134 Dr. Grecory on the 
can be expected 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 6 careful experiments will usually 
give the interval to a degree of correctness 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 approxi- 
mative estimates are required, be of but small consequence. When 
the distance exceeds four miles, this method of approximating to 
it can only be employed under favorable 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 aie 
I think, conclude unhesitatingly : 
ist, That sound moves uniformly; at least, in a horizontal 
direction, or one that does not deviate greatly from horizontality. 
2d, That the difference in intensity of a sound makes no ap- 
preciable 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 swm of their separate velocities gives the apparent 
* 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. 
