346 



standards, at ports which the vessels may visit, entails many incon- 

 veniences, and is in many respects a far less satisfactory method. 

 The limitation here recommended is not, however, to be understood 

 as applicable in the case of other establishments than Kew, where 

 a special provision may be made for an equally careful and correct 

 examination. 



At land stations, in addition to proper measures to assure the 

 correctness of the barometer and consequent comparability of the 

 observations, care should be taken to ascertain by the best possible 

 means (independently of the barometer itself), the height of the 

 station above the level of the sea at some stated locality. For this 

 purpose the extension of levels for the construction of railroads will 

 often afford facilities. 



It may be desirable to indicate some of the localities where the 

 data, which tables such as those which have been spoken of would 

 exhibit, are required for the solution of problems of immediate 

 interest. 



1. It is known, that, over the Atlantic Ocean, a low mean 

 annual pressure exists near the equator, and a high pressure at the 

 north and south borders of the torrid zone (23 to 30 north and 

 south latitudes) ; and it is probable that from similar causes similar 

 phenomena exist over the corresponding latitudes in the Pacific 

 Ocean : the few observations which we possess are in accord with 

 this supposition ; but the extent of space covered by the Pacific is 

 large and the observations are few ; they may be expected to be 

 greatly increased by the means now contemplated. But it is parti- 

 cularly over the Indian Ocean, both at the equator and at the 

 borders of the torrid zone, that the phenomena of the barometric 

 pressure, not only annual but also monthly, require elucidation by 

 observations. The Trade-winds, which would prevail generally 

 round the globe if it were wholly covered by a surface of water, are 

 interrupted by the large continental spaces in Asia and Australia, 

 and give place to the phenomena of monsoons, which are the indi- 

 rect results of the heating action of the sun's rays on those conti- 

 nental spaces. These are the causes of that displacement of the 

 trade-winds, and substitution of a current flowing in another direc- 

 tion, which occasion the atmospheric phenomena over the Indian 

 Ocean, and on the north and south sides of that Ocean, to be 



