Report on Marine Meteorological Observations. 371 



the average pressure on the surface of the globe generally ; and that 

 anomalous differences, also of considerable amount, exist in the 

 mean annual pressure in different parts of the arctic ocean. These 

 all require special attention, with a view to obtain a more perfect 

 knowledge of the facts, in regard to their amount, geographical 

 extension, and variation with the change of seasons, as well as to 

 the elucidation of their causes. 



Dry Air and Aqueous Vapour. 



The apparently anomalous variations which have been noticed to 

 exist in the mean annual barometric pressure, and in its distribution 

 in the different seasons and months of the year, are also found to exist 

 in each of the two constituent pressures which conjointly constitute 

 the barometric pressure. In order to study the problems connectec^ 

 with these departures from a state of equilibrium under tbeir most 

 simple forms, — and generally for the true understanding of almost all 

 the great laws of atmospheric change, — it is necessar)' to have a 

 separate knowledge of the two constituents (viz. the pressures of 

 the dry air and of the aqueous vapour) which we are accustomed to 

 measure together by the barometer. This separate knowledge is 

 obtained by means of the hygrometer, which determines the elas- 

 ticity of the vapour, and leads to the determination of that of 

 the dry air, by enabling us to deduct the elasticity of the vapour 

 from that of the whole barometric pressure. It is therefore 

 extremely desirable that tables, similar to those recommended under 

 the preceding head of the barometer, should be formed at every 

 land station, and over the ocean at the centres of geographical spaces 

 bounded by certain values of latitude and longitude, for the annual, 

 monthly, and season pressures, — 1 . Of the aqueous va])our ; and 

 2. Of the dry air; each considered separately. Each of the said 

 geographical spaces will require its appropriate ledger for each of 

 the twelve months. 



It may be desirable to notice one or two of the problems con- 

 nected with extensive and important atmospherical laws which may 

 be materially assisted by such tables. 



1^. By the operation of causes which are too well known to re- 

 quire explanation here, the dry air should alwaj's have a minimum 

 pressure in the hottest months of the year. But Ave know that 

 there are places where the contrary prevails, namely, that the pres- 

 sure of the dry air is greater in summer than in winter. We also 

 know that when comparison is made between places in the same 

 latitude, and haviij-- the same, or very nearly the same, differences of 

 temperature in summer and in winter, the differences between the 

 summer and winter pressures of the dry air are found to be subject 

 to many remarkable anomalies. The variations in the pressure of 

 the dry air do not therefore, as might be at first imagined, depend 

 altogether on the differences between the summer and winter tem- 

 peratures at the places where the variations themselves occur. The 

 increased pressure in the hottest months appears rather to point to 

 the existence of an overflow of air in the higher regions of the 



