Mr Meikle on the Theory of the Air-Thermometer. 333 



of temperature ; but it has more generally been admitted, par- 

 ticularly of late, that the expansions of gaseous bodies under a 

 constant pressure follow the true law of the influx of heat. In 

 proof of the latter opinion, it has been alleged, that, when a solid 

 is expanded by heat, its cohesion, being greater at lower tempe- 

 ratures, resists the expanding power so much the more ; and 

 therefore, in the lower parts of the scale, the increments of vo- 

 lume produced by equal additions of heat are smaller than at 

 higher temperatures ; or, in other words, that the expansion of 

 solids proceeds at an accelerated rate, whilst the increase of heat 

 is uniform. Something of the same kind, though in an inferior 

 degree, is said to take place in liquids ; but gaseous bodies be- 

 ing supposed to have no cohesion between their particles, are 

 accounted free to obey the true law of temperature. 



With this species of argument I am by no means satisfied, 

 because I really cannot pretend to see the force or meaning of 

 it ; nor do I think we are likely soon to arrive at any certain 

 conclusion, by reasoning on principles which are less known per- 

 haps than the thing to be proved. Does analogy not rather 

 render it probable, that the particles of gases attract each other 

 with forces varying inversely as the squares of their distances ? 

 Many other objections, and reasonable ones, too, might be pro- 

 posed, to which no solid answer can be given. Such, however, 

 serve to shew on what a slippery foundation a very general opi- 

 nion may sometimes rest, — an opinion, in the present instance, 

 with which many speculations must either stand or fall. In- 

 deed, if authorities are allowed to have any weight in settling a 

 disputed point, then I believe by far the greater number of the 

 more enlightened are not only favourable to the common theory 

 of the air-thermometer, but scarcely entertain a doubt on the 

 subject. 



It must, however, be admitted, that conjectures or opinions, 

 though supported by numerous and respectable authorities, are 

 not to be compared with experimental evidence, or with conclu- 

 sions deduced from such evidence by sound reasoning. I have, 

 therefore, with the view of approaching a little nearer to cer- 

 tainty in a point which is any thing but settled, been induced 

 to attempt an investigation of the theory of the air-thermometer 

 on its own principles, divested as much as possible of hypothe- 



