442 



Prof. J. A. E wing. 



[Feb. 10, 



a pendulum 20 feet long, suspended as favourably to steadiness as 

 possible, has been set swinging during even a very slight earthquake, 

 through arcs about three times as long as the actual motion of the 

 earth's surface. By using a pendulum of as great length as this, how- 

 ever, and by making it trace a continuous record in the manner de- 

 scribed above, we may distinguish between that part of the record 

 which is due to the swing of the pendulum and that part which is the 

 true register of the earth's motion, the waves produced by the swing 

 of the pendulum being of much longer periods than those due to the 

 undulation of the earth. The record thus shows the short earth- 

 quake waves superposed upon a long sinuous curve due to the swing 

 of the pendulum. 



In this way, a long pendulum seismograph may (though with diffi- 

 culty) be made to give good results ; and one designed by the author 

 has been erected in the University of Tokio, thanks to the liberality 

 of the Japanese directors. More recently, however, the writer has 

 designed and constructed another seismograph, which he believes to 

 be novel, and which is certainly far superior to the long pendulum 

 seismograph in simplicity and cheapness of construction, ease of use, 

 and accuracy of results. To describe this new instrument is the 

 object of the present paper. 



Figs. 1 and 2 show as much of the apparatus as is needed for the 

 purpose of explanation. The scale in fig. 1 is about one-third, and in 

 fig. 2 about one-sixth, of full size. 



A solid round cylindrical brass bob a is pivotted between two 

 conical points of steel, so as to be capable of revolving freely about a 

 vertical axis lib at the short end of a light horizontal lever. The 

 horizontal lever consists of two parts — a light brass frame c and a long 

 pointer of straw d, the extreme end of which is sharp-pointed, and 

 presses gently against a revolving glass plate / (fig. 2), whose surface 

 is smoked. The lever cd is pivotted about a vertical axis ee, between 

 two conical steel points, the lower one working in a hole in the base ; 

 the upper one, the point of a set- screw fixed in the top of an inverted 

 stirrup, which is in one piece with the base of the instrument. In 

 fig. 1, one side of this stirrup is, for the sake of clearness, supposed to 

 be removed. The base stands, supported by three levelling screws, 

 on the top of a post, which is firmly driven into the ground, and then 

 cut off short a few inches above the surface. The instrument is fixed 

 to the post by a " geometrical clamp " (see Thomson and Tait's 

 "Nat. Phil.," 2nd edition, vol. i, § 198). The sockets for the feet of 

 the levelling screws are a pyramidal hole, a V slot, and a plane surface ; 

 and the pressure necessary to give steadiness is produced by a single 

 screw bolt near the centre, which is not shown in the drawing. By 

 this arrangement the axis ee is placed and held in a vertical position. 

 It moves with the earth during a shock. There are two precisely 



