236 



NATURAL SCIENCE. 



April, 



springs, used by Zollner, and replaced them by agate cups resting on 

 steel points. As this instrument is one with which some most 

 valuable investigations have been, and are still being, made, I propose 

 to describe it in some detail. I will conclude this section with the 

 following summarised classification of the different forms of the 

 horizontal pendulum (quoted from the Report of the British Asso- 

 ciation Earth Tremors Committee for 1895) : — 



"I. The pendulum in which the rod or mirror is suspended by 

 two wires. These may be again sub-divided : (a) The pendulums of 

 Close and H. Darwin, and practically also of Delaunay, and Lord 

 Kelvin and the Darwins, in which the centre of gravity of the rod or 

 mirror lies between the two points of attachment of the suspending 

 wires ; (b) The pendulums of Hengeller, Perrot, and Zollner, in which 

 it lies outside them. 



" 2. The pendulums of Gerard and Milne, in which the rod is 

 supported by one wire and on one steel point. 



" 3. The pendulum of von Rebeur-Paschwitz, which is supported 

 on two steel points." 



Fig. 5. — Pendulum of von Rebeur-Paschwitz. 



It should be added that Professor Ewing's horizontal pendulum 

 seismograph, though constructed (in 1880) for a different purpose, also 

 serves for indicating slow tilts of the ground. It resembles closely 

 the last-named instrument. 



The Horizontal Pendulum of Dr. E. von Rebeur-Paschwitz. — An outline 

 of the pendulum and stand which supports it is given in Fig. 5. The 

 pendulum, D E F, is made of thin brass tubes, and is in the form of an 

 isosceles triangle. Two of the tubes, D E, are prolonged beyond the 

 base of the triangle, and to the ends, P Q, agate cups are attached. 

 These rest against two extremely fine steel points, C C, projecting 

 from cylinders which rotate very stiffly about horizontal axes. The 

 cylinders are attached to a strong rectangular frame, A, screwed to 

 the stand, B, of the whole instrument. The upper steel point is 

 directed away from, and the lower one towards, the centre of gravity, 

 G, of the pendulum, and there is thus no tendency to slipping 

 between the steel points and the agate cups. The line joining the 

 steel points, or axis of rotation, is 68 mm. in length, and though very 



