634 On the Method af judging by the Ear 
building happen to intervene between a person 
in motion, and the place from which a loud noise 
“proceeds, he no longer hears the sound in its 
proper direction, provided he passes very near 
the edifice, though at the same time he is con- 
vinced that the noise still continues, by hearing 
it reflected from another quarter. Let him, in 
the next place, advance in a right line perpen- 
dicular to the side of the building, and he will 
ina short time lose the echo, and recognize the 
original sound. Perhaps it may be imagined, 
that this change will not be observable before 
a right line, drawn from the head of the hearer 
to the sounding body, barely touches the top 
of the edifice: but this supposition, which is 
true when applied to the phenomena of sha- 
dows, cannot be relied on in the present instance, 
For as often as a sound meets with interruption, 
those pulses which pass nearest the obstructing 
body propagate themselves afresh into the air 
lying on the contrary side of the impediment, 
by means of the lateral communication of mo- 
tion; on which account, an intercepted sound 
recurs at a less distance from the obstacle, than 
that which a geometrician would assign to the 
event, by drawing the right line specified above, 
By the way, we may observe, that the principle 
which Signor Venturi has lately denominated 
the Jateral communication of motion to fluids, 
