234 NATURAL SCIENCE. April. 



Perrot (10). In 1869, again, an instrument closely resembling 

 Hengeller's, but with improvements in detail, was made by Zollner 

 (16). To this astronomer is due the credit of first successfully 

 proving and drawing public attention to the value and extraordinary 

 delicacy of the horizontal pendulum. 



It may be well, perhaps, at this stage to explain briefly the 

 principle of the instrument. Let D and E (Fig. 2) be the two points 

 of support, E H a vertical, and D H a horizontal line, the angle 

 D E H being greatly exaggerated in the figure. It is obvious that 

 the pendulum, in a state of rest, must always lie in a vertical plane 

 passing through the axis of rotation D E ; and also, in order to be in 

 stable equilibrium, that the centre of gravity of the ball and rod must 

 lie on the side of the vertical E H towards which the axis E D 

 inclines. 



Now, suppose the point D to receive a very slight displacement 

 (small even compared with the length D H) in a direction perpen- 

 dicular to the plane of the paper. Then the whole pendulum will 

 turn through the same angle as the line D H does, and, obviously, 



V 



E' 

 Fig. 2. — The Principle of the Horizontal Pendulum. 



the smaller D H is the greater will be the angle of deflection corre- 

 sponding to a given displacement of the point D. Again, the longer 

 the vertical line H G (which is practically equal to D G) the greater 

 will be the displacement of the point D due to a given tilt of the 

 axis. Hence, the smaller the angle D E H, the greater will be the 

 deflection of the pendulum caused by a given tilt of the ground; in 

 other words, the greater will be the magnifying power of the 

 instrument. 



If the displacement of the point D is not perpendicular to the 

 plane of the paper, it may be resolved into two components, one 

 parallel and the other perpendicular to that plane. The former 

 component merely alters very slightly the horizontal distance between 

 the points of support, and therefore produces a minute, and in most 

 cases hardly perceptible, change in the sensitiveness. The latter 

 component is that which causes the pendulum to be deflected. Thus, 

 the instrument is only adapted for measuring tilts of its axis in 

 a direction perpendicular to the plane of rest. To measure a tilt 



