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1831. be again developed, and it is easy in this way to separate many colours from each other. The experiment in illustration of Newton's theory of colour, by painting the head of a top and spinning it, is well known; by the means just described the experiment can be still further extended, and the colours separated one from another, even while the whole system remains in motion.

The combination of other forms than wheels by the apparatus described, page 294, produces very beautiful effects. The application of colours here also is so evident as to need no illustration. The variation of the proportion of the interval to the remaining pasteboard causes many curious appearances, especially when the shadows produced in sunlight are observed.

Since the printing of the paper, a friend has referred me to the article 'Animalcule' in Brewster's Encyclopedia, where an opinion on the appearance of these creatures is given, nearly the same as that I have ventured. Speaking of the opinions of those who suppose them to be true revolutions, it is said, “ Yet notwithstanding our respect for the skill and talents of such renowned naturalists, we cannot deny that we think the production of the vortex is more probably effected by the simple motion of the fibrillæ—it may ensue from their rapidly bending in regular or alternate succession, or by some analogous means."

Trevelyan's Experiments on the Production of Sound during the Conduction of Heat.

[Read Friday evening, April 29, 1831.]

had remarked that when a heated poker was laid down upon a table, so that the knob rested upon it, whilst the hot part was supported by an interposed block of cold lead, regular musical notes were frequently produced. By extending the experiments, he found that a better form than that of a poker might be used for the hot metal: a piece of brass about four inches long, one inch and a quarter broad, and half an inch thick, should have a groove of one-eighth of an inch in width, formed down the middle of one of the broad faces, and then that face bevelled from the edges of the groove on each