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plied and recorded by two pointers projecting from the pendulums, 

 and so set that in their oscillation their ends move with exceed- 

 ingly little friction on the surface of a circular glass plate, which 

 at the time of the earthquake is made to rotate, and they thus trace 

 a pair of magnified records of the earth's horizontal motion in two 

 components, side by side, on the surface of the smoked glass plate, 

 which may then be varnished and used as a negative for taking 

 photographic copies. The vertical component is recorded on the 

 same revolving glass by a pendulum standing on the opposite 

 side. In this there is a massive Jpar pivotted on a horizontal line, 

 about which it oscillates freely in a vertical direction. The bar 

 is held up by a pair of long spiral springs, and by adjusting the 

 point at which the pull of the springs is applied to the bar, so that 

 it may be below a horizontal plane through the fulcrum, the equili- 

 brium of the bar is made as nearly neutral as is desirable. The bar 

 is pivotted and the pull of the springs applied by hard steel 

 points working in hard steel centres. A light steel bar is fixed 

 vertically to the edge of the pendulum so as to multiply the 

 motion, and carries at its base a horizontal index, so jointed that 

 a vertical motion of the rod is converted into a horizontal motion 

 of the index in a direction radial to the circumference of the 

 smoked plate. Another piece of apparatus is for setting the 

 glass plate in motion so soon as an earthquake commences. It 

 consists of clockwork which, when in action, drives a small 

 friction roller over the surface of the glass plate near its edge, 

 thereby giving it a uniform speed of rotation. The speed of the 

 clockwork is controlled by a balanced centrifugal governor, 

 furnished with vanes which dip into a vessel containing castor 

 oil. The clock is started at the commencement of the earthquake 

 by means of an electro -magnetic detent, which acts whenever an 

 electric circuit is closed by the small seismoscope which stands on 

 the other side. This seismoscope consists of a metal disc, with a 

 small cavity in the centre, contained within an iron cylindrical 

 box, to the top of which it is suspended by three fine wires. From 

 the cavity in the disc a pin passes through a hole in a platinum plate 

 attached to the brass upright of the seismoscope. The slightest 

 disturbance will cause a movement of the disc and consequently 

 of the pin, which will close an electric circuit by contact with the 

 platinum plate. This happens during the preliminary tremors of 

 the earthquake and causes the recording plate to begin revolving 

 before the principal motions are felt. The plate is supported in 

 such a way as to allow it to be removed and replaced without 

 disturbing the rest of the apparatus. Two spare plates are sup- 

 plied with the seismograph. From the known rate of rotation 

 of the plate, together with the undulating lines drawn by the 

 pointers on its surface, whose magnification is also known, the 

 velocity of displacement and rate of acceleration of a point on the 

 earth's surface may be determined, as well as the direction of each 

 displacement. In this instrument the horizontal motions are 

 multiplied by about 4'1 and the vertical by about 2. 



