BY A. E. BLACKMAX. 



379 



every precaution be taken to ensure the isochronism of 

 each as well — perhaps the technicalities of this can be 

 left. I might mention that the best form of suspension 

 for isochronism is a double spring. The pendulums should 

 be m.ade of materials the least aiTected by changes of 

 temperature, and be operated in vacuo — ^the clocks being 

 wound by electricity. The pendulum springs should be 

 of exactly the same strength, the pendulums of identical 

 form, weight, and distribution of the same. The clock 

 trains should he exactly alike, and be driven by the same 

 actual weight — the principle of uniform tension of the 

 driving cord or chain ensuring equal power to the move- 

 ments, and, consequently, perfectly equal impulse to the 

 pendulums. 



The place of experiment should be upon a plain, with 

 no hills, or even tall buildings, above the horizon. There 

 should be no sources of vibration near, such as railways, 

 cart roads, etc. Even the masses of the clock trains, the 

 protecting structures must be considered and gravitation- 

 ally balanced. 



The centrifugal force of the earth's rotation must be 

 allowed for; also we should have to consider the gravita- 

 tional effects of a periodic character due to the sun and 

 moon at least. There may be yet more disturbing factors 

 to deal with — perhaps the electro-dynamic forces due to 

 the earth's magnetic lines of force. 



From an astronomical point of view the gaseous 

 matter is as much a portion of the mass as the liquid 

 and solid portion. Our method does not weigh the 

 atmosphere, this being a spherical shell, within v/hich we 

 must of necessity perform our experiments, its gravi- 

 tational forces are s'elf-destructive, as far as we are 

 concerned. Hov.ever, this can be easily figured out and 

 added in. Knowing the area of the surface of the earth, 

 the average pressure at the surface and law of relation 

 between volumes and pressures of gases — Boyle's law — 

 we can easily measure its mass ; of course, this is not the 

 mere product of surface and pressure as it would be were 

 the earth's surface a plane. 



