SEISxMOMETRY. 37 



toothed bar axled at one end, and held up above a pin 

 projecting from the face of the pendulum bob. When 

 this falls it catches the projecting pin and holds the 

 pendulum. 



Another way of relieving the toothed bar is to hold 

 up the opposite end to that at which it is axled by resting 

 it on the extremity of a horizontal wire fixed to the bob 

 of a conical pendulum — for example, one of the indices of 

 a conical pendulum seismograph. The whole of this 

 apparatus, which may be constructed at the cost of a few 

 pence, can be made small enough to go inside an ordinary 

 clock case. 



The difficulty which arises with all these clock-stop- 

 ping arrangements is that it is difficult for observers 

 situated at distant stations to re-start their clocks so that 

 their difference in time shall be accurately known. Even 

 if each observer is provided with a well-regulated chrono- 

 meter, with which he can make comparisons, the rating of 

 these instruments is for all ordinary persons an extremely 

 troublesome operation. 



In order to avoid this difficulty the author has of late 

 years used a method of obtaining the time without 

 stopping the clock. To do this a clock with a central 

 seconds hand is taken, and the hour and minute hands 

 are prolonged and bent out slightly at their extremities 

 at right angles to the face, the hour hand being slightly 

 the longest. Each hand is then tipped with a piece of 

 soft material like cork, which is smeared with a glycerine 

 ink. A light flat ring, with divisions in it corresponding 

 to those on the face of the clock, is so arranged that at 

 the time of a shock it can be quickly advanced to touch 

 the inked pads on the hands of the clock and then with- 

 drawn. This is accomplished by suitable machinery, which 

 is relieved either by an electro-magnet or some other 



