CONNECTION OF ELECTEICITY AND MAGNETISM. 255 



and it is asked^ by tbe advocates of mediate action, whether, in those 

 cases in which we cannot perceive the intermediate agency, is it not 

 more philosophical to admit the existence of a medium which we cannot 

 at present perceive than to assert that a body can act at a place where 

 it is not? 



To a person ignorant of the properties of air, the transmission offeree 

 by means of that invisible medium would appear as unaccountable as 

 any other example of action at a distance, and yet in this case we can 

 explain the whole process and determine the rate at which the action is 

 passed on from one portion to another of the medium. 



Why, then, should we not admit that the familiar mode of communi- 

 cating motion by pushing and pulling with our hands is the type and 

 exemplitication of all action between bodies, even in cases in which we 

 can observe nothing between the bodies which appears to take part in 

 the action ? 



Here, for instance, is a kind of attraction with which Professor Guth- 

 rie has made us familiar. A disk is set in vibration, and is then brought 

 near a light suspended body, which immediately begins to move toward 

 the disk, as if drawn toward it b^^ an invisible cord. What is this cord ? 



Sir W. Thomson has pointed out that in a moving fluid the pressure 

 is least where the velocity is greatest. The velocity of the vibratory 

 motion of the air is greatest nearest the disk. Hence the pressure of 

 the air on the suspended body is less on the side nearest the disk than 

 on the opposite side 5 the body yields to the greater pressure, and moves 

 toward the disk. 



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 op- 

 posite sides of the suspended body are rendered unequal, and it moves 

 toward the disk in consequence 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 

 indifterence 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, l!«rewton's law of gravitation, which every astronomical 

 observation 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 



