230 LECTURES TO SCIENCE TEACHERS. 



four equal parts, and then each of these into five, corre- 

 sponding to minutes. I then connect the corresponding 

 points at the opposite edges, and so divide my field into 

 rectangular spaces. 



Now on the space so divided it is obvious that the position 

 of any body (say a train) travelling along a definite course 

 from South Kensington to Bayswater in twenty minutes or 

 less can be readily shown. To do this I have to set off on 

 the vertical or ordinate corresponding to each minute the 

 actual position of the train at the end of such minute, 

 according to the distance it has travelled from its starting- 

 point. Thus, the train has arrived at Gloucester Road in 

 four minutes ; I therefore indicate its position at the end of 

 four minutes by measuring off three-quarters of a mile, 

 viz., the actual distance between South Kensington and 

 Gloucester Road, along the fourth ordinate. This done, I 

 connect the point so found with the starting-point. If the 

 train has in its journey from South Kensington moved at a 

 uniform rate, then the line is a straight one. If, as would 

 certainly be the case, it started slowly and slowly stopped, 

 that would be marked thus : (See Fig. 1.) 



It is obvious that on the same plan the motions of any 

 number of trains, whether going in the same or in opposite 

 directions, could be represented. In fact I have been told 

 that a graphic method such as I have been describing has 

 been employed in the construction of the time-tables of 

 some of the French railways. 



Let me pass from this example to one in which the 

 graphic method is used to register a fundamental physical 

 fact or law, e.g., the law of falling bodies an example 

 which I choose because it was the first motion to which the 

 graphic method was applied in physics. You have before 

 you a cylinder (Fig. 2) which revolves on a vertical axis at a 

 uniform rate, viz , forty times in a minute. Here is the fall- 

 ing body, which, in order to accomplish the result we want, 

 we have suspended by a thread in such a position that when 

 it falls its motion is recorded on the blackened surface of the 

 cylinder. The clockwork having been set in motion, I will 

 now apply a lighted match to the thread. At the moment 

 that it is burnt through, the weight will fall, and its motion 

 will be recorded on the surface of the blackened cylinder. 

 It will describe on that surface a line which will resemble 



