Barometric Pressure. 33 



these upper strata absorb a considerable amount of heat. This 

 diurnal heating action of the sun on the upper strata would 

 harmonise far better with the general uniformity of the daily 

 barometric oscillation along the different parallels of latitude, as 

 well as with its general independence of weather. We need not 

 quite exclude local influences, but these seem to be more of a 

 secondary character. 



" Inasmuch as the -periodical action of the sun's rays on the 

 uffer strata of the atmosphere, recurring day by day, must 

 produce periodical movements of great regularity (an oscillation 

 of the entire mass of the atmosphere), it is easy to see that this 

 can explain the typical character of the diurnal barometric 

 oscillation, while the local differences of the earth's surface 

 represent the modifying element. 



" If a limited mass of fluid is set in simple pendulum-like 

 oscillations, their amplitudes are governed by the given conditions 

 of the fluid or the gas (dimensions, temperature). If the impulse 

 is a single powerful one, such as that which gives rise to seiches 

 in lakes, it is perfectly immaterial how it goes on : the mass of 

 water takes up always the same pendulum movement in which it 

 can move in virtue of its dimensions (the length and depth of its 

 basin). 



"If the impulse recurs periodically, then oscillations of that 

 period are forced to occur, even if these do not coincide with any 

 any of the forms of oscillation which belong to free waves. This 

 holds if the impulse represents a simple sine wave. In other 

 cases the following must be considered. Fourier showed 

 mathematically that any periodical form of oscillation (or wave 

 of any form) can always be resolved into a sum of simple 

 pendulum oscillations (waves), and that their number of 

 oscillations are i, 2, 3, times as great as the number of 

 oscillations of the given form of movement, and only in one 

 siti^le manner. When any periodically recurring impulse of any 

 form is resolved by Fourier's harmonic analysis into pendulum 

 oscillations, each portion of these produces a forced oscillation 

 of the same period in the mass of fluid. But the amplitudes of 

 these forced waves do not preserve the same proportion to each 

 other as those of the waves which produce them. If the period 

 of an exciting wave is nearly the same as that of a free wave 

 in the liquid, the resulting forced oscillation will attain a 

 disproportionally great amplitude. 



" This principle may be applied to the constant oscillations 

 of our atmosphere which are produced by a periodical impulse, 

 i.e., by the variation of temperature which recurs uniformlv day 

 by day. If the atmospherical envelope of our earth, with its 

 conditions of space and of temperature, is most easily set in 

 oscillations of a semi-diurnal period, the semi-diurnal portion of 

 its exciting cause, the diurnal temperature wave, will be the most 

 active. It does not matter whether this semi-diurnal temperature 

 wave has a real independent existence." 



