OnciUutiun uf the Barometer. 3*21 



like other undulations, such as waves on the surface of water, 

 will spread to an indefinite distance on each side. But suppose 

 that, before they have passed beyond B andB', the action of the 

 sun upon A ceases, and terrestrial radiation produces a counter- 

 action. The column A contracts ; and the columns B and B', 

 jiossessing superior height and weight, will force back a portion 

 of air upon A, while other portions overflow upon C and C. 

 There will soon be a minimum of pressure at the bases of B and 

 B', and a maximum at the bases of A, C, and C, as shown by 

 the line 3°. If the sun again act upon A, and even, indeed, if 

 it do not again interfere, the undulatoiy motion will proceed, 

 and at the next step we shall have the condition of things shown 

 by 3^, and so on indefinitely. 



14. Upon several mechanical conditions will depend the rate 

 at which the undulations will spread. It is plain that if the 

 action of the sun recurs at intervals exactly equal to those occu- 

 pied by the passage of each undulation, its effect will be to in- 

 crease and maintain the undulatory motion, which otherwise 

 would die out by friction. From this cause, too, the undulations 

 will continually decrease in height as they spread. If the inter- 

 vals are not equal, the sun will soon counteract the undulatory 

 motion. 



15. Next, instead of supposing the sun to act only on A, let 

 it travel along, say over B, C, D, &c., and at such a rate as 

 exactly to keep pace with the undulations. After acting for six 

 hours upon A, and producing a minimum there as in 1°, the sun 

 will begin to act upon B just as a maximum of pressure has oc- 

 curred, and its effect will be greatly to increase the overflow of 

 air upon A and C, and assist the production of a minimum in B 

 and of maxima in A and C. It will further reach C just as the 

 inaxinmm has there occurred, its efi'ect being precisely similar to 

 that upon A and B, and so on indefinitely. The condition, 

 however, still is that the sun moves exactly at the same rate as 

 the undulations; otherwise it will 



Uiutralize its own effect. 



16. To apply this reasoning to 

 the actual condition of the atmo- 

 sphere, we have only to suppose the 

 stratum of atmosphere extended 

 round the earth, of which the sec- 

 tion is a circle, as in fig. 2, and the 

 Kun to revolve round the earth, as 

 it apparently does in a period of 

 twenty-four hour.s, in the direction 

 A, B, C, D. Its action upon A will 

 produce a minimum of pressure there 



