370 Royal Society : — 



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

 greatly increased by the means nor*' contemjilated. 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 

 different from those in corresponding localities over, and on either 

 side of the equator in the Atlantic Ocean, and (probably generally 

 also) in the Pacific Ocean. 



It is important aUke to navigation and to general science to know 

 the limits where the phenomena of the trade-winds give jilace to 

 those of the monsoons ; and whether any and what variations take 

 place in those limits in different parts of the year. The barometric 

 variations are intimately connected with the causes of these variations, 

 and require to be known for their more perfect elucidation. 



The importance, indeed, of a full and complete knowledge of the 

 variations which take place in the limits of the trade-winds gene- 

 rally in botli hemispheres, at different seasons of the year, has 

 long been recognized. On this account, although the present 

 section is headed " Barometer," it may be well to i-emark here, that 

 it is desiraljle that the forms supplied to ships should contain 

 headings, calling forth a special record of the latitude and longitude 

 M'here the trade-wind is first met with, and where it is first found 

 to fail. 



2°. The great extent of continental space in Northern Asia 

 causes, by reason of the great heat of the summer and the ascending 

 current produced thereby, a remarkable diminution of atmospheric 

 pressure in the summer months, extending in the north to the Polar 

 Sea, and on the European side as far as Moscow. Towards the 

 east it is known to include the coasts of China and Japan, but the 

 extent of this great diminution of summer pressure beyond the 

 coasts thus named is not known. A determination of the monthly 

 variation of the pressure over the adjacent parts of the Pacific Ocean 

 is therefore a desideratum ; and for the same object it is desirable 

 to have a more accurate knowdedge than we now possess of the pre- 

 vailing direction of the wind in different seasons in the vicinity of 

 the coasts of China and Japan. 



30. With reference to regions or districts of increased or dimi- 

 nished mean annual pressure, it is known tliat in certain districts in 

 the temperate and polar zones, such as in the vicinity of Cape Horn 

 extending into the antarctic polar Ocean, and in the vicinity of Ice- 

 land, the mean annual barometric pressure is considerably less than 



