EARTHQUAKES IN SCOTLAND AND IRELAND. 4f 



presses on the wooden board by a small leaden weight resting on its upper 

 end, inside of a metal tube containing the pencil. 



Three wooden rods are fixed to this spherical segment, on its outer edge, 

 at equal distances, and unite above the basis, so as to form a point of suspen- 

 sion for the pendulum. 



The instrument is fixed by three feet to the floor of a room, and, with the 

 help of adjusting screws, the chalk is brought to the centre of the concave 

 segment which is to be marked by its vibrations. The concentric circles, 

 which are marked 1, 2, 3, &c., from the centre of the segment, indicate the 

 number of inches that the lower extremity of the pendulum is thrown from 

 the centre ; and the cardinal points show the direction from or to which the 

 shock has proceeded. 



2. The Inverted Pendulum Seismometer. — (1.) The smallest of the instru- 

 ments made on this principle has a pendulum thirty-nine inches long, and is 

 fixed into a brass socket at its lower end. The connexion between the pen- 

 dulum and the socket consists of a strong elastic wire, which, by means of a 

 pinching screw, can be either raised or depressed in the socket, so as to in- 

 crease or diminish the length and sensibility of the pendulum. There is a 

 leaden ball near the top of the pendulum from three to four pounds in weight : 

 it has a hole through its centre so as to allow the pendulum rod to pass freely 

 through it, and it can be fixed at any part of the rod by means of a pinching 

 screw. At the upper extremity of the pendulum there is a soft lead pencil, 

 which rests on an elastic wire contained in a brass tube. The pencil is thus 

 pressed against a white surface of paper, forming the segment of a sphere, 

 having a radius of thirty-nine inches. The paper is pasted on a piece of 

 copper beaten into the proper shape. This copper segment rests on four 

 upright iron rods which are fixed into the base of the instrument. The base 

 consists of four corresponding flat iron bars, which cross in the middle, and 

 support at that point the socket above described, to which the elastic wire of 

 the pendulum is fixed. 



There are on the white segment of this instrument concentric lines in red 

 ink, an inch apart, and numbered from the centre, so as to indicate the num- 

 ber of inches that the pendulum is by any shock thrown off" its centre. There 

 are also on this segment, as on that of all the instruments, points of the com- 

 pass to indicate the directions of the shocks. 



The instrument is fixed firmly to the floor of the room where it is set. By 

 means of three adjusting screws, which affect the socket, the upper extremity 

 of the pendulum is brought to the centre of the segment to be marked by it. 



Any further description of this instrument is rendered unnecessary in con- 

 sequence of a paper by Professor Forbes, published lately in the Transactions 

 of the Royal Society of Edinburgh, where the mechanism and mathematical 

 properties of it are very clearly pointed out. 



(2.) The other instrument constructed on this principle has a pendulum 

 ten feet eight inches in length. The spherical segment, on which the vibra- 

 tions of its point are intended to be marked, is not, as in the instrument just 

 described, supported on upright rods fixed to its base, but is suspended over 

 the pendulum by a strong hold-fast of iron fixed into a wall. In other re- 

 spects, the mechanical construction of this instrument is much the same as 

 that of the former one. 



The above instruments were sent to Comrie, a small town in Perthshire, 

 where shocks have been very frequent during the last fifty years, and where 

 the earthquake of October 1839 was felt more strongly than in any other 

 part of Scotland. They were given in charge to Mr. Peter Macfarlane, Post- 

 master at Comrie, a very intelligent person, who had been assiduous in 



