i8g6. 



THE HORIZONTAL PENDULUM. 



235 



completely, it is necessary to have two such pendulums placed with 

 their planes at right angles to one another. 



Returning to the history of the horizontal pendulum, another 

 change was made, again without any knowledge of previous work, by 

 the Rev. M. H. Close, of Dublin (1, 7). In 1869, shortly before 

 Zollner, this gentleman constructed the pendulum shown in outline in 

 Fig. 3. A B is a rod, suspended by two fine wires A C and B D, the 

 axis, C D, being, as before, very nearly vertical. The only difference 

 in principle between this form and Hengeller's is that the centre of 

 gravity of the rod lies between the two points of attachment, A B, of 

 the wires, and consequently the points of supports, C D, are both 

 above the rod. In Hengeller's pendulum, the centre of gravity lies 

 outside the line joining the points of attachment, and the points of 

 attachment are therefore on opposite sides of the rod. Precisely 

 similar in principle to Mr. Close's form is the new bifilar pendulum 

 of Mr. Horace Darwin (3, 4, 5), which is a modification of the 

 celebrated instruments erected in the Cavendish Laboratory at 

 Cambridge in 1880 and 1881 (2). 



Fig. 3. — Close's Pendulum 



Fig. 4. — Gerard's Pendulum. 



In the year 1851, Mr. A. Gerard, of Aberdeen 1 (6), devised 

 another form of the horizontal pendulum, represented in Fig. 4. C D 

 is a long beam, ending at D in a sharp point, which rests against 

 the wall of the room in which the instrument is erected. B C is a 

 copper wire, at one end carrying a heavy weight, W, and at the other 

 fastened to a ring in the wall at B, so that the line B D is very nearly 

 vertical. Forty years afterwards the same idea occurred to Professor 

 J. Milne (8, 9), who has made much use of a similar, but smaller and 

 more delicate, instrument, in connection with his admirable researches 

 on earth tremors in Japan. 



In 1888 and 1889 the horizontal pendulum underwent a further 

 modification at the hands of the late Dr. E. von Rebeur-Paschwitz, of 

 Merseburg, Saxony (4, 11, 12, 13), who removed the fine wires, or 



1 For the reference to Gerard's paper I am indebted to Kennedy's "A Few- 

 Chapters in Astronomy " (7), a valuable work which deserves to be better known. 



S 2 



