226 SIXTH RKI'OUT — 1836. 



nature is such that they cannot be shown to exist by the evidence 

 of the senses alone. The phfenomena of capiUury attraction ap- 

 pear to have principally led to hypotheses respecting the consti- 

 tution and molecular action of liquids. The first ^vl•iters on the 

 subject considered it sufficient to treat the liquid as incompres- 

 sd)le, and attribute to its molecules, and to those of the contain- 

 ing solids, an attracting force, sensible only at insensible distances 

 from the attracting centres; on which supposition the problem 

 does not materially differ from those that belong to the common 

 theory of inelastic fluids. Poisson conceived it necessary to 

 treat the question with more distinct reference to the molecular 

 constitution of bodies, and to the repulsive, as well as attractive, 

 forces which keep the molecules separate from each other in 

 jjlaces of equilibrium. The views of this eminent mathematician 

 respecting the constitution of fluid bodies, particularly as applied 

 in his New Theory of Capillary Action, formed, together with 

 an exposition of the theories of preceding writers, the main sub- 

 ject of my second Report. I propose, in the present essay, to 

 speak of some other instances of the application of mathematics, 

 in explanation of the phaenomena of rest or motion of fluids, and 

 carefully to distinguish, as heretofore, the calculations derived 

 from hypotheses merely from those that set out from experi- 

 mental facts. The mechanical theory of the atmosphere, and 

 of the propagation of sound in it, as affected by the development 

 of heat, will principally claim our attention. In conclusion, I 

 shall take occasion to add some supplementary remarks on sub- 

 jects contained in the preceding Reports, and to notice any ad- 

 ditions that may have been very recently made to this depart- 

 ment of science. 



Mechanical 27teori/ of the Atmosjihere. — The pressure of a 

 perfectly elastic fluid when at rest, and everywhere of the same 

 temperature, varies in the same proportion as its density. This, 

 the well-known law of Boyle and Mariotte, was recently proved 

 to be true, for pressures amounting to twenty-seven times the 

 mean atmospheric pressure, by a committee of the French In- 

 stitute appointed for ascertaining the elastic force of steam, in 

 some preliminary experiments for executing the purpose of the 

 commission*. The modification this law must receive to take 

 in the effects of change of temperature (the fluid still remaining 

 at rest) was first stated by Dalton as a result of experiment, and 

 confirmed very shortly after by the experiments of Gay-Lussac t« 



• Memoires de I'Academie des Sciences, torn. x. p. 207. 



t The paper of Dalton was read before the Manchester Philosophical Society 

 in October, 1801, and was published in 1802. Gay-Liissac's experiments ap- 

 peared iu the Anmdes de C/iimie, 1802, torn, xliii. p. 137. 



