121 Annual Variation of Atmospheric Pressure 



rendered it possible to treat of the atmosphere as a whole. 

 I will now endeavour to shew the importance of being en- 

 abled to take such general views, selecting as an example the 

 annual variation of the barometer. 



The study of the annual barometric variation had long been 

 singularly neglected, while the diurnal barometric variation 

 had had devoted to it an attention quite disproportioned to 

 its subordinate interest in reference to the general move- 

 ments of the atmosphere. This otherwise incomprehensible 

 mistake is excused by the localities where nature had been 

 first interrogated. As the diurnal variation had manifested 

 itself with great distinctness and regularity in tropical 

 America, it naturally presented itself as an object of interest 

 in Europe also. The annual variation, on the other hand, is 

 inconsiderable, both in Europe and the tropical parts of 

 America; and thus, while atmospheric phenomena were 

 treated simply as facts of which the periodicity alone was to 

 be investigated, without seeking for physical causes, it was 

 natural that a phenomenon, in which opposite effects result- 

 ing from two different causes counterbalance each other, 

 should altogether escape notice. It is, perhaps, more re- 

 markable that no surprise should have been excited when 

 the atmospheric pressure was not found to diminish from 

 winter to summer with increasing heat. 



When, by the labours of Prinsep more particularly, the 

 phenomena of the tropical atmosphere in Hindostan became 

 more known, there was seen to be a great difference between 

 the barometric variation there and in tropical America ; in- 

 asmuch as the Indian observations shewed a decidedly well- 

 marked annual variation. A new error was now fallen in- 

 to, and it was supposed that the phenomenon did not extend 

 beyond the torrid zone, and that it was an immediate conse- 

 quence of the periodical change of wind, i. e. of the monsoons. 

 This erroneous view was completely refuted when the baro- 

 metric relations at the Siberian stations became known ; for 

 it was then found that, north of the Himalaya (which in the 

 supposed hypothesis must have formed the limit of the phe- 

 nomenon), the annual barometric variation was exhibited on 

 a large scale, and over a region so extensive that the shores 



