216 On the Theory oF Wa aler-Spouts. 
air is compressed, the excess of heat will begin to be diffused 
among the surrounding bodies as soon as it is developed: and 
when the pressure is removed, the air cannot return to its original 
bulk till it be supplied with heat. 
If the heat given out during compression could be retained by 
non-conductors round the compressed air, ready to be imbibed 
whenever the pressure should be removed, then undoubtedly the 
elastic force would be as the density; but in all cases the heat 
is free to move to other bodies; and when air is compressed into 
half the space it occupies in its natural state, according to Mr. 
Dalton, its ten nperature is increased fifty degrees: and as the 
rate of cooling is nearly as the excess of temperature, the clastic 
force will be diminishéd in proportion to the quantity of heat 
given out: consequently, under great compressing forees the 
elastic force must increase much slower than the density, but the 
precise effect will differ acgording to the time in which the cha ge 
is produced, 
To take into consideration the imperfect elasticity of -air, neal 
‘the effect of the vapour it contains, would render the calculation 
of the velocity of sound extremely intricate ; but an approximate’ 
value of the modulus of elasticity might be obtained from experi- 
ment. 
If Newton’s measure of the elastic force of air had been cor- 
rect, the velocity of sound calculated from it should have been 
above the result of experiment and not below it; because he sup- 
poses the air to be perfectly elastic. His hypothetical supposition 
respecting the magnitude of the sclid particles of air is unsup- 
ported by experience or analogy: and though his reasoning re- 
specting the effect of vapour in some measure coincides with the 
ingenious speculations of Mr. Dalton, yet it is not the less di- 
stant from the real laws of the elasticity of a mixture of gaseous 
bodies. 
I am, sir, yours, &c. &e. 
Sopt. 21, 1818. THomas TREDGOLD. 
XXXV. On the Theory of Water-Spouts, By Mr, Gaviy 
INGLIS. 
To Mr. Tilloch, 
DEAR sin,—T HAVE been observing with no small degree of 
interest the various statements regarding the laws and principles 
of that wonderful and often alarming phenomenon, the water= 
spout, a satisfactory theory of which can only be deduced from 
a collection and collation of facts resulting from actual observa- » 
tion, and furnished’ by those who may ‘have had an hie 
Q 
