416 Mr. H. F. Blanford. The Diurnal Barometric 



From this table it appears that only in one or two of the winter 

 months at Batavia and Melbourne does the forenoon maximum of 

 pressure coincide so nearly with the moment of most rapid heating 

 as at Prague and Yarkand. In all cases, except in the midwinter 

 months at Melbourne, the former follows the latter by an interval 

 which averages 31 minutes at Melbourne, 35 minutes at Batavia, 

 1 hour and 8 minutes at Calcutta, and 1 hour and 48 minutes at 

 Bombay. But it is to be noticed that, at all the stations, this interval 

 is shortest in the winter and greatest in the summer. It is true that 

 the computed temperature epochs may be 10 minutes or so in error, 

 owing to the merely approximative character of the method adopted, 

 and that, for an hour or more afterwards, the change in the rate of rise 

 is very small, not exceeding a few tenths of a degree per hour ; but 

 the retardation of the barometric maximum is too systematic to be 

 explained away by any such considerations. There are, however, 

 others of a very obvious character. 



The hypothesis attributes the increase of pressure in the forenoon 

 to the mean increase of tension in the atmosphere up to a very great 

 height, not to that of the lowest stratum only. And since this latter is 

 heated much more rapidly than the higher strata, and that, owing to 

 variations in the character of the earth's surface, the rates of heating 

 in contiguous areas of the lower strata themselves vary indefinitely, 

 the convective movements, which are set up in consequence, produce 

 innumerable small modifications in the form of the local temperature 

 curves, which will to a great extent eliminate each other when a 

 mean is taken of those of higher and lower strata ; and the general 

 form of this curve for the greater mass of the superincumbent 

 atmosphere must be much more constant than that deduced from the 

 thermometer readings of our observatories. Generally, as was 

 assumed by Lamont in his discussion of the problem, the critical 

 phases of the former will be later than those of the latter. This 

 retardation will be greatest where the diurnal range of temperature 

 is greatest, and especially at such intertropical stations as Bombay 

 and Calcutta. 



The diurnal march of the temperature at such an observatory as 

 the Colaba Observatory at Bombay, must be influenced in a high 

 degree by the local influx of cooler air from the neighbourhood. 

 Situated on a narrow point of land, and surrounded, in all directions 

 but one, by many miles of sea, the atmosphere is scarcely ever calm, 

 and a wind from any quarter other than between north and north- 

 west comes directly from the sea close at hand, the movement of the 

 air increasing with the rise of temperature. To this circumstance, in 

 all probability, it is due that this rise undergoes a slight check at an 

 earlier hour than at any of the other stations. It is very slight, as is 

 shown in the following table, which gives the amount of the rise for 



