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



overlying strata. Having regard to the slow rate at which this wave 

 is generated, the rise of temperature, even in such a climate as that 

 of Northern India, not exceeding £° or 6° in the hour of most rapid 

 heating, equivalent to an increment of less than T ^th of the initial 

 pressure, it appears to me that the rate of propagation will be sensibly 

 that due to half the height of a homogeneous atmosphere, or a little 

 more than two-thirds the rate of the sound-wave. This rate will be 

 continually retarded as the wave advances through the loftier and 

 colder strata, being proportional to the square root of the absolute 

 temperature of each stratum. And it will depend on the thickness of 

 the atmospheric sheet heated, the amount of the heating, and on the 

 thickness and temperature of the cold external strata, whether the 

 retardation may not be such as to allow of the tension of the lower 

 strata becoming such as is indicated by the barometer at the time of 

 the forenoon maximum. Under such circumstances, the instant of 

 maximum pressure should coincide with that of the most rapid rise 

 of temperature and vaporisation. 



I do not think that our knowledge of any of these fundamental 

 conditions is such as to justify a rejection of the hypothesis on a priori 

 grounds, and it may therefore be worthy of inquiry how far it is in 

 accordance with verifiable observation. At Calcutta, the atmospheric 

 pressure at 9 h. 30 m. a.m. is about g^o-th greater than at the time of 

 the morning minimum ; an increase which would be produced by heat- 

 ing: the air in a closed vessel less than 2°. A retardation of about 

 half an hour in the dissipation of the increased pressure produced by 

 heating and evaporation would suffice to produce the observed effect. 



Dr. Sprung, in his admirable manual, the ' Lehrbuch der Meteoro- 

 logie,' published in 1885, has referred to the above hypothesis,* and 

 has tested the coincidence of the critical phases of temperature and 

 pressure by the summer results of the hourly observations and auto- 

 graphic registers of the Prague Observatory, from 1842 to 1861, 

 which have been recomputed by Professor Augustin. The result of 

 this test appears to be satisfactory. At Prague, on the mean of the 

 summer months, the forenoon barometric maximum occurs a little 

 after 8 a.m., and nearly coincides with the most rapid rise of tempera- 

 ture, t 



In India there is no station at which the forenoon maximum falls 

 at so early an hour at any season ; but at Yarkand and Kashghar, 

 according to Dr. Scully's valuable observations, in the summer, it 

 occurs even earlier than at Prague, while in the winter it is as late as 

 the mean epoch at Calcutta. It is true we have only fifteen series of 



* Op. cit., p. 336. 



f As computed from the figures given by Dr. Sprung, by the application of the 

 mecnod of differences (see footnote below), the barometric maximum occurs 

 nineteen minutes later than the instant of most rapid heating. 



