102 GENEKAL OBSEKVATIONS ON THE WINDS. 



Therefore, in estimating the height of the barometer, account must be 

 taken of the amount (or weight) and elasticity (or tension) of the vapour, 

 and siibtracted from the height of the mercury, to give the true weight of 

 the dry air. With a dew-point temperature of 87-35°, the pressure of 

 moisture is equal to the weight of 1-26 inch of mercury, and must be 

 subtracted from the height shown by the barometer, as before stated. This 

 was the view held by Dalton, Ure, Eegnault, Daniell, Sir Henry James, 

 Alexander Buchan, &c. 



In opposition to this, Professor Patton, of Bombay, maintained that 

 moisture did displace an equal or equivalent volume of air, and that, there- 

 fore, it was only the difference of their amount which should be applied 

 as a correction, and he estimated the amount of vapour above stated to be 

 equal to a pressure of only 0-518 of an inch of mercury. But the first 

 theory is thought to be the most feasible.* 



(12.) Leaving the field of conjecture, we come to the actual condition of 

 the atmosphere which covers the North Atlantic Ocean in particular, and 

 the whole earth generally. Its elevation, or weight, is ascertained by the 

 barometer, as is well known. According to the decrease in the height of 

 the mercury on ascending to great elevations, it is calculated that at 15 

 miles the air is rarefied to about 25,000 times, and that at 80 or 90 miles a 

 perfect vacuum exists. It presses with a mean force of 14-73 lbs. per square 

 inch, and forms one 1,125,000th part of the mass of the whole earth. 



The Trade Winds do not reach to more than 3 miles in height, and it is 

 probable that all the phenomena of clouds and vapour occur beneath the 

 height of 4 to 5 miles. 



(13.) If the surface of the earth were evenly covered with land or water, 

 or a combination of both, the phenomena of the Trade and Anti-Trade 

 Winds would form symmetrical zones around the globe ; but the relative 

 proportions are very different in the two Hemispheres, being 100 land to 

 150 water in the Northern, and 100 land to 628 water in the Southern.f 

 There is a still greater contrast, if we take the rational horizon of London 

 as a great circle dividing the earth into two Hemispheres. It will be then 

 seen that London is in the centre of that half which includes all the land, 

 except Australia ; and the other half includes nearly all the water of the 

 globe. From this cause the line of meeting between the N.E. and S.E. 



* See " Abstracts of Meteorological Observations by the Royal Engineers, 1853-4," 

 by Sir Henry James, R.E., F.R.S. ; "The Ilaudy Book of Meteorology," by Alexander 

 Buchan, M.A., 1868, pp. 160 — 162 ; also, the later works of Professor Tyndall, &c. 



t The dry land, as far as is known, is estimated to occupy 49,806,000 square statute 

 miles. If this is increased to 51,000,000 for the unknown Polar regions, it will allow 

 146,000,000 of square miles to be covered by the Ocean. — Sir J. Herschel. 



In a Paper on "The Height of the Laud and the Depth of the Ocean," read by 

 Dr. John Murray before the Royal Society of Edinburgh, and published in the Scottish 

 Geographical Magazine, 1888, pp. 1 — 41, it is estimated that the area of the dry land i3 

 about 55,000,000 square miles, and of the Ocean 137,200,000 square miles. The mean 

 Height of the land above sea-level is 2,250 ft., and the mean Depth of the Ocean is 2,080 

 fathoms, so that the Ocean could cover the whole surface of the globe to a depth of about 

 2 miles. It is also estimated that the rivers and floods annually carry 3-7 cubic miles 

 of solid matter to the Ocean, and at this rate it would take 6,340,000 years to wash the 

 whole of the dry land into the sea. 



