It seems to us that the truth here expressed is not unmixed with error. In the very first sentence, we think that accuracy would require the suppression of the word _not_. The intensity of sound depends not only upon the density and elasticity of the air whose pulsation is an antecedent condition, but also upon the density and elasticity of the air _through which_ the pulse is transmitted. While it is true that a pulse given to the denser column or stratum of air may be transmitted through a _rarer_ medium with greater resultant force than if its origin and direction were reversed, it by no means follows that the intensity of sound is unaffected by the density of the air _in_ which it is heard. We apprehend the truth to be that the pulse given to highly rarefied air is very feeble; and its secondary effect upon a denser and more elastic fluid, correspondingly slight; while the pulse from the denser air would be transmitted with greater--but still diminished--force, through the rarer atmosphere _in which_ it reaches the ear. An _absolute vacuum_ could not transmit the pulse given through a column or stratum of elastic fluid. A _rarefied atmosphere_ could but transmit it with a force always varying with its own elasticity. And were it possible to preserve one’s consciousness within the exhausted receiver of an air-pump, we doubt if the most sensitive ear could be made to hear the roar of a cataract without. “A feeble sound becomes instantly louder as soon as the air becomes more dense;” but _not as loud_ as if the same initial pulse were _immediately_ given to the denser air. In the case of two cannon equally charged, one of which is fired on the top of a mountain, and the other in a valley below it, to say that “_owing to its origin_, the sound generated in the denser air is louder than that generated in the rarer,” _sounds_ much like saying it is _because_ it is. If it be more than this, it is wrong. It is a clear case of _non causa pro causa_. The origin [of the _pulse_] of sound is in either case the same: the explosion of equal charges of gunpowder, in guns supposed to be of like material and equal size. The _effects_ are not the same, because the effect of a force depends upon its _transmission_ as well as upon its origin. Does the atmosphere “_accompany_” peals of thunder? Or does this expression convey a distinct idea of the office of the atmosphere in the production of sound? We understand that the atmosphere receives the pulse or blow, and that its transmission to the ear is due to the elastic force of the intermediate air. It is not the _homogeneousness_ of air, but its _elasticity_ which transmits the pulse. And though, in architecture, the object sought is a uniformly elastic air throughout the _auditorium_, it does not follow, nor is it even desirable, that the _maximum effect at a given point_ should be obtained by it.