166 Mr. E. Threlfall on the 



A tank, measuring a yard each way, was constructed and 

 filled with windows of strong plate glass about a foot square 

 in each of two of its adjacent sides, This tank was filled 

 with water. It was at first intended to introduce small 

 charges of fulminate of mercury contained in glass bulbs of 

 \ inch in diameter, and capable of being fired electrically. 

 The bulbs were to be surrounded by water coloured by some 

 dye-stuff', and the distribution of this after the explosion was 

 to be observed. However, this was soon found to be un- 

 necessary, for the debris of explosion afforded sufficient 

 guide. One observer (Mr. William Walker) was generally 

 stationed at one of the windows ; I fired the charge by 

 means of a contact-key in connection with a small coil, and 

 looked in from the top, or side, as seemed most desirable. 

 We soon found that the general effects were pretty regular 

 in some ways, and irregular in others. When a small glass 

 globe one half-inch in diameter had been filled with pure 

 fulminate of mercury shaken well down, but not pressed, the 

 ends of a pair of insulated wires twisted together were passed 

 in and pushed down to measurement till the points were as 

 nearly as possible about the middle of the globe. A 

 little melted paraffin was run round the neck of the bulb to 

 make it water-tight, and the whole was lowered by means of 

 the wires to a constant depth of about 18 inches. At first 

 we were astonished to find that the debris of explosion had 

 the appearance of being shot down to the bottom; not in a 

 jet, but with exactly the rolling motion that smoke has in 

 coming out of a chimney, — as if, in fact, there w r as vortex 

 motion of some sort. I was anxious to try instantaneous 

 illumination, but found I had not the apparatus to produce a 

 bright enough spark. I shall hope, however, to be enabled 

 to do this at some future time. Noticing the constancy of 

 the downward action of the explosion, it occurred to me 

 that it was produced by the want of symmetry introduced 

 by the neck and wires of the cartridge. In order to test 

 this I turned the next bulb on its side, and then the debris 

 seemed to move with its peculiar rolling motion away from 

 the neck. In fact, the appearance presented to the unaided 

 eye w r as that of a more or less definite column of rolling 

 white smoke shot out with great velocity, and coming to rest 

 very rapjdly when about five inches from the centre, as if 

 acted on by an irresistible force. I also made some ex- 

 periments by exploding a charge in the centre of a Florence 

 oil-flask filled with red dye and immersed in the water. The 

 dye was shot out with the debris, and the flash appeared to 

 be suddenly stopped some two or three inches outside where 



