“In the following January my father gave a second and last lecture on the same subject. Emboldened by our past success, we proceeded to experiments involving greater risk of failure; among others a thunder-cloud, which, to effect its discharge (whereby a model building was to be blown up with gunpowder), had to be moved by electric influence through a distance of not less, I think, than eight or ten feet. But the crowning illustration, with which the lecture concluded, was a revolving planisphere of my construction, four feet in diameter, and representing all stars, of not less than the fourth magnitude, within forty degrees of the South Pole. Wishing that the various magnitudes should appear in the illustration, I devised an arrangement for that purpose. For producing the sparks to represent stars of the first magnitude, I cut the approaching edges of the tinfoil into a round shape, and placed them about one-twelfth of an inch asunder; for those of the second magnitude I gave the edges a pointed shape, also reducing the space between them to a minimum; for stars of the third and fourth magnitudes, while retaining the same arrangement, I produced further obscuration by covering the one with a single thickness, and the other with two thicknesses, of thin paper. To represent the Magellanic clouds was a more difficult matter; but here also I hit upon an expedient. Piercing the disc, in the proper places, with holes proportionate to the size and in the form of the respective nebulae, I placed behind each hole in a plane parallel to that of the disc, and distant about half an inch from it, a piece of paper somewhat more than sufficiently large to correspond with the perforation; and I so arranged that this paper was illuminated by sparks at the back of the disc. When I add that the planisphere thus illuminated was at the same time kept in constant and equable revolution, I shall perhaps be regarded as justified in the belief entertained at the time that the whole result was a more exact representation of the starry heavens than had ever before been produced. The applause previously given to my Great Bear was more than redoubled on sight of my Southern Sky, and the lecture terminated amidst the congratulations of friends, my father being, of course, greatly pleased, myself sufficiently elated, and the whole family triumphant. I may add that a full description of my planisphere will be found in the ‘Philosophical Magazine’ for October, 1818.”[36] In 1816 he devised and constructed an alarum water-clock:—