This can be successfully shown to a large number of people at once only by using an electric arc, but there is no occasion to produce this light if not more than one person at a time wishes to see the evolution of the drops. It is then merely necessary to make the fountain play in front of a bright background such as the sky, to break it up with a tuning-fork or other musical sound as described, and then to look at it through a card disc equally divided near the edge into spaces about two or three inches wide, with a hole about one-eighth of an inch in diameter between each pair of spaces. A disc of card five inches in diameter, with six equidistant holes half an inch from the edge, answers well. The disc must be made to spin by any means very regularly at such a speed that the tuning-fork, or stretched string if this be used, when looked at through the holes, appears quiet, or nearly quiet, when made to vibrate. The separate drops will then be seen, and everything described in the preceding pages, and a great deal more, will be evident. This is one of the most fascinating experiments, and it is well worth while to make an effort to succeed. The little motor that I used is one of Cuttriss and Co.'s P. 1. motors, which are very convenient for experiments of this kind. It was driven by four Grove's cells. These make it rotate too fast, but the speed can be reduced by moving the brushes slightly towards the position used for reversing the motor, until the speed is almost exactly right. It is best to arrange that it goes only just too fast, then the speed can be perfectly regulated by a very light pressure of the finger on the end of the axle. _Mr. Chichester Bell's Singing Water-jet._