204 Mr. T. Gray on Instruments for Measuring 



the amplitude-indications rendered nearly absolute. It may 

 be objected that this friction will cause the amplitude for each 

 individual motion to be shown too small. But I find on trial 

 that the permanent displacement which this is capable of pro- 

 ducing is almost inappreciable, and that the displacement due 

 to a motion of the glass plate backwards and forwards at a 

 period nearly corresponding with the period of the pendulum 

 is not very great, even if two or three times repeated. 



The improvement in the mode of attaching the pointers was 

 called for by the fact that, even with the lightest pointers which 

 could be conveniently made, the rotational energy imparted 

 to them by the shock made them swing too far round. This 

 I got over by hanging the pointers by a bifilar suspension 

 from the lower side of the pulleys. When the pulleys and 

 pointers are so arranged, the pull on the pendulum can be 

 made almost infinitesimal ; and yet, since the indices are not 

 forced to move with the pulley, but rather tend to check its 

 motion, the indications are very nearly accurate. 



I may remark that the indications of these machines, a con- 

 siderable number of which have been in use for some time, 

 tend to show that there is no definite direction of movement, 

 or, in other words, that the successive impulses contain vibra- 

 tions in different azimuths. No doubt much of this is due to 

 the existence of direct and transverse vibrations, such as we 

 may expect from the theory of vibrations in elastic solids. 



Besides exercising a controlling power on the pendulum, 

 the friction-point can be made to describe very interesting 

 curves on the glass plate if it be previously smoked. These 

 curves in some of the larger earthquakes have indicated a very 

 complicated motion of the earth. 



IV. Double-Lever Seismograph*, 



In the course of my experiments on the horizontal-lever 

 arrangement, I was informed by Prof. Milne, to whom I had 

 shown my experiments, that a very similar arrangement had 



* This paper is for the most part extracted from an account of a seis- 

 mograph constructed on the principle described, which was communicated 

 to the Seismological Society of Japan at its meeting on January 26, 1881. 

 The only differences between the instrument here described and that 

 figured in the ' Transactions ' of the Society, are some variations in size and 

 in details of construction which have been suggested by experience. The 

 instrument there described was made very small, the horizontal levers 

 and frame carrying them being all surrounded by the ring R, which had 

 in that case an internal diameter of about 12 centimetres. It was found 

 inconveniently compact, and was besides difficult to construct accurately. 

 As the instrument now described is almost wholly of wood, its first cost 

 is very small. 



