558 



NA TURE 



[April 14, 1 88 1 



nishes as the range of the surface-temperature increases, 

 and it is certain that the ranges of the temperature at all 

 the stations as we ascend above thi: soil, will approach 

 always nearer to each other the higher we go ; so that if 

 we knew in what way the relation between the oscillations 

 is produced, and thence, in all probability, the part of the 

 atmosphere which is chiefly in question, we might find a 

 more constant relation between A/ and A/ at all the 

 stations. 



I will now proceed to make some remarks which may 

 aid in the study of these questions. I would first observe 

 with relation to the stations in the preceding table that 

 the value of /■ is only 0024 at Chittagong, while it is nearly 

 o'029 at Saugor Island. I have no doubt, however, that 

 this difference is due to some local or instrumental cause. 



In the first article I have, as already stated, used only 

 observations made in first-class observatories. Every 

 one acquainted with the difficulties of obtaining good 

 observations in India, especially at out-stations, will 

 understand that the mere printing of their results cannot 

 give them any certain weight. Thus from the tables of 

 means given in Mr. Blanford's work, it appears that the 

 value of k at Vizagapatam is 0'032, while at Cuttack less 

 than 3° to the north, it is o'o25. This great difference 

 would lead to the conclusion that there must be some 

 remarkable local cause, or that there is some error in the 

 observations ; if the former, then farther research as to 

 this cause would be of the highest importance ; the latter 

 seems to me the most probable explanation. The follow- 

 ing e.xample, that of a station so well known as Madras, 

 will show some reason for this belief. The results for 

 Madras, given in the table, are those employed in the 

 first article, and deduced from observations made in the 

 Magnetic and Meteorological Observatory from 1841 till 

 1S55. In Mr. Blanford's tables, however, means are 

 given from observations for nine years. These means 

 give results markedly different from the others. The 

 following summary for the months of December, January, 

 and June, will prove this : — 



Height fif Barometer. 

 December. J.inuary. June. 



29*954 29-998 29-691 



Group of Years. 



1841-4S 

 1846-50 

 1851-SS 



Means ... 



■957 

 •992 



■967 



■994 



•693 

 ■702 



■695 



1S68-71 -978 -967 -671 



1872-76 -955 '926 -802 



Mr. Blanford's 1 



means for nine > '965 '944 744 



years ) 



Now the three means for January, each deduced from 

 five years' observations, do not deviate from the mean of 

 the whole more than '008 inch, while the corresponding 

 extreme deviation for June is 'o 7 inch. Whereas the 

 means from nine years, according to Mr. Blanford's tables, 

 differ by o'ojo inch from the means of fifteen years, and 

 the annual range, which is 0^299 by the fifteen years' 

 observations, becomes only 0200 by the last series. 

 Judging from the means for four years — given by Mr. 

 Blanford in his valuable memoir in the P/ul. Trans, on 

 the winds of North India— whieh I believe to be part of 

 the nine years, the means for five years, 1S72-76, give a 

 range of only 0T24 inch from January to June ; but even 

 the mean pressure for the place and the annual law of 

 variation is altered by the nine-year series, the maximum 

 pressure occurring in December instead of January. It is 

 essential m such investigations that some confidence can 

 be placed in the observations, and if this can be done 

 anywhere it is certainly in observations made at such a 

 station as Madras. I have employed previously means 

 deduced from fifteen years' observations in the Magnetic 

 and iMeteorological Observatory ; but these means differ 

 in a manner so extraordinary (for a South Indian station) 



from the means given by Mr. Blanford from nine years' 

 observations, that this demands attention and explana- 

 tion.' 



We see that the annual oscillation of pressure increases 

 with that of temperature, and as shown in the article 

 already cited, that this relation holds for two places on 

 opposite sides of the Ghiits, nearly at the same height 

 within sixty miles of each other, for which the annual 

 oscillations at one are twice those for the other. 



Let us now see what our knowledge from laboratory 

 experiments of the laws of expansion and equilibriimt of 

 gases would induce us to conclude with reference to the 

 stations in question. 



In the first place, let us remember that the yearly mean 

 temperature at Pallamcottah is about 7° Fahr. greater 

 than at Trevandrum, while the yearly mean pressure is 

 the same. We conclude at once from this result that the 

 mean pressure does not depend on the mean temperature. 



I stated that the mean temperature in Januaiy at Pal- 

 lamcottah, on the east side of the Ghats, was 4^ Fahr. 

 greater than at Trevandrum, sixty miles distant on the 

 west side of the mountains, and that the mean pressure 

 was 0065 greater at the former than at the latter. Sir 

 John Herschel has shown that according to our know- 

 ledge of the expansion of gases, there should be a single 

 diurnal atmospheric tide due to one side of the earth 

 being warmer than the other, and this due to the e.xpan- 

 sion of the gases in the warmer half causing the centre of 

 gravity of the air to be higher than in the other, should 

 produce a sliding motion of the warmer air towards the 

 colder in twelve hours. 



If such a result is what may be supposed to occur in 

 the period of a day, there cannot be the smallest doubt 

 that if a mass of the atmosphere has a lower pressure and 

 a lower temperature than another mass at a short distance, 

 the former should flow towards the latter, and produce 

 ecjuilibrium within twenty-four hours. Here, however, 

 we find that there is a dift'erence of pressure of 0065 inch, 

 which remains during a whole month at a distance of 

 sixty miles, and this continued difference of pressures is 

 not to be explained by any known property of gases. 



We see, indeed, the remarkable fact that the oscillation 

 of the monthly mean pressure at one station proceeds 

 quite independently of that at the other, and both in a 

 constant ratio with the variation of the temperature of the 

 atmosphere in the lowest stratum.'-^ All this is inexpli- 

 cable by any heating action alone. 



I have omitted all notice of the aqueous vapour in the 

 atmosphere. It is obvious, however, that this cannot 

 explain the annual oscillation of pressure, since it is just 

 when the tension of vapour is greatest, that the baro- 

 metric height is least. The introduction of more vapour 

 into the atmosphere does not make the whole lighter, but 

 heavier ; and when we adopt the arithmetical operation 

 of Dove and subduct the vapour pressure from the baro- 

 metric height, we find the oscillation to be greater, not 

 less, than before. 



It is not easy to determine the variation of vapour 

 pressure from the indications of the dry- and wet-bulb 

 thermometers, and the exact result of the total vapour 

 above the barometer cannot as ) et be determined, but its 

 amount will not explain the annual law, nor will it explain 

 the independent oscillations in question. I have, how- 

 ever, long ago suggested that the humidity of the air may 

 be in cjuestion, and as the oscillations of dryness or 

 humidity of the lower mass are probably related to the 

 temperature, the oscdlations of pressure may also be 

 related to them. 



Sedgwick has said, in his inaugural discourse on the 



' Mr. Broun asked me if I knew what years made up the nine, but I 

 know of no one e.xcept Mr. Blanford himself who can give a satisfactory 

 answer to this question, and he is in India. 'I he " Vade Mecum " being an 

 official work, ought certainly to be trustworthy.— E. D. A. 



= And probably of the mean temperature of a layer of some considelable 



