1876.] On the Length of Electric Sparks. 167 



(2) that when the sky was clear and there was a heavy dew, the sound 

 could invariably be heard as far against a light wind as with it, and in 

 some cases much further. On one occasion, when the temperature at 1 

 foot above the grass was 38° and at 8 feet 47°, and the speed of the wind 

 was 1 foot per second at 5 feet above the grass, the bell was heard 440 

 yards against the wind and only 270 with it. 



Since, therefore, on the nights of the experiments at Yillejuif and 

 Montlhery it is stated that the sky was clear, that there was dew, and 

 the temperature recorded at the two stations shows the diminution to 

 have been downwards, it is argued that the effect of the wind to render 

 the sound less audible at Villejuif was completely balanced by the down- 

 ward refraction of temperature. 



Another phenomenon recorded by Arago is, that while the reports of 

 the guns at Montlhery as heard at that station were attended with 

 prolonged echoes, this was not the case with those at Yillejuif. It is 

 thought that this difference is sufficiently accounted for by the fact that 

 while Montlhery is surrounded by high hills with precipitous or wooded 

 sides, which must produce echoes, the country in front of Villejuif is 

 very flat and has not a tree upon it for miles. 



In concluding the paper reference is made to the Appendix to the last 

 Report of the American Lighthouse Board, in which Dr. Henry, the 

 Chairman, gives an account of his experiments, extending over thirty 

 years, and the conclusions to which they have led him, both of which 

 are in favour of the apparent stoppage of the sound being due to 

 refraction. 



IV. " On the Length of the Spark from a Battery of 600, 1200, 

 1800, and 2400 rod-Chloride-of- Silver Cells, and some Phe- 

 nomena attending the Discharge of 5640 Cells.-" By Warren 

 De La Hue, D.C.L., F.B.S., and Hugo W. Muller, Ph.D., 

 F.R.S. Received January 6, 1876. 



On the 24th February, 1875 *, we had the honour of communicating 

 to the Society, in conjunction with our friend Mr. Spottiswoode, an 

 account of some experiments to ascertain the cause of stratification in 

 electrical discharges in vacuo. These experiments were made with a 

 battery of 1080 cells of powder chloride of silver, which was described; 

 we have now in action 3240 such cells, and have recently completed 

 2400 rod-chloride-of-silver cells t, making our total force 5640 cells in 

 action. To these will be shortly added another unit of 1080 cells 

 powder chloride, and two other units of 1200 rod chloride, making a 

 total of 9120 cells. 



"We have more recently made a verbal communication to the Society of 



* Proc. Eoy. Soc. no. 160, 1875. t Ibid. p. 357. 



