36 . 



revolution in the orbit and rotation on the axis, and even calculat- 

 ed the precise point at which it must have been applied, so as to 

 accord with the existing motions and velocities" of the earth and 

 the moon.* 



But Bernouilli's hypothesis proceeds on the assumption, com- 

 mon to him with other mathematicians and astronomers, that the 

 planets and their satellites move ahoutfreel?/ in space, under the 

 single condition of revolving in certain definite orbits, a condi- 

 tion which affects their centres only, leaving their circumferences 

 and all the other particles of the mass perfectly free. But if 

 there be any truth at all in the Newtonian doctrine of gravita- 

 tion, or in the common doctrine of the terrestrial tides, all the 

 particles of the circumference and the mass alike are affected by 

 the mutual attractions of the different revolving bodies, those 

 particles that are nearest the external attraction being more, and 

 those farthest off being less affected than the centre itself, which 

 nevertheless, consistently with Bernouilli's hypothesis, must be 

 considered to be the only part affected at all, the attracting power 

 passing by and through the intervening particles without giv- 

 ing token of its presence. Moreover, the rotatory effect of the 

 oblique impulse which he postulates could be permanent only in 

 the case of a body revolving freely in space, that is to say, with 

 only its mathematical centre attracted to the centre of the orbit. 

 But the earth is not such a body, and there is not within our 

 reach any projectile whatever, that can be made to move along an 

 orbit or a curve unaffected by the earth's attraction of gravitation 

 acting upon every particle of its mass from without, and direct- 

 ing or controlling its movements ; and it appears to me that 

 without such an external cause of rotation as gravity supplies, 

 the rotating tendency produced by the oblique impulse would 

 cease as soon as the movement was communicated to the whole 

 mass, which would then fly forward with all its particles in one 

 direction only ; for, as already remarked, they cannot move 

 backwards. All that is wanted is a power to keep them back, 

 and that is more likely to be found without than within the mov- 

 ing body. It is a negative power not very likely to be found in 

 any revolving body. Though, therefore, a stone, or ball, or quoit, 

 thrown from the hand with a force acting obliquely on its centre 

 of gravity, turns round on that centre, yet every calculation re- 



• " Jean Bernouilli, dans un memoire de dynamique, ou il considere les centres 

 spontanea de rotation, fait voir qu'une force de projection applique non pas au 

 centre de la terre, mais un peu plus loin du soleil, et cela de tI^ du rayon, donne- 

 roit a la terre supposee ronde et homogenee, deux mouvemens assez confoimes a 

 ceux que Ton observe ; pour la lune j^^. Si Timpulsion primitive eut ete applique 

 a des plus grandes distances de chaque centre, le mouvement de rotation seroit plus 

 rapide. Encyclopedic^ Art. Rotation. 



