178 ANNUAL OF SCIENTIFIC DISCOVERY. 



chief novelty of the investigation consists in introducing the supposition 

 of the pendulum being, not a mere mathematical point, but a physical 

 agglomeration of particles. By adopting this more general view of the 

 subject, M. Hansen has succeeded in deducing several results of a hidden 

 character which had hitherto escaped notice. The most important of these 

 consists in the fact, that a rotatory motion of the pendulum about its axis is 

 capable of exercising a very sensible influence on the azimuthal motion of 

 the plane of oscillation. M. Hansen illustrates his results by a variety of 

 striking examples, and he concludes his valuable essay by investigating the 

 motion of a pendulum of a novel construction invented by himself, with 

 the view of obviating certain disadvantages attending the usual form. 



INTERESTING EXPERIMENTS WITH THE PENDULUM. 



A series of interesting experiments with the pendulum have been 

 recently made by Prof. Airy, the Astronomer Royal of England, assisted 

 by a number of eminent astronomers, in a pit of Harton. colliery, South 

 Shields, 1,260 feet deep. The observations consist in noting the vibrations 

 of an invariable pendulum on the surface, and another at the bottom of 

 the mine, both being mounted on firm iron stands, in a manner similarly 

 to each other. These pendulums hang on knife-edges, resting on agate 

 planes, thus sustaining little resistance from friction. If swung in vacua, 

 the vibrations would probably continue for 24 hours, and in their state as 

 used, though liable to hinderance from atmospheric causes, yet the vibra- 

 tions will continue at least eight or nine hours. Corrections are applied 

 to the results for the effect of temperature, and also for buoyancy, or the 

 effect produced by the pressure of the air on the pendulum. The vibra- 

 tions are counted by the assistance of a clock, which is mounted imme- 

 diately behind the detached pendulum ; and thus, by the aid of the clock, 

 the number of vibrations in a certain time can be easily noted. To the 

 centre of the bob of the clock pendulum is attached a small oval-shaped 

 disk, covered with gold-leaf, and illuminated by a lamp. It is necessary 

 in the adj ustments that this disk, when stationary, should be hid by the 

 detached pendulum, and that there should be a slit in the clock-case, 

 which should also be just covered by it. A line, therefore, drawn through 

 the centre of the telescope which is placed at a little distance, through the 

 detached pendulum, the slit in the clock-case, and the illuminated disk on 

 the clock pendulum, should be a straight line. Suppose the two pendu- 

 lums set swinging, we should soon perceive that one was vibrating faster 

 than the other, and that the disk would be gradually approaching the 

 detached pendulum until it would be completely hid, and both pendulums 

 would be going exactly together. This is called a coincidence, and is 

 carefully noted to the nearest second of time. When the illuminated disk 

 has reappeared, which is generally in a few seconds, one pendulum will 

 still continue gaining on the other until another coincidence takes place. 

 The time is again noted ; and thus we have the interval of coincidence, or 



