The great difficulty presented by the science of meteorology lies in the intricate combination of causes producing atmospheric variations, and the impossibility of determining by experiment the relative efficiency even of the most important agents of change. As Sir W. Herschel well observed, we are in the position of a man who hears at intervals a few fragments of a long history narrated in a prosy, unmethodical manner. 'A host of circumstances omitted or forgotten, and the want of connection between the parts, prevent the hearer from obtaining possession of the entire history. "Were he allowed to interrupt the narrator, and ask him to explain the apparent contradictions, or to clear up doubts at obscure points, he might hope to arrive at a general view. The questions that we would address to Nature, are the very experiments of which we are deprived in the science of meteorology.' l
It is, therefore, but seldom in the study of this science that we meet with phenomena to which we can assign a definite cause, or which we can explain on simple principles. Even those marked phenomena, which appear most easily referable to simple agencies, present difficulties on a close investigation which compel us at once to recognise the efficiency of more causes than one. For instance, the phenomenon of the trade-winds, as explained by Halley, appears at first sight easily intelligible; but when we look on this phenomenon as a part merely - as indeed it is - of the marvellously complex circulation of the earth's atmosphere-when we come to inquire why these winds blow so many days in one latitude, and so many in another, or why they do not blow continually in any latitude-when we consider the character of these winds as respects moisture and temperature, the variation of the velocity with which they blow, and of the quantity of air they transfer from latitude to latitude-we encounter difficulties which require for their elucidation the comparison of thousands of observations, or which baffle all attempts at elucidation.
1 Kaemtz's Meteorology.
There is, however, one atmospheric phenomenon -that which I have selected for the subject of this paper -which presents a grand simplicity, rendering the attempt at a simple solution somewhat more hopeful than is usually the case with meteorological phenomena. The discovery of this phenomenon formed one of the most interesting results of Captain Sir J. C. Boss's celebrated expedition to the Antarctic Ocean. He found, as the result of observations conducted during three years, that the mean barometric pressure varied in the following manner at the latitudes and places specified:-
Height of the barometer
Cape of Good Hope and Sydney
Kergnelen and Auckland Isles
We see here a gradual increase of barometric pressure, from the equator to about 30° south latitude, and from this point at first a gradual diminution-so that in about 40° south latitude we find the same pressure as at the equator, and thence a more rapid diminution. The rate of change is illustrated graphically in Fig. 1, which represents the height of the barometer above 28 1/2 inches for different southern latitudes. In the northern hemisphere there is a similar increase of pressure as we leave the equator, a maximum is there also attained in about latitude 30°; but from this point towards the poles there is a marked difference in the rate of diminution of pressure in the two hemispheres. The following table by Schow is sufficient to indicate this:-
There are minor irregularities in this table, due, doubtless, to local peculiarities, the arrangement of land and water being so much more complicated in the northern than in the southern hemisphere. Neglecting these (as in Fig. 2, which represents for the northern hemisphere the relations corresponding to those exhibited for the southern hemisphere in Fig. 1), we see that there is a much greater resemblance between the rise and fall of barometric pressure as we proceed northwards than as we proceed southwards. In fact, the curve is almost exactly symmetrical on either side of 30° north latitude to the equator on one side, and to latitude 60° on the other. From 60° the pressure continues to diminish for awhile, but appears to attain a minimum in about latitude 73°, and thence to increase. In the southern hemisphere, if there is any corresponding minimum, it must lie in a latitude nearer the south pole than any yet attained.
The most marked feature in the comparison of the two hemispheres is the difference of pressure over the southern and northern zones, between latitudes 45° and 75°. This is a peculiarity so remarkable, that for a long time many meteorologists considered that the observations of Captain Ross were insufficient to warrant our concluding that so important a difference really exists between the two hemispheres. But not only has Captain Maury - from a comparison of 7,000 observations - confirmed the results obtained by Ross, but, in meteorological tables published by the Board of Trade, the same conclusions are drawn from 115,000 observations, taken during a period of no less than 13,000 days. In fact, it is now shown that the difference is yet greater than it had been supposed to be from the observations of Captain Ross. From a comparison of observations made in the Antarctic Seas with those of Captain Sir Leopold McClintock, it appears that the average difference of barometric height in the northern and southern zones, between latitudes 40° and 60°, is about one inch.
Figs. 1 and 2 exhibit a relation midway between these later results and those tabulated above.
Assuming an average difference of only three-quarters of an inch in the northern and southern zones, between latitudes 40° and 60°, let us consider what is the difference of pressure on these two zones of the earth's surface. The area of either zone is 21,974,260.5 square miles, and the pressure on a square mile due to a barometric height of three-quarters of an inch is about 670,000 tons, therefore the pressure on the northern zone, between the latitudes named, exceeds the pressure on the southern zone by no less than 14,500,000,000,000 tons. Including all latitudes within which there has been ascertained to be a difference of barometric pressure in the two hemispheres, we shall probably be within the mark if we say, that the atmospherical pressure on the northern hemisphere is 20,000,000,000,000 tons greater than the atmospherical pressure on the southern hemisphere.