Mr. J. P. Joule on a remarkable appearance of Lightning. 127 



able to perceive the effect of increasing or diminishing the 

 pressure by less than an atmosphere, in lowering or elevating 

 the freezing-point of water. 



If a convenient minimum thermometer could be constructed, 

 the effects of very great pressures might easily be tested by 

 hermetically sealing the thermometer in a strong glass, or in 

 a metal tube, and putting it into a mixture of ice and water, 

 in a strong metal vessel, in which an enormous pressure might 

 be produced by the forcing-pump of a Bramah's press. 



In conclusion, it may be remarked, that the same theory 

 which pointed out the remarkable effect of pressure on the 

 freezing-point of water, now established by experiment, indi- 

 cates that a corresponding effect may be expected for all 

 liquids which expand in freezing ; that a reverse effect, or an 

 elevation of the freezing-point by an increase of pressure, may 

 be expected for all liquids which contract in freezing ; and 

 that the extent of the effect to be expected may in every case 

 be deduced from Regnault's observations on vapour (pro- 

 vided that the freezing-point is within the temperature-limits 

 of his observations), if the latent heat of a cubic foot of the 

 liquid, and the alteration of its volume in freezing be known. 



XIII. On a remarkable appearance of Lightning. 

 By J. P. Joule, F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



N the 16th inst., after a very sultry morning, this town 

 was, in common with a large tract of country, visited at 

 4? o'clock by a thunder-storm accompanied with heavy rain. 

 In the evening of the same day, about 9 o'clock, we had an op- 

 portunity of witnessing a most magnificent display of electrical 

 discharges, which continued almost uninterruptedly for the 

 space of one hour, accompanied, however, by only a few drops 

 of rain. I had never before seen lightning of such an extra- 

 ordinary character. Each discharge appeared to emanate 

 from a mass of clouds in the south-west, and travelled six or 

 ten miles in the direction of the spectator, dividing into half a 

 dozen or more sparks, or zigzag streams of light, in some in- 

 stances the termination of each of these sparks being, as re- 

 presented in the adjoining sketch, again subdivided into a 

 number of smaller sparks. I did not observe any of the dis- 

 charges to strike the ground; and from the interval of time 

 between the appearance of those which crossed the zenith and 



