24(3 I'llILOSOPHICAL TRANSACTIONS. [aNNO 1777, 



climates, or zones of the earth, where the observations were made. On this 

 supposition it was natural for the mind to form to itself some general hypothesis, 

 which might serve to account for the appearances; and the first that presented 

 itself was the following: that the atmosphere surrounding our globe might 

 possibly be composed of particles, whose specific gravities were really different; 

 that the lightest were placed at the equator; and that the density of the others 

 gradually increased from thence towards the poles, where the heaviest of all had 

 their position. 



It is a well known and established (act, that in the middle latitudes, a north 

 or north-easl wind constantly raises the barometer, and generally higher as its 

 continuance is longer. The contrary happens when a south or south-west wind 

 blows; for it is commonly lowest when the duration and strength of the wind 

 from that quarter have been the greatest. Thus the north-east wind, by blow- 

 ing for any length of time, brings into the middle latitudes a mass of air 

 heavier than that which naturally appertains to the region, and raises the baro- 

 meter above its mean height. The continuance of a south-wester carries off 

 the heavy air, deposits a much lighter body in its stead, and never fails to sink 

 the barometer below its mean height: hence, in the middle parts of Europe, 

 there is a difference of about 24- inches between tiie highest and lowest states of 

 the barometer. But supposing it to be only 2 inches, the difference of pressure 

 still amounts to -J^ part of the whole weight of the atmosphere. Now it is 

 evident from the Peruvian observation, that the greatest fluctuation of the 

 barometer, which is at the level of the sea, does not exceed 0.226 of an incli, 

 or -j4-3 part of the whole pressure; and if the change should be no greater at 

 the poles, which is not improbable, it follows, that the measurement of heights bv 

 means of the barometer, in middle latitudes, will be more precarious and uncer- 

 tain than in the torrid and frigid zones. 



Such in general were the first ideas which the comjjarison of the operations of 

 the barometer with the effects of the north-east and south-west wind* on that 



* I have been well infoinied, that in China the north-west wind raises the barometer most, and is 

 highly electrical ; it is at die same time the driest and the coldest ; and at Canton, under tlie^northern 

 tropic, tliere is frequently ice. On the east coasts of Nortli America the severity of the north-west 

 wind is universally remarked ; and there can scarcely be a doubt, that the inhabitants of California, 

 and other parts on the west-side of that great continent, will, like those on the west of Europe, feel 

 the strong etfects of a north-east wind. The extraordinary dryness and density of the wind from the 

 north pole, seems therefore to be occasioned by its passing over the continent of Europe and Asia on 

 one side, and that of North America on the other. Those who live on the east and west coasts of 

 South America, will find the driest and coldest winds come to them respectively from the south-west 

 and south-east. As the winds seem to be colder, drier, and denser, in proportion to the extent of 

 land they pass over from the poles towards tlie equator, so they appear to be more moist, warm, and 

 light, in proportion to the extent of ocean they jjass over from the equator towards the poles. Henc« 

 (lie humidity, warmth, and lightness, of the Atlantic wind to the inhabitants of Europe. 



