544 APPENDIX. 



cause the barometer to rise above the mean, as the motion diminishes, 

 which must take place some time in the night. This rise will be small, 

 however, compared with that at 9 or 10 A. M. As the barometer now 

 stands above the mean, it must necessarily descend to the mean at the 

 moment when the air is neither increasing nor diminishing in tempera- 

 ture, which will be a little before sunrise. If this is a true explanation 

 of the four daily fluctuations of the barometer, it will follow that the 

 morning rise ought to be greater at considerable elevations, provided 

 they are not too great, because some of the air will be lifted above the 

 place of observation ; and such was found to be the case by Col. Sykes, 1 

 in India. As this morning rise of the barometer depends on the increas- 

 ing elasticity of the air, and this increasing elasticity, on heat, Mr. Espy 

 proposed to the mathematicians to calculate how much the whole atmos- 

 phere is heated from sunrise till the time when the barometer stands 

 highest, the actual rise of the barometer being given. In this way, as 

 refraction is affected by temperature, meteorology may assist astronomy. 



Professor Forbes doubted the correctness of Mr. Espy's views of the 

 cause of the great daily fluctuation of the barometer at elevated stations; 

 for, towards two or three o'clock, the heat being greatest, its effect in 

 lifting up the inferior air to and above the elevated station should then 

 be greatest, whereas that time of the day was nearer to the time of min- 

 imum height of barometer than its maximum. 



To this Mr. Espy answers, that at very great elevations, it is probable 

 from the theory there would be only two fluctuations in a day, the max- 

 imum at the moment of the greatest heat, and the minimum, at the mo- 

 ment of the greatest cold. But at moderate elevations, such as from 

 1800 to 2000 feet, the effect would be to prolong the time of maximum a 

 few minutes at the upper station, so that it might still be rising above, 

 after it began to descend below. 



Experiments were wanting to determine this matter. 



The principle itself, as a vera causa, is too plain to admit of doubt. It 

 is the mere application of the law that "action and reaction are equal 

 and in opposite directions." 



It might, however, be illustrated in the following manner. 



Let a person balance himself in a pair of large scales, in a stooping po- 

 sition. Now he cannot raise himself erect, without causing the scale in 

 which he stands to descend, he cannot come to rest in an erect position 

 without his scale again rising, to equilibrium. He cannot stoop down 

 again, after a balance, without his scale rising, at the moment his motion 

 downwards commences, he cannot diminish his velocity downwards, 

 without increasing his pressure on the scale beyond his natural weight, 

 and causing his scale to descend again beyond equilibrium; and finally 

 he cannot come to rest without producing equilibrium; and thus he 

 will produce four oscillations of the balance by one upward and one 

 downward motion of his body, corresponding to one expansion and one 

 contraction of the air in the day. 



1 Col. Sykes found the nocturnal falling minimum tide from 10 11, P. M., 

 to 4 - 5, A. M., at Poona, .0181 inches from about 2000 to 1800 feet high ; and 

 the diurnal rising tide, from 4 5, A. M., to 9 10, A. M., .0445 inches; and 

 at the height of 4500 feet, he crives these same tides .0240 and .0636, and at the 

 height of 6407 feet, .0433 and .0490, as observed by Dr. Walker and M. Dal- 

 jnahoy. fRoy. Phil. Trans, for 1835 p. 196.] 



