SOUND. 319 



was necessary to add the velocity of the wind to that of 

 the sound ; when in opposite directions, this was to be 

 subtracted. The force of the sound was found to cause 

 no change in the velocity, the feeblest report reaching 

 the ear in an equal space of time with the loudest. 



The velocity of sound as determined by the French 

 Academicians, is less by 35 feet than the commonly re- 

 ceived computation of Derham, 1142. The Florentine 

 Academy make it 1148. Farther experiments are neces- 

 sary to determine which of these is nearest to truth ; or 

 whether the velocity does not vary in different times, 

 places, and temperatures. Some late philosophers have 

 doubted the uniformity of the velocity. Yet Derham, 

 whose experiments have been conducted with great care, 

 says, ' That in all weathers, whether the sky be clear 

 and serene, or cloudy and turbid, whether it snows or 

 rains, thunders or lightens, whether hot or cold, winter 

 or summer, whether the mercury in the barometer rises 

 or falls, in all changes of atmosphere, wind only excepted, 

 the velocity of sound is neither more nor less ; only 

 the sound will be more or less loud. 



As before stated, some other philosophers have sup- 

 posed that they could perceive a difference in the velocity 

 of sound, occasioned by various circumstances ; such as 

 the state of the weather, the loudness of the sound, &c. 

 But the experiments of Derham, which lead to a different 

 result, were performed with greater care, and with more 

 accurate instruments. The opinions of the other philos- 

 ophers, too, might be expected to be influenced by the 

 strong expectation which any one would have of finding 

 such a difference. For example, every one would ex- 

 pect that a very loud and intense sound would force itself 

 through the air with greater velocity than a faint and 

 dull one. Yet the result of Mr Derham's experiments, 

 showed that the sounds of all bodies, such as guns, bells, 

 hammers, &c, have the same degree of velocity. ' He 

 compared the strokes of a hammer, and the report of a 

 gun, at the distance of a mile, (that being the greatest at 

 which he could hear the sound of a hammer,) and he 

 found that the sound of both reached him at the same 



