412 Dr. Gregory on the 



in the higher temperatures, than my experiments seem to in- 

 dicate. 



The above practical rule, so far as it may be entitled to 

 confidence, may be useful, 1st, to the military man in deter- 

 mining the distance of an enemy's camp, of a fortress, a bat- 

 tery, &c. ; 2d, to the sailor, in determining the distance of 

 another ship, &c. ; 3d, to the land-surveyor in ascertaining 

 the length of base lines, &c. in conducting the survey of a 

 lordship or county ; 4th, to the philosophic observer, in ap- 

 preciating the distances of thunder-clouds during a storm. 

 Yet in either of these applications the rule must be regarded 

 as approximative only; because few practical men can be ex- 

 pected to possess a time-measurer for less intervals than tenths 

 of seconds (if, indeed, so small) : and an error of a tenth of a 

 second, will occasion a mistake of from 37 to 40 yards in the 

 estimate of the distance. Beyond this, however, the error 

 need scarcely ever extend ; because a mean of 5 or G careful 

 experiments will usually give the interval to a degree of cor- 

 rectness far within the limits just specified. Indeed, an error 

 of from 30 to 40 yards in a distance of three or four miles, 

 will, on most occasions, where such approximative estimates 

 are required, be of but small consequence. When the di- 

 stance exceeds four miles, this method of approximating to 

 it can only be employed under favourable circumstances of a 

 very quiescent atmosphere, &c. : on which account, I felt 

 scarcely any desire to extend my own experiments to stations 

 more remote from each other, than those which I selected on 

 Shooter's Hill and Blackheath. 



Combining the results of experiments here recorded with 

 those which have been formerly deduced by Derham and 

 others, we may, I think, conclude unhesitatingly : 



1st, That sound moves uniformly; at least, in a horizontal 

 direction, or one that does not deviate greatly from horizon- 

 tality. 



2d, That the difference in intensity of a sound makes no 

 appreciable difference in its velocity*. 



3d, Nor, consequently, does a difference in the instrument 

 from which the sound is emitted. 



4th, That wind greatly affects sound in point of intensity ; 

 and that it affects it, also, in point of velocity. 



5th, That when the direction of the wind concurs with that 

 of the sound, the sum of their separate velocities gives the ap- 

 parent velocity of sound ; when the direction of the wind op- 



* The consecution of the notes in a tune, notwithstanding the difference 

 in their intensity, being uninterrupted when heard at a distance, furnishes 

 an elegant and decisive confirmation of this proposition. 



poses 



