JSIov. 7, 1878] 



NATURE 



induced me to make a more careful study of the Indian 

 observations at present before me. 



Mr. C. Chambers had noticed previously the appearance 

 of " a periodicity of very small range, which nearly corre- 

 sponds in duration with the decennial sun-spot period 

 (" The Meteorology of the Bombay Presidency." London, 

 1878, p. 12). It is obvious, whatever may be the results 

 derived for a single station, that they can have little value 

 if contradicted by those obtained elsewhere. I have 

 shown that from Singapore to Simla the daily mean pres- 

 sure varies in nearly the same way, the maxima as well 

 as the minima occurring simultaneously or nearly so over 

 all India {Proc. Roy. Soc, 1876, p. 24). Are then ^& years 

 of maximum and minimum pressure also the same over 

 India ? The question has considerable interest inde- 

 pendently of the existence of a decennial law. The fol- 

 lowing table contains the differences (A B) of yearly mean 

 barometric height in thousandths of an inch from the 

 means derived from the whole series of observations at 

 each station. I possess at present imfortunately only an 

 incomplete series for Trevandrum and the variations for 

 1841-48, and for 1853-64 are those from the means for 

 each of these series of years, the barometers having been 

 different. I have also added the differences of yearly 

 mean temperature in tenths of a degree Fahr. from the 

 means for each series of years during which the thermo- 

 meter preserved the same position. The different series 

 are separated by bars . 



^ There is an error of — 20 in the mean pressure given in the Madras 

 observations for 1846 (here corrected). 



" The variations for lk>mbay are taken from Mr. C. Chambers's " Meteo- 

 rology cf the Presidency of Bombay," pp. 11 and 32. 



An examination of the quantities AB will show that all 

 the principal deviations from the mean pressures occur 

 in the same years and have the same sign at all the 

 stations. The agreement of the variations for Trevan- 

 drum and Bombay in the six years 1859-64 is also very 

 remarkable. 



We see that there was a maximum of pressure in 1845 

 at the first three stations, a minimum at Madras and 

 Bombay in 1849 (best marked at the former station), a 

 maximum at the last three stations in 1855 ; a mini- 

 mum in 1862, and a maximum in 1864 at Trevandrum 

 and Bombay (the latter station showing a maximum 

 also in 1868). These means, then, confirm in a general 

 way for all India the result obtained by Mr. C. Chambers 

 for Bombay,' that on the whole the mean atmospheric 

 pressure is greatest near the epochs of minimum, and 

 least near the epochs of maximum sim-spots, though 

 there is no exact agreement between the two classes of 

 variations from year to year. 



And as the maximum pressure in 1855, shown at all 

 the three stations, is considerably before the epoch of 

 minimum sun-spots, it would be difficult to conclude 

 that the epochs of one phenomenon lag on those of the 

 other. 



I consider this result to be one of very great import- 

 ance. The exactness with which the mean barometric 

 height has been obtained with standard instruments in 

 first-class observatories, and the general agreement of 

 the years of maxima and minima over India, give a 

 weight to it which cannot be extended to an element 

 so variable with locality as the rainfall. Although 

 the barometric variations are not sufficient to ex- 

 plain those of the amoimt of rainfall (which is so dif- 

 ferent in different years, and for the same years at the 

 different stations), yet they give a probability to the 

 existence of similar laws in the variations of the me- 

 teorological elements which I believe was previously 

 wanting. 



Mr. F. Chambers's note was written chiefly with the 

 object of deducing from the barometric means evidence 

 that the emission of solar heat is greatest in years cf most 

 sun-spots. He says : " It is well known that in Central 

 Asia the annual variation of the barometric pressure is 

 greater than in any other portion of the globe, and it is 

 imiversally admitted that this variation is due to the great 

 variation of temperature between summer and winter, 

 the barometer being low when the temperature is high, 

 and vice versa. If, therefore, the absolute heat of the 

 sun is subject to considerable variations, we ought to 

 find the barometric pressure in Central Asia respond- 

 ing to those variations just as it does to the annual 

 variations of temperature ; in other words, the sum- 

 mer barometric minimum should be lowest in those 

 years when the sun is hottest, and the winter maxi- 

 mum should be highest in those years when the sxm is 

 coldest" 



As I do not admit that the annual barometric oscillation 

 is due to variation of temperature, I do not accept the 

 conclusion : for though there is an apparent relation be- 

 tween the two, within certain geographical limits, and 

 imder certain local conditions (which are altogether inde- 

 pendent of the heat emitted by the sun), there is no evi- 

 dence that the one is due to the other. The true con- 

 clusion is that the local conditions which are favourable 

 to a large oscillation of the monthly mean atmospheric 

 pressure are favourable to a large oscillation of the 

 monthly mean temperature : always within certain geo- 

 graphical limits, for beyond these the relation does 

 not hold. 



The subject is, I think, a very interesting one, and 

 merits consideration. The relation of the oscillations of 

 the monthly means will be seen from the ranges in the 

 following table : — 



' They also add another epoch, 1P45. 



