"78 Notes concerning the Phenomena 



swing towards it first, then back again, continuing this 

 vibration. When a sheet of white paper was waved near it 

 the balance was caused to swing round several times. 



" Such irregular behaviour on the part of the balance at 

 once suggests electrical action as a disturbing cause, and 

 that there is abundance of electrical disturbance within the 

 tube is very certain. Indeed, very little experience of the 

 action of the Sprengel pump is sufficient to show that besides 

 its function of an air-pump it is also an electrical machine. 

 In warm weather especially sparks can be drawn by pre 

 senting the knuckle to the surface of the mercury in the 

 lower reservoir, and in a dark room, when a tolerably 

 attenuated atmosphere is reached, flashes of sheet lightning 

 are seen to quiver from time to time through all the 

 airways. 



" There can be little doubt that these discharges are 

 implicated in some of the irregular behaviour of the pith- 

 balls, which has just been described, so that for delicate 

 experiments we must either have the tube hermetically 

 sealed off and detached from the pump after exhaustion, or 

 at least allow it sufficient repose for the restoration of the 

 electrical equilibrium. 



" When these precautions are taken the behaviour of the 

 balance unci;' the influence of heat or cold is perfectly 

 consistent aria' regular ; the balls receding from a source of 

 heat and approaching a source of cold. 



"Touching the question of the causation of these phenomena, 

 in the first place I think it will be ceded that we may have 

 approach without attraction properly so called, and diverg- 

 ence without repulsion, — the weathercock is not attracted 

 in the direction of the wind, but is forced into the position 

 of least resistance. 



" There are some beautiful experiments that bear upon this 

 point, by Dr. Frederick Guthrie, in reference to the approach 

 of vibrating bodies towards one another, as though in 

 obedience to an attractive force. He shows how a delicately 

 poised piece of card or handful of cotton wool moves towards ■ 

 a tuning-fork during the vibration of the latter. Also, how 

 & paper drum, suspended horizontally, approaches a drum 

 made by stretching parchment across the wide end of a funnel, 

 when the latter is made to vibrate by rapidly forcing air into 

 and drawing it out of the funnel. 



"He also advances in clear terms an explanation of these 



