202 Mr. T. Gray on Instruments for Measuring 



value, while by the arrangement shown in the figure it can be 

 made to write any motion of sufficient magnitude to be appre- 

 ciated. 



The point p' is, of course, placed in the instantaneous axis, 

 of which the position is easily found in the case of this instru- 

 ment. Let h be the height of the instantaneous axis above 

 the plate P, R the external radius, and r the internal radius 

 of the cylinder, then 



3B 2 + r* 

 2R ' 



from which it is easy to see that, in the case of a thin cylinder, 

 the instantaneous axis is nearly on the inner surface of the 

 cylinder. In the case of the instrument actually constructed, 

 S=8 and r = 7, and therefore 



, 192 + 49 



16 



= 15*06 nearly. 



III. Pendulum Seismograph. 



About a year ago (Jan. 1880) I began to make experiments 

 on horizontal levers pivoted at one end and loaded at the other, 

 with the view of obtaining a body in approximately neutral 

 equilibrium, which by its inertia would give a means of record- 

 ing the motions of the earth during an earthquake. The 

 levers which I used were about 30 centimetres long between 

 the load and the pivots; and the load was generally about 

 5 kilogrammes. This mass remained steady even when an 

 oscillatory motion of two or three centimetres was given to 

 the pivoted end of the lever, if that motion was at right angles 

 to the length of the lever. When the motion was oblique, 

 however, there was a tendency for the mass to change its 

 position, probably due to the direct impulses given to it not 

 quite neutralizing each other. This change of position is not 

 of vital importance if the successive motions be recorded on a 

 moving plate ; but as I was not at that time inclined to use 

 clockwork in connexion with the machine, and it formed a 

 great objection when a static record was used, I did not go 

 further in the matter. The experimental machine, however, 

 proved an excellent instrument for class-room illustrations of 

 inertia; and as such I have since used it in my lectures on that 

 subject. It is, of course, easy to give an arrangement of this 

 kind sufficient stability to prevent permanent displacement ; 

 but when so arranged it has little, if any, advantage over an 

 ordinary pendulum, and has some marked disadvantages. 



I came to the conclusion at that time that the ordinary ver- 



