362 Messrs. Milne and Gray's Experiments 



smoked smooth paper, on which the points of the indices of a 

 seismograph rest. 



(c) Continuous-motion Receivers. — These are simply strong 

 clocks, arranged for the purpose of driving a drum about 

 9 inches, or a glass plate about 18 inches in diameter, at the rate 

 of about one revolution in two minutes. So long as no earth- 

 quake occurs, the pointers of the seismograph continue to move 

 along the same line. When an earthquake occurs, the apparatus 

 is in motion and ready to receive the whole record. It is yet 

 a question whether this is necessary, as it is quite possible to 

 start an apparatus before an ordinary seismograph (giving, 

 say, a multiplication of 10) is able to show any movement on 

 the somewhat broad line which has been previously formed. 

 Certainly in earthquakes, when the motion is considerable, 

 there is no use for such an apparatus. 



With apparatus for this purpose it is difficult to produce 

 uniform motion, as it is evident that an escapement-method of 

 governing is not suitable. Conical pendulums, troughs of 

 liquid with dipping vanes, after the pattern of Airy's governor, 

 have been tried, but, though acting well when there is no dis- 

 turbance, are likely to be affected by the shock, and hence may 

 not act well when they are wanted. It appeared that a better 

 arrangement would be, to reverse the action of the Airy 

 governor and cause the trough of liquid to rotate, the vane 

 remaining stationary. The liquid, from the mere fact of its 

 rotation, would not be so likely to be affected by the shaking ; 

 and, besides, the trough, in consequence of its comparatively 

 great mass, would have a fly-wheel governing-action. If the 

 vane be placed near the outside of the trough, the governor 

 can be made to have great power, because the raising-up of 

 the liquid by centrifugal force immerses a greater area of the 

 vane. It was found on trial that an arrangement of this kind 

 makes an excellent governor, which we think might be taken 

 advantage of for many other purposes. 



5. Time- Takers. 



For taking the time of a shock, the simplest plan is for an 

 observer, when the shock is felt, to quickly draw out his 

 watch and observe the second, minute, and hour successively, 

 and then in as short a time as possible to compare his watch 

 with a standard clock. 



One of the chief objects in taking time accurately is to 

 compare the times of arrival of the shock at two or more places 

 some distance apart; as this is the most reliable method of finding 

 the direction from which a shock has come, and in some cases 

 its origin. The only satisfactory method of comparing the 

 times taken at two different places is by an electric signal. The 



