290 THE POPULAR SCIENCE MONTHLY. 



teresting to review the state of our knowledge on this obscure subject, 

 and to show on what sides the question is accessible to science. 



The form of the planets is itself an index, to a certain point, of the 

 mode of their origin and their actual condition. These slightly flat- 

 tened globes that wheel about the sun have been subject to the same 

 laws that shape the drop of water and the grain of shot. It is impos- 

 sible not to believe that they are specimens on a vast scale of the 

 equilibrated form assumed by free fluid masses through the action of 

 internal forces which assemble and unite their molecules. All these 

 spheroids have been or still are liquid drops that have become flattened 

 by reason of their rotary motion. Newton was led to infer the flatten- 

 ing of the poles from the idea that the earth had originally been in a 

 liquid state, as the centrifugal force due to rotation tends to swell 

 the equatorial at the expense of the polar regions. By the operation 

 of the same force that impels a stone when swung in a sling to free 

 itself, and that causes grindstones to burst when turned too rapidly, 

 the particles of a revolving sphere tend to fly from the axis of rotation, 

 and this centrifugal force, nil at the poles, increases as the equator is 

 approached, and there attains its maximum intensity. The effect of 

 this is to diminish weight, substances being a little less heavy at the 

 equator than at the poles. 



Imagine the earth completely liquid : the equatorial portion, driven 

 by centrifugal force, will be elevated while the poles will be depressed. 

 To better comprehend this, let us imagine a siphon, the two arms of 

 which, joined at the center, issue, one at one of the poles and the other 

 at the equator. The two liquid columns therein can remain in equi- 

 librium, as the globe revolves, only on condition that the equatorial 

 column, which is exposed to the action of centrifugal force, be longer 

 than the polar column, which has lost nothing of its weight from this 

 cause. The sphere becomes a flattened spheroid. This change of form 

 can be demonstrated by turning rapidly on its vertical axis a sphere of 

 clay or of flexible steel circles, as used in illustration of physics. As 

 the pliant mass solidifies more or less completely, this flattened form 

 is preserved. 



That there is a discrepancy between weight at the equator and at 

 the poles, more marked as we approach or recede from one or the 

 other, may be shown by noting, by the tension of a spring, the weight 

 of the same mass under different latitudes ; but a more positive means 

 of ascertaining this fact is furnished by the oscillations of the pendu- 

 lum, which are retarded as the force of the earth's attraction dimin- 

 ishes. The astronomer Richer, having been sent. to Cayenne in 1672 

 to observe the planet Mars, remarked that a timepiece regulated at 

 Paris lost ten and a half minutes daily at Cayenne. It was this cir- 

 cumstance, at first inexplicable, that led Newton to suspect that the 

 earth was a flattened spheroid. 



It will be evident that an exact knowledge of the figure of the earth 



