322 On the Semidiurnal OscUlutiun of the Barometer. 



and maxima at B and B'; and, assuming the undulations thus 

 produced to travel at such a rate as would carry them round the 

 globe in twenty-four hours, the sun will then commence to act 

 upon B at tlie moment at which its maximum of pressure has 

 happened. The succeeding minimum at B will therefore be in- 

 creased, and maxima will be produced at A and C. During 

 the same time also the maximum pressure at B' will occasion 

 an overflow upon A and C, thus assisting the direct action of 

 the sun. The same uiidulatory motion will ensue as the sun 

 passes round to C and D ; and it is evident, as a simple mathe- 

 matical consequence of the supposed conditions, that at each 

 point of the circle maxima and minima of barometric pressure 

 will succeed each other at intervals of six hours. This is repre- 

 sented in fig. 3, in Fig. 3. 

 which the ellipse is 

 the shape assumed by 

 the atmosphere, and 

 is supposed to rotate 

 round the earth in 

 the same period as 9i 

 the sun. 



17. If, lastly, we 

 suppose that an at- 

 mospheric undula- 

 tion produced by the 

 sun's action does not travel at the same rate as the sun, the 

 question becomes more complex, and the curve produced loses 

 its symmetry. The sun probably travels a little the faster, 

 so as perpetually to overtake the preceding undulation before it 

 is quite accomplished. The eiFect of this may perhaps be best 

 understood from fig. 4, in which is shown the partial interference 

 of the maximum (M) Fig. 4. 



immediately produced 

 by the sun, with the 

 maximum {m) occa- 

 sioned by the sun's 

 action twelve hours 

 previously. The fur- 

 ther prosecution of 

 this problem belongs 

 rather to the mathe- 

 matician than the me- 

 teorologist; but it is 

 not hard to see from this figure why the night minimum is in 

 most places deferred till sunrise (5 a.m.), and why it is less 

 strongly marked than that occasioned directly by the sun's rays. 



