356 Mr. T. Gray on an Improved 



a suitable governor. A somewhat simpler arrangement is 

 obtained by using a single drum covered with paper, or a 

 smoked glass or metal cylinder, and giving to this cylinder a 

 slow motion of translation in the direction of its axis, so that 

 the record takes the form of a spiral line round it. As, how- 

 ever, the rate of motion must be such as to give the time of 

 occurrence with fair accuracy within a second of time, it is 

 difficult to obtain a good record on a cylinder of moderate 

 size, which will extend over more than twelve hours with 

 this arrangement. It is of course easy to adapt the apparatus 

 to be used either way, if that were desirable; but the con- 

 tinuous ribbon of paper is so much the better form of re- 

 ceiving-surface that the description given in this paper, in so 

 far as it refers to earthquakes, only includes that form. The 

 spiral record has some advantages in apparatus adapted to 

 record slow changes of level of the earth's surface ; and it will 

 be again referred to in that connection. For such purposes 

 the rate of motion may be made excessively slow ; rnd hence 

 the records for a considerable length of time may be written 

 on one sheet. 



At the time of occurrence of an earthquake, the rate of 

 motion of the paper is automatically greatly increased, and a 

 chronographic reed is simultaneoasly set into vibration, and 

 made to mark equal intervals of time on the ribbon, thus 

 showing accurately the rate of motion at any instant. The 

 actual rate of motion of the paper on the slow speed may be 

 varied from about a quarter of an inch to an inch per minute, 

 and on the fast speed from about 25 to 50 inches per minute, 

 with the present form of instrument. This change of speed 

 is generally obtained by including in the driving clockwork 

 two governors, one of which can be automatically thrown out 

 of gear, either electromagnetically or mechanically. The 

 latter method has beei found the best and the simplest in 

 practice. The arrangement commonly used is described 

 below, page 361, and need not be more particularly referred 

 to here than in a general statemert of the operations it is in- 

 tended to perform. At the time of an earthquake three 

 operations take place simultaneously. One is the introduction 

 in train with the clockwork of an adjusting mechanism which 

 is intended to readjust the starting apparatus, whatever that 

 may be, so thati t may be in readiness for another earthquake 

 should that occur. Another is to throw out of gear the slow- 

 speed governor, or, if that method is adopted, to work a change- 

 wheel lever, so as to shorten the train between the driving 

 power and the governor. A third is to close the circuit of 

 the chronographic reed, so as to cause it to mark time on the 





