﻿334 Mr. J. K. Laughton on Barometric Differences 



head is to some extent lifted ; the air all around, or principally 

 in one direction, is to some extent driven back ; and equilibrium 

 of elastic force being maintained, equilibrium of density is also 

 maintained by a slow and for the most part imperceptible change 

 between the hotter and colder masses ; for geographically, and 

 more especially at sea, the large areas of hot and cold air are not 

 separated from each other by any sharp outline ; the differences 

 of temperature, and therefore the differences of density, are very 

 gradual, and any motion that differences of density may cause 

 must be of an extremely gentle nature. 



This statement is very contrary to the common idea and the 

 generally received theory ; but it is easy to show that any velo- 

 city which can possibly be generated by such differences of den- 

 sity is altogether inappreciable. The most familiar illustration 

 of this may be taken from the trade-winds. Premising that the 

 account given of these winds in every book on meteorology with 

 which I am acquainted is more or less incorrect in its geographical 

 details, I will, so far as the data on which my calculation is based, 

 follow Mr. Buchan, the clearest and latest exponent of the influ- 

 ence of density. He says that in the Atlantic the north trades 

 prevail between latitudes 9° and 30°*. According to his charts, 

 the mean pressures on these parallels may be taken as 29*95 

 and 30' 15, and the mean temperatures as about 80° and 72° F. 

 He gives no hygrometrical estimate ; but wet-bulb temperatures 

 of 79° and 6S° will probably not be very far wrong. These ele- 

 ments give us, from Mr. Glaisher's Tables, the weights of a cubic 

 foot of air on each parallel as 509*5 and 522*7 grains respectively, 

 showing a difference between the two of 13*2 grains. Bearing 

 in mind that a cubic foot of air on the parallel of 30° presses on 

 the cubic foot of air immediately south of it, and not on a cubic 

 foot on the parallel of 9°, it follows that the difference of weight 

 between two contiguous cubic feet is ^ of -^ of ^ of 13*2 grains^ 



or 1 of a grain nearly. The acceleration due to this pressure i s 



. I . q nearly, or rather less than .„*.,. of a foot per second: 



600,000X500 ~ J t 900,000 r > 



which, taking no account of friction, or the absorption of motion 

 by compression, elasticity, or in any other way, would be equi- 

 valent to about 8 inches in an hour. It can scarcely be a mat- 

 ter of opinion whether a force so small is in reality capable of 

 overcoming the numerous natural hindrances to the motion it 

 would tend to generate. 



The fact is that we bring to the study of meteorology ideas as 

 to the motion of air which we have formed from noticing the 

 draughts in a room, or the strong currents generated in the pro- 

 cess of ventilating coal-mines ; we forget the effect not only of 



* 



Handy Book, p. 214. 



