ON HYDRAULIC PRESSURE, 307 



ball moves slowly, its effect at any instant is in some degree divided through- 

 out all that part of the atmosphere, which the sound of the report has reached; 

 and if the ball follows the sound very speedily, it is obvious that the portion 

 of the air before the ball, which partakes of the effect, must be very small. 

 The sound is observed to be propagated with a velocity of about 11 30 feet in 

 a second, and a cannon ball may be discharged with a velocity of aOOO; but 

 one half of this is very speedily lost, so as to be wholly useless with regard to 

 the effect of the ball. If, therefore, we wish to increase the range of a 

 cannon ball, we must increase its weight ; for the resistance increases only 

 in proportion to the surface of the ball, while the weight is determined by its 

 solid content. 



It is not easy to explain, in a manner perfectly satisfactory, the reflection of 

 a cannon ball, or of a stone, which strikes the surface of the sea, or of a piece 

 of water, in an oblique direction. We may, however, assign some causes 

 which appear to be materially concerned in this effect. In the first place the 

 surface of the water, acting at first for some time on the lower part of the ball, 

 produces, by its friction, a degree of rotatory motion, by means of which the ball, 

 as it proceeds, acts upon the mass of water which is heaped up before it, and 

 is obliged by a similar friction to roll upwards, so that it mounts again to a 

 much greater height than it could possibly have attained by the mere hy- 

 drostatic pressure of the water at a depth so inconsiderable. But a more 

 powerful cause than this appears to be the continual succession of new sur- 

 faces which are to be depressed, and which may be supposed to react on the 

 ball, so as to produce the same effect, as a more intense pressure would have 

 done, if it had continued stationary; and the mutual action of the water and 

 the ball may be compared to the impulse of an obliqvie stream, moving with 

 the velocity of the ball, which would impel it much more powerfully than the 

 simple hydrostatic pressure at a much greater depth. It happens in this case, 

 as in many others, that the effects which appear to be the most familiar to us, 

 do not by any means admit the clearest and simplest explanation. 



