Page:Supplement to the fourth, fifth, and sixth editions of the Encyclopaedia Britannica - with preliminary dissertations on the history of the sciences - illustrated by engravings (IA gri 33125011196181).pdf/25

Rh The extensive branch of Pneumatics has been elucidated in a number of articles, containing many new facts and views; most of them contributed by Professor Leslie. Under the article Acoustics he has examined the later experiments and opinions as to the theory of Sound. Though the Encyclopedia contains an account of the invention and construction of Balloons, it was judged proper to resume the consideration of these machines, in order to afford an opportunity of tracing the progress of discovery with greater precision, and of explaining the calculation of their ascent and stability. This has been done in the article Aeronautics, where, also, the more remarkable of the later aërial voyages are described. With a view, in like manner, more fully to explain the successive steps which led to the discovery of atmospheric pressure, and to state and examine the various formulas that have been proposed for determining heights by the Barometer, there have been given two articles on that instrument, and its uses in measuring elevations. The connected subjects of Climate, Cold, Dew, and Meteorology, rank under Pneumatics, in so far as they can be considered as belonging to Natural Philosophy; but the phenomena to which they relate depend upon principles which place them, partly, within the province of Chemistry. In the very valuable series of articles here given upon these subjects, the author has accordingly drawn his reasonings and illustrations from either department. The last of them contains a copious explanation, of the theory and applications, of all the different instruments capable of being employed in Meteorological Observations; a portion of it calculated to be eminently useful to those engaged in such observations, or in directing the proper preparations for making them. Besides the above articles by Professor Leslie, there are some others, by different writers, on subjects connected with the present head. Such are the articles on Blowing Machines; on the application of the Steam-Engine to the propulsion of Vessels; and on the new Steam-Engine proposed by Mr Perkins.

Of the numerous discoveries made during the present century relative to the theory of Colours, and the Double Refraction and Polarisation of Light, an account will be found, in two articles on those interesting branches of Optics, by Dr Young and M. Arago. There is also an instructive article, written by Professor Leslie, containing an historical review of those optical controversies and discoveries which, after the long intermission consequent on the death of Newton, terminated in the construction of the Achromatic Telescope. c