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274 unexceptionable equivalent for it can be offered. These quotations shew, amid many suggestive observations, that knowledge concerning the air was then very unsettled; and yet they prove, it may be assumed, that air was implied in the passage referred to.

Note 2, p. 101. An echo is produced whenever, &c.] This passage is obscure, both from its elliptical wording and the want of adequate exemplification; but, in attributing to the air elasticity and capability of being reflected, it seems to suggest that the atmosphere only is the cause of sound and, therefore, of echoes. So, according to modern science, "an echo is sound reflected from a distant surface and repeated to the ear; although several other conditions are required for its production." In another treatise, it is assumed that reflexion of the air (ἡ ἀνάκλασις) is the immediate cause of an echo; and since an echo is reflexion, "must there not be, for its production, air confined, impacted and communicating, as one mass, with that which is to be reflected?" But an echo, whether or not audible, ought, as the text states, looking at the properties of the air, to be a constant occurrence; for as light is continually reflected from bodies, and thereby casting shadows by which light is distinguished, so sound, owing to the air's elasticity, must be often reflected and, therefore, repeated, in varying degrees of intensity, according to the nature of the surface on which it may have impinged. That age, in fine, was