Maxima and certain Conditions of Temperature, &c. 423 



The general average of all the hours is 412 mm. per hour, but the 

 quantity recorded between 9 and 10 p.m. is only 231 mm., or little 

 more than half, and that between 10 and 11 A.M. 288 mm., or little 

 more than three-fifths of this average. It is to be observed that the 

 forenoon minimum of Batavia falls an hour later than that of 

 Calcutta, whereas the evening and principal minimum is an hour 

 earlier. This is exactly what might be expected from the combina- 

 tion of a double diurnal oscillation with one of single period, the 

 latter having its maximum in the former case at night, in the latter 

 in the daytime. 



The Melbourne hourly rainfall tables show great variations in 

 different months, and admit of very little definite conclusion, except 

 that, as at Batavia, there is more rain at night than in the day. It 

 is then only in the warm and nearly saturated atmosphere of Bengal 

 and Java, in their respective rainy seasons, that these diurnal inter- 

 ruptions of the rainfall about the hours of the two barometric 

 maxima are decidedly manifested. But in these two cases they are 

 most marked ; and this circumstance, taken in conjunction with the 

 corresponding cloud variation, which is shown by so many stations, 

 points strongly to a causal connexion between the diurnal variation 

 of pressure and the condensation of atmospheric vapour in the cloud- 

 forming strata of the atmosphere, which, I think, we can scarcely fail 

 to recognise. 



The mere fact that an increase of atmospheric pressure, from what- 

 ever cause arising, is accompanied with a dissipation of cloud and a 

 diminution of rainfall, would not perhaps call for special remark. 

 But it is to be observed that whereas the nocturnal barometric maxi- 

 mum, at all the stations here dealt with, is less pronounced than that 

 of the forenoon, the concomitant effects in the clearing of the atmo- 

 sphere and in the check in the rainfall are much greater in the former 

 case than in the latter. They seem to point to a forcible compression 

 of the atmosphere, and dynamic heating of the cloud-forming strata. 

 And some such temporary effect does not seem impossible, even at a 

 time when the earth's surface and the air immediately in contact with 

 it are cooling rapidly. Moreover the temperature curves of Prague, 

 Calcutta, and Batavia all show a very slight irregularity about 10 p.m., 

 which indicates a slight check in the fall of temperature about that 

 hour greater than takes place either in the preceding or subsequent 

 hour, and which may possibly be the manifestation of such an action 

 in the lowest atmospheric stratum. Slight as it is, the fact that it 

 occurs at the same hour in all these curves, and that it coincides with 

 the evening pressure maximum and the strongly marked minima of 

 cloud and rainfall, is at least significant. 



Wben we tabulate the differences of the first and second orders of 

 the hourly means of the original observations, at the three stations 



