PRELUDE TO THE SOLUTION OF PROBLEMS. 25 



Jatter opinion is very emphatically expressed by Bacon. 1 " The colli- 

 sion or thrusting of air, which they will have to be the cause of sound, 

 neither denotes the form nor the latent process of sound ; but is a 

 term of ignorance and of superficial contemplation." Nor can it be 

 justly denied, that an exact and distinct apprehension of the kind of 

 motion of the air by which sound is diffused, was beyond the reach of 

 the ancient philosophers, and made its way into the world long after- 

 wards. It was by no means easy to reconcile the nature of such motion 

 with obvious phenomena. For the process is not evident as motion ; 

 since, as Bacon also observes, 2 it does not visibly agitate the flame of a 

 candle, or a feather, or any light floating substance, by which the slight- 

 est motions of the air are betrayed. Still, the persuasion that sound 

 is some motion of the air, continued to keep hold of men's minds, and 

 acquired additional distinctness. The illustration employed by Vitru- 

 vius, in the following passage, is even now one of the best we can 

 offer. 3 " Voice is breath, flowing, and made sensible to the hearing by 

 striking the air. It moves in infinite circumferences of circles, as 

 when, by throwing a stone into still water, you produce innumerable 

 circles of waves, increasing from the centre and spreading outwards, 

 till the boundary of the space, or some obstacle, prevents their outlines 

 from going further. In the same manner the voice makes its motion 

 in circles. But in water the circle moves breadthways upon a level 

 plain ; the voice proceeds in breadth, and also successively ascends in 

 height." 



Both the comparison, and the notice of the difference of the two 

 cases, prove the architect to have had very clear notions on the sub- 

 ject ; which he further shows by comparing the resonance of the 

 walls of a building to the disturbance of the outline of the waves of 



O 



water when they meet with a boundary, and are thrown back. " There- 

 fore, as in the outlines of waves in water, so in the voice, if no obstacle 

 interrupt the foremost, it does not disturb the second and the following 

 ones, so that all come to the ears of persons, whether high up or low 

 down, without resonance. But when they strike against obstacles, the 

 foremost, being thrown back, disturb the lines of those which follow." 

 Similar analogies were employed 'by the ancients in order to explain 

 the occurrence of Echoes. Aristotle says, 4 "An Echo takes place, 

 when the air, being as one body in consequence of the vessel which 

 bounds it, and being prevented from being thrust forwards, is reflected 



Hist. Son. el And. vol. ix. p 68. 2 Ibid. 3 De Arch. v. 3. " DeAnimd ii. S 



