1852.] OF THE ROYAL INSTITUTION. 217 
enters in any way into the exercise of this power, whatever the 
distance between the acting bodies, as that from the sun to the 
earth, or from star to star. We can hardly conceive of this force in 
one particle by itself; it is when two or more are present that we 
comprehend it : yet in gaining this idea we perceive no difference in 
the character of the power in the different particles; all of the same 
kind are equal, mutual, and alike. In the case of gravitation, no 
effect which sustains the idea of an independent or physical line of 
force is presented to us ; and as far as we at present know, the line 
of gravitation is merely an ideal line representing the direction in 
which the power is exerted. 
Take the Sun in relation to another force which it exerts upon the 
earth, namely, its illuminating or warming power. In this case 
rays (which are lines of force) pass across the intermediate space ; 
but then we may affect these lines by different media applied to 
them in their course. We may alter their direction either by re- 
flection or refraction; we may make them pursue curved or angular 
courses. We may cut them off at their origin and then search for 
and find them before they have attained their object. They have a 
relation to ¢ime, and occupy 8 minutes in coming from the sun to the 
earth : so that they may exist independently either of their source or 
their final home, and have in fact a clear distinct physical existence. 
They are in extreme contrast with the lines of gravitating power in 
this respect ; as they are also in respect of their condition at their 
terminations. The two bodies terminating a line of gravitating 
force are alike in their actions in every respect, and so the line 
joining them has like relations in both directions. The two 
bodies at the terminals of a ray are utterly unlike in action; one is 
a source, the other a destroyer of the line; and the line itself has the 
relation of a stream flowing in one direction. In these two cases 
of gravity and radiation, the difference between an abstract and a 
physical line of force is immediately manifest. 
Turning to the case of Static Electricity we find here attractions 
(and other actions) at a distance as in the former cases; but when 
we come to compare the attraction with that of gravity, very striking 
distinctions are presented which immediately affect the question of a 
physical line of force. In the first place, when we examine the 
bodies bounding or terminating the lines of attraction, we find them 
as before, mutually and equally concerned in the action; but they 
are not alike: on the contrary, though each is endued with a force 
which speaking generally is of the like nature, still they are in such 
contrast that their actions on a third body in a state like either of 
them are precisely the reverse of each other, — what the one attracts 
the other repels; and the force makes itself evident as one of those 
manifestations of power endued with a dual and antithetical condi- 
tion. Now with all such dual powers, attraction cannot occur unless 
the two conditions of force are present and in face of each other 
through the lines of force. Another essential limitation is that 
