i 7 8] BAR ferent degrees of temperature of the air. Among the trowelling barome- ters, that of M. de Luc, improved by J. F. Ltrz, is the most useful and compendious. — A icind-laro- meter has also been contrived by Mr. Wilkes, of which we pro- pose to give an account under the article, Wind. A great variety of other useful instruments have been invented since the days ofToRRic.ELLi, upon similar principles, and with mecha- nical additions more or less com- plicated ; but it must be acknow- ledged, that the simple barometer of his invention is the most exact, though not the most sensible ba- lance for weighing the atmosphere, and has therefore been most gene- rally adopted. Rationale. — M. de Luc, not sa- tisfied with the different hypothe- ses brought forward by Wallis, Halle. Leibnitz, Maikan,&c. to explain the variations of die ba- rometer, lias satisfactorily refuted the conjectures of all his predeces- sors, and endeavoured to establish his own, which is founded on a supposition, that a column of air, loaded with vapours, is lighter than a column of pure air of equal bulk. He consequently asserts, 1 . That the density of air is the immediate and o)i 1 1/ cause that supports the mercury ; and '1 . That the more elastic the air is, the less does it press and weigh upon its base : but though there is considerable ingenuity in this explanation, it is by no means conclusive. Soon after the publication of this theory, M. Begjdemn, in 17/3, endeavoured to prove, that the variation of the whoie mass of the atmosphere, as well as that of the spring or elasticity in a part EAR. of this mass, are. the two general causes of the variations of the ba- rometer ; and that the primitive causes of this remarkable eii"e:-r are, heat, cold, dryness, and moisture, wit their different combinations. We cannot enter into the parti- culars relative to the opera! ion of these various causes, but shall briefly observe, that heat dilates the air; cold contracts it, and draws together those parts which it is the property of heat to sepa- rate : hence, the natural effect of the former is expansion ; die con- sequences of the latter, condensa- tion, compression of its spring, and an increase of pressure, on ac- count of which, the mercury rises in the barometer. Respecting the effects of dryness and moisture, it deserves to be re- marked, that, if the latter diminishes the pressure of the air, by relaxing its spring, it on the other hand loads it with watery particles, which * very considerably augment its mass, so that it may be difficult to deter- mine how far the moisture of the air e .Ktrts its influence on the rise, and fall of die barometer. The same observation may be applied to dry- ness, which dispels the watery par- ticles diat increased the weight of the air ; so that there appears to be a perpetual conflict between die effect of spring and mass, between elasticity and weight ; and experi- ence alone can inform us whether, in this contest of the elements, the effect of elasticity is much superior to that of weight. Such is the plausible explanation of Beguelin, and we have only to regret, that the result of his inquiries, still leaves the subject involved in much doubt and difficulty j because even diis theory does not account for the Sudden changes of die atmosphere. Never-
