214 On the Modulus of Elasticity of Air, 
afternoon, precisely; where those gentlemen who have used the 
buoy and knipper, and reported in its favour, are particularly 
requested to attend, to witness this trial of their comparative 
accuracy.—11th July 1818.” 
Accordingly on Thursday the 16th instant, in pursuance of this 
advertisement, I proceeded at the time, and to the place ap- 
pointed; and in the company of several naval officers and gentle- 
men of high respectability, resorted to experiments which have 
been duly reported to the Lords of the Admiralty, the Commis- 
sioners of the Navy, and the Board of Longitude. In one ex- 
periment made with the buoy and knipper in three fathoms wa- 
ter, it appeared by the knipper to be four fathoms; and on re- 
peating this experiment in three and a half fathoms, it appeared 
to be sever, the buoy running up the line; so that this instru- 
ment will show a depth of water, in a current, or when a ship is 
under way in a driving sea, so much greater than the real depth, 
as to endanger in the most imminent degree both the ship and 
crew, 
In another instance, when the buoy and knipper were carried 
in a second boat, and the lead dropt with thirty fathoms of line 
across the current, the lead hung in the knipper, and would not 
sink; this experiment was repeated with fifteen fathoms of line 
across the currrent, when the same result took place—the lead 
not sinking. 
In repeating these experiments frequently with my machine, 
fhe real depth was given in every instance, without any varia- 
tion, I am, sir, 
Your obedient servant, 
Coventry, July 20, 1818. Epwarp Massry, 
XXXIV. On the Modulus of Elasticity of Air, and the Velocity 
of Sound. By Mr. Tuomas Trepeotp. 
To Mr. Tilloch. 
Sir, — Wren Sir Isaac Newton investigated the propagation 
of sound, he considered the weight of an uniform atmosphere to 
be equal to the mean elastic force of common air*: and this 
mean elastic force has been considered to be the same as the 
modulus of elasticity of later writers. But the modulus of elas- 
ticity is defined to be a column of the same substance, capable 
of producing a pressure on its base, which is to the weight pro- 
ducing a certain compression, as the length is to the diminution 
of the length. Now this differs materially from the mean elastic. 
bd Principles of Natural Philosophy, props. 47, 48, 49, and 50. book ii, 
. force 
