ACTION AT A DISTANCE. 313 



The disk, therefore, does not act where it is not. It sets the air next it 

 in motion by pushing it, this motion is communicated to more and more distant 

 portions of the air in turn, and thus the pressures on opposite sides of the 

 suspended body are rendered unequal, and it moves towards the disk in conse- 

 quence of the excess of pressure. The force is therefore a force of the old 

 school a case of vis a tergo a shove from behind. 



The advocates of the doctrine of action at a distance, however, have not 

 been put to silence by such arguments. What right, say they, have we to 

 assert that a body cannot act where it is not ? Do we not see an instance of 

 action at a distance in the case of a magnet, which acts on another magnet not 

 only at a distance, but with the most complete indifference to the nature of 

 the matter which occupies the intervening space ? If the action depends on 

 something occupying the space between the two magnets, it cannot surely be a 

 matter of indifference whether this space is filled with air or not, or whether 

 wood, glass, or copper, be placed between the magnets. 



Besides this, Newton's law of gravitation, which every astronomical obser- 

 vation only tends to establish more firmly, asserts not only that the heavenly 

 bodies act on one another across immense intervals of space, but that two 

 portions of matter, the one buried a thousand miles deep in the interior of 

 the earth, and the other a hundred thousand miles deep in the body of the 

 sun, act on one another with precisely the same force as if the strata beneath 

 which each is buried had been non-existent. If any medium takes part in 

 transmitting this action, it must surely make some difference whether the space 

 between the bodies contains nothing but this medium, or whether it is occupied 

 by the dense matter of the earth or of the sun. 



But the advocates of direct action at a distance are not content with 

 instances of this kind, in which the phenomena, even at first sight, appear to 

 favour their doctrine. They push their operations into the enemy's camp, and 

 maintain that even when the action is apparently the pressure of contiguous 

 portions of matter, the contiguity is only apparent that a space always inter- 

 venes between the bodies which act on each other. They assert, in short, that 

 so far from action at a distance being impossible, it is the only kind of action 

 which ever occurs, and that the favourite old vis a tergo of the schools has no 

 existence in nature, and exists only in the imagination of schoolmen. 



The best way to prove that when one body pushes another it does not 

 touch it, is to measure the distance between them. Here are two glass lenses, 



VOL. ii. 40 



