200 Mr. T. Gray on Instruments for Measuring 



very light and. rigid; at the upper end the rod is flattened and 

 hinged just above the bend by a piece of tough Japanese paper 

 glued to its upper side. This gives a very light, flexible, and 

 sufficiently strong hinge. The plate P may be in a plane 

 which, by proper starting-apparatus, will be set in motion at 

 the beginning of an earthquake, or it may be taken as part of 

 a roller or of a circular plate kept continuously in motion by 

 proper mechanism. 



This instrument possesses the advantage of being compact, 

 of writing the actual motion, and of being capable of recording 

 with approximate accuracy earthquakes of much greater mag- 

 nitude than can be recorded by most of the instruments in 

 ordinary use. 



The multiplication given by an instrument of the sort may 

 be determined experimentally ; or it may be approximately 

 calculated by taking a point at a height equal to seven fifths 

 of the radius of the large sphere S as nearly steady. In a 

 locality where the motion of the ground is considerable, the 

 lever I must of course be made to give little, or perhaps no 

 multiplication. 



In several instruments of this class which have been con- 

 structed for the observation of earthquakes in Japan, a seg- 

 ment only of the sphere S has been used, with an independent 

 mass placed on a pivot at the centre of oscillation (arranged 

 in this case to be above the centre of the sphere), and of such 

 magnitude as to make the equilibrium nearly neutral. This 

 method of construction allows the radius of the sphere to be 

 much increased ; but it introduces a difficulty of adjustment, 

 a complication of parts, and generally a slight want of sym- 

 metry, which causes a little uncertainty in the interpretation 

 of the records. 



The static records* given by this machine are very interest- 

 ing. These are simply enlarged representations of the motion 

 of the earth; and are in many cases very curious, having gene- 

 rally a resemblance to the curves obtained when two harmonic 

 motions, not in the same direction, are combined. 



II. Rolling- Cylinder Seismograph. 



In this instrument (fig. 2) a pair of hollow cylinders, C, C, 

 made perfectly equal in thickness and in length by turning in a 

 lathe, are placed, with their axes horizontal and mutually at right 

 angles, on a smooth plane plate, P, furnished with levelling- 

 screws, S. An arm, A, fixed to the plane, and therefore forced 

 to move with it, passes over the top of one of the cylinders, 



* That is, the records taken with the plate on which the motions are 

 recorded at rest relatively to the earth, 



