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gating the causes thereof, it is desirable to obtain, by means of 

 barometric observations strictly comparable with each other, and 

 extending over all parts of the globe accessible by land or sea, 

 tables, showing the mean barometric pressure in the year, in each 

 month of the year, and in the four meteorological seasons, on land, 

 at all stations of observation, and at sea, corresponding to the 

 middle points of spaces bounded by geographical latitudes and longi- 

 tudes, not far distant from each other. 



The manner of forming such tables from the marine observations 

 which are now proposed to be made, by collecting together observa- 

 tions of the same month in separate ledgers, each of which should 

 correspond to a geographical space comprised between specified 

 meridians and parallels, and to a particular month, is too obvious 

 to require to be further dwelt upon. The distances apart of the 

 meridians and parallels will require to be varied in different parts 

 of the globe, so that the magnitudes of the spaces which they 

 enclose, and for each of which a table will be formed, may be 

 more circumscribed, when the rapidity of the variation of the par- 

 ticular phenomenon to be elucidated is greatest in regard to 

 geographical space. Their magnitude will also necessarily vary 

 with the number of observations which it may be possible to collect 

 in each space, inasmuch as it is well known that there are extensive 

 portions of the ocean which are scarcely ever traversed by ships, 

 whilst other portions may be viewed as the highways of a constant 

 traffic. 



The strict comparability of observations made in different ships 

 may perhaps be best assured, by limiting the examination of the 

 instruments to comparisons which it is proposed to make at the Kew 

 Observatory, before and after their employment in particular ships. 

 From the nature of their construction, the barometers with which 

 Her Majesty's navy and the mercantile marine are to be supplied 

 are not very liable to derangement, except from such accidents as 

 would destroy them altogether. Under present arrangements they 

 will all be carefully compared at Kew before they are sent to the 

 Admiralty or to the Board of Trade ; and similar arrangements may 

 easily be made by which they may be returned to Kew for re-examina- 

 tion at the expiration of each tour of service. The comparison of 

 barometers when embarked and in use, with standards, or supposed 



