1824.] Mr. Daniell's Reply to Z. 27 



suddenly to a very great amount." From this extract I am 

 almost tempted to believe that Z has not done me the honour to 

 read my Essay, but only to review it. I have taken much pains 

 to show at p. 31, et seq. that such a sudden condensation of a 

 large quantity of vapour is not by any means requisite to pro- 

 duce an effect upon the barometer, but on the contrary, that a 

 gradual partial increase of temperature in the strata of an atmo- 

 spheric column of only two degrees is sufficient to depress the 

 mercury 063 in. If this part of my illustration should not have 

 been overlooked, I can only at present join issue upon the 

 opinion ; pledging myself to examine impartially and thankfully 

 any argument which Z may hereafter bring against my con- 

 clusions. 



My reviewer objects in the third place to an error into which 

 not only myself but Mr. Leslie has fallen, viz. that the particles 

 of air in passing over the surface of the globe do not for a 

 moment cease to gravitate, and that no horizontal movement of 

 them will produce the slighest derangement in a perpendicular 

 direction. He observes, " Now it is well known that any 

 body, to which a projectile motion of five miles per second has 

 been imparted, would revolve around the earth like a planet, and 

 would cease to exert any pressure on its surface. Any less 

 velocity must produce a proportional decrease of weight in the 

 particles of air, which is known to move at the rate of from 60 to 

 100 miles per hour." 



As this argument, if correct, is indeed decisively subversive 

 of my theory, I trust that I may be allowed to illustrate it some- 

 what particularly. According to the proposition set forth by Z, 

 a loaded waggon presses less upon the earth when at rest than 

 when in motion upon the road ! A steam boat must rise out of 

 the water in proportion to the velocity communicated to it by its 

 engine ! ! And many vessels must doubtless have been upon the 

 point of upsetting from this ill-understood cause ! ! ! To these 

 bodies, it is true, we cannot communicate a projectile motion of 

 five miles per second, but the former will move about five miles 

 per hour, and the latter probably ten, and " these lesser degrees 

 of velocity must produce a proportional decrease of weight." 

 Why a cannon ball indeed when projected from a gun does not 

 'mount in the air like a soap-bubble, is not quite obvious ; for such 

 a result one might undoubtedly expect from the theory of Z. 

 But to be serious : — With a laudable desire of correcting my 

 blunders, it is clear that the reviewer has fallen himself into an 

 error of no trivial importance. It is not true that any body to 

 which a projectile force of five miles per second has been 

 imparted, would revolve around the earth like a planet, unless 

 litis motion were to take place in vacuo, and were unopposed by any 

 pressure whatever: and no body that ever pressed at all upon the 

 burface of the earth would cease to exert such pressure in con- 



