitation ; for the latter acts in the 

 direction of a radius of the terrestrial sphere, while the former is 

 parallel to an equatorial diameter : on account of this obliquity of 

 action, the diminution in the force of gravity, arising from the 

 diurnal rotation of the earth, is everywhere proportioned, not to 

 the cosine of the latitude simply, but to its square. 



