146 



NATURE 



[Dec. 24, 1874 



interest by meteorologists, as not only model mnno- 

 grams of the subject discussed by them, but as further 

 developing and occasionally opening up certain lines of 

 inquiry which lead to practical applications of the science. 

 In these respects the Report for 1873 is the best, as well 

 as the most suggestive. Its outstanding feature is the 

 discussion of the deficient rainfall of the Presidency during 

 1873, so disastrous by the famine which followed it ; and 

 the developing in the course of the discussion of a principle 

 which, if confirmed by future observations, " will enable 

 us to some extent to forecast our [Indian] seasons, or at 

 least to speak with some confidence to their probable 

 character for some months in advance." 



From the increased number of stations now in connec- 

 tion with the department, and from the additional data 

 obtained from the meteorological superintendents of the 

 Governments of Ceylon, the Upper Provinces, Central 

 India, and Berar, it is possible to form a conception 

 of the geographical distribution of pressure, temperature, 

 rain, &c., over one-half of India and its seas. The sum- 

 maries of all the observations made over the region 

 during the past seven years form an admirable feature of 

 the Report. We very cordially join in the hope expressed 

 that the observations which have been made in the Presi- 

 dencies of Bombay and Madras will in future be acces- 

 sible, and that those made in the Punjab will be put on 

 such a footing as to be trustworthy and comparable. As 

 regards the last-named region, in all the annual reports 

 we have seen (down to i S70) the barometric observations 

 are given uncorrected for temperature and unaccom- 

 panied with the readings of the attached thermometer ! 

 When, on making the annual survey of the meteorology 

 of India, the north-west, west, and south of the country 

 can be included, it will be possible to write the history of 

 the two monsoons of the year, and probably to point out 

 the determining causes of their irregularities. 



" The principal meteorological characteristics of the 

 year 1S73 were an excessive temperature, in Oude and 

 the North-western Provinces more especially ; an un- 

 usually low pressure of the atmosphere in the same 

 region, and probably also in the south-east corner of the 

 Bay of Bengal, while in Eastern Bengal pressure was 

 persistently high ; great unsteadiness in the winds, indi- 

 cating the predominance of local causes in affecting the 

 air currents, while the normal monsoon current from the 

 south-west set in nearly a month later than usual, and 

 ceased nearly a month earlier ; lastly, a general defi- 

 ciency of moisture in the atmosphere, as is betokened 

 both by the hygrometric observations, the comparative 

 absence of cloud, and the great deficiency of rainfall." 



The usual characteristics of the Indian summer mon- 

 soon, based on the past seven years' observations, are 

 thus stated : — 



" In ordinary years the winds of the south-west mon- 

 soon blow, on the one hand from the Arabian Sea, on the 

 other hand from the Bay of Bengal to7i>:rrds a line lyiii,'^ 

 to tlie south of the Ganges, at no great distance, and 

 parallel to that river. A barometric depression begins to 

 appear in or near this region in April, and by the time the 

 rains set in in June it is well established ; the pressure 

 decreasing along it from east to west where this trough, 

 as it may be termed, merges in the great barometric 

 depression of the Punjab and the Bikaneer Desert. To the 

 south of this line the winds from the Arabian Sea blow 

 across the Central Provinces, chiefly from the west. To 

 the north of it, those from the Bay of Bengal, turning 



with the Gangetic Valley, blow in an opposite, or easterly, 

 direction, their line of meeting being along this trough." 



Bengal being thus dependent, as regards its rainfall, on 

 the aerial current which blows from the Bay of Bengal up 

 the valley of the Ganges, it is evident that whatever 

 weakens this current or directs it to the northward will have 

 a serious influence on the rainfall. Now, in 1873 the trough 

 described above did not occupy the usual position to 

 the south of the Ganges, but a position considerably to the 

 north-west, in Oude and Rohilcund, immediately under 

 the hills. A change in the direction of the wind neces- 

 sarily followed this change in the position of the area of 

 lowest atmospheric pressure ; and in strict accordance 

 with the now well-known relation of wind to pressure, there 

 was an unusual prevalence of westerly winds over the 

 greater part of Bengal during June and July, and the rain- 

 fall consequently was deficient. 



The observations made in the Andaman and Nicobar 

 Islands show the existence of a barometric depression 

 over the south-eastern portion of the Bay of Bengal, the 

 effect of which would be to deflect a large portion of the 

 monsoon current of the Bay of Bengal towards Sumatra 

 and the Tenasserim and Burmah coasts. Thus, then, the 

 monsoon current, on which Bengal is dependent for its 

 rainfall, was not only deflected northward from its usual 

 track during 1S73, but was also weakened in force by 

 being partially drained away to the south-east in the 

 direction of Burmah. 



In the examination of the rainy seasons of 1S6S, 1869, 

 and 1873, Mr. Blanford has the merit of first drawing 

 attention to the existence of local and persistent variations 

 of pressure, which appear as a local exaggeration or 

 partial suppression of the great annual variation — the 

 pressure remaining for many months, sometimes through 

 two or more consecutive seasons, either higher or lower 

 than the average, relatively to other parts of the country, 

 over a more or less extensive track. It, is to these per- 

 sistent irregularities in the distribution of atmospheric 

 pressure that the irregularities in the distribution of the 

 rainfall must be ascribed, and it is to the further in- 

 vestigation, by future observations, of the characteristic 

 feature of persistency in this class of barometric variations 

 that we look with hope to the realisation of a great 

 triumph awaiting meteorology, viz., the prediction, for 

 some months in advance, of the general character of the 

 coming seasons of India, and thereafter a gradual exten- 

 sion of the principle to other countries. 



As regards the humidity, the only data of observation 

 published in the Report are the dry-bulb observations. 

 To these are added the computed values for the elastic 

 force of vapour and the relative humidity. In future 

 issues of the Reports we should recommend that the wet- 

 bulb observations be also published. In a country of 

 such extreme climates as India, it is eminently desir- 

 able to have the whole observed facts relative to the 

 humidity before us, particularly since, from the present 

 defective state of our hygrometric tables as regards dry 

 hot climates, computed values can be regarded only 

 as rough approximations. In estimating the state of 

 the sky, a clear sky is entered as 10, and a sky com- 

 pletely covered with cloud as o. It might be well in 

 future to adopt the recommendation of the Vienna Mete- 

 orological Congress on this head, by which a clear sky is 



