33 



siphons on a band of paper, which is kept moving uniformly by 

 a train of clockwork. The siphon which records the vertical 

 component is actuated by a mass supported on the free end of a 

 horizontal lever, which is held in position by a knife-edge and 

 two flat springs bent through a large angle, so as to give the 

 lever a long period of free oscillation. The period is still further 

 increased by means of a small flat spring fixed to the frame- 

 work below the knife-edge and attached to the lever in such a 

 way as to give the requisite amount of negative stability. The 

 siphons which record the horizontal components are actuated by 

 conical pendulums, so arranged as to have long periods of 

 oscillatory motion. 



The time of occurrence and the period of oscillation of the 

 earth are obtained from the position of the record on, and the 

 rate of motion of, the paper-ribbon. These are checked by a 

 separate timepiece, which closes an electric circuit and makes a 

 mark on the paper-ribbon every 10 minutes and a distinguishing 

 mark every hour. These marks, besides giving the rate of motion 

 of the ribbon and the time at which any part passed the driving 

 drum, furnish a datum-line from which the position of the siphon- 

 records may be measured ; and thus any change of position of 

 the bobs of the pendulums, occasioned say by a change of level, 

 can be observed. The conical pendulums can be so adjusted as 

 to be extremely sensitive to change of level, and hence the 

 instrument may be used to record slow pulsations of the ground. 



The instrument here exhibited is suited for motions of small 

 amplitude. 



99. Horizontal pendulum seismograph, with vertical 

 motion seismograph, designed by Prof. J. A. Ewing. 



Made by the Cambridge Scientific Instrument Gompanv. 

 E. 142. 1888. 



This instrument consists of two parts, both of which make 

 records on the same revolving sheet of glass. It is only adapted 

 for small motions which require to be magnified in the record. 

 The principle of the instrument is to provide three pendulums 

 which, while the earth is still, are perfectly free to move, each in 

 one direction, such directions being mutually at right angles ;. 

 and conversely, if the earth moves in any of those directions r 

 they will not partake of the motion, but be relatively displaced. 

 The two horizontal motions are provided for by two vertical bars> 

 contained within cylinders, which are set on a tripod with level- 

 ling screws. These bars are pivotted at the base by sharp steel 

 points resting in cone-shaped cups ; but at the top their points 

 rest in V-shaped grooves, whose directions in the two pendulums 

 are at right angles. In this way motion in one direction only 

 can be recorded by each. The pendulums are furnished with an 

 adjustment by which they are put in nearly neutral equilibrium 

 and stand at right angles to each other. The motion is multi- 



