450 Scientific Proceedings, Royal Dublin Society. 



inverted. True causes never precede their effects; they are always 

 strictly simultaneous with them. The science of Dynamics recog- 

 nises true causes only. All change of the motion of a body is in 

 that science attributed to forces acting while the change is taking 

 place ; and the persistence of a body in motion while no forces are 

 acting on it is due to the inertia of the body, i.e. the body itself is 

 the cause of it. It is because the inertia of a body is a sufficient 

 cause for its continuing in motion that time can elapse between 

 events in nature. Whether the motion changes or does not 

 change, the effect and its true cause are accurately simultaneous. 

 The dispute as to whether action takes place at a distance does not 

 disturb this statement. Everyone who does not suppose that the 

 sun attracts the earth from a distance and without lapse of time, 

 supposes that some medium pervading the intervening space com- 

 municates the action ; and it is not the distant body, but the sur- 

 face of this medium where it touches the body acted on, that upon 

 this view can alone be recognised in the science of Dynamics as the 

 true immediate cause of the changes of motion of the second body. 

 Thus, in all cases, dynamical effects arise along with, and not 

 after, their causes. But in popular language, and indeed in all 

 but very carefully strict language, many events are spoken of as 

 caused by events that have preceded them. Thus, in the usual 

 loose way of talking, we may speak of a ball's having been re- 

 acted on by the ground as the cause why it is now ascending, 

 although a moment's reflection would show that, in strict lan- 

 guage, the reaction of the ground has caused only those changes 

 of motion that occurred while the ground was pressing against the 

 ball, and that the ball's afterwards continuing to ascend is due to 

 its inertia. Sometimes the two classes of causes are distinguished 

 as immediate and remote. Now the change which we have sup- 

 posed the universe to undergo would in no way affect immediate, 

 that is, true causes ; but all that we now recognise as an antecedent 

 or quasi-cause would, to the spectator looking on at the universe 

 from without, be changed into the effect, and that which is now 

 the effect would to his apprehension occur first and become the 

 cause. 



These seem the first lessons which the study we have entered 

 upon impresses upon us. But it is capable of giving further in- 

 struction. Hitherto we have supposed the altered universe looked 



