WEATHER MAPS.] METEOROLOGY 157 Whipple has also sorted the winds according to the eight points of the compass, with results of the greatest interest. If N.W., N., N.E., and E. winds be grouped together as polar, and S.E., S., S.W., and W. winds as equatorial winds, the mean hourly velocity of the polar winds, for the same gradients, is I l miles in excess of the equatorial winds. Now, since polar winds pass into lower latitudes, the surface of the earth over which they blow is warmer, whereas the surface is colder than the equa torial winds which blow over it. It follows that the increased velocity of polar winds is referable to the same conditions which result in the diurnal increase in the wind s velocity and the greater velocity for the same gradients of winds when the annual temperature is rising, since in all these cases the winds blow over a surface of a higher temperature than their own. It is evident from these considerations that for the development of the law of the relation of the wind s velocity to the barometric gradient with an exactness sufficient to warrant us in expressing that relation in a general mathematical formula much yet remains to be done. In truth, as regards the various formulae submitted by Ferrel, Mohn, Hann, Everett, and others, we have no choice but to allow the justness of Strachan s criticism (Modern Meteorology, p. 98) that the theoretical values furnished by the formulae do not accord with the actual values, and that therefore a satisfactory formula is yet to be found. Ere such a formula need be looked for, the conditions must be fulfilled for the preliminary work of sup plying the observational data required. The "Challenger" observations prove that, with gradients substantially the same, the velocity of the wind is greater on the open sea than near land ; and we have seen that the velocity varies with the hour of the day, and generally is increased as the temperature of the surface rises above that of the air blow ing over it, and diminished as the temperature of the surface falls below that of the air. It is evident that observations on the open sea will afford data for the simplest solution of the problem ; but on land the diurnal, seasonal, and non- periodic changes of temperature greatly complicate the problem, and render necessary for its solution observations specially designed for the purpose. It is not easy to see how these can be obtained but by carrying out the plan pro posed in 1875 by Stevenson of establishing strings of well- equipped meteorological stations planted sufficiently close that the barometric gradients may be determined within the limits of accuracy required. Observations made twelve times daily for a year, at stations so arranged, would supply the observational data for the solution of this funda mental problem in meteorology. Till some such proposal be carried out, the problem remains unsolved, for barometric gradients based on the widely separated existing stations are too uncertain and rough and the wind observations are wanting in that comparability which alone can satisfy the inquiry. Weather and Weather Maps. Weather is the state of the air at any time as respects heat, moisture, wind, rain, cloud, and electricity; and a change of weather implies a change in one or more of these conditions. Of these changes the most important as regards human interests are those which refer to temperature, wind, and rain ; and, as these are intimately bound up with the distribution of atmospheric pressure, the latter truly furnishes the key to weather changes. These relations are well shown by the International Monthly "Weather Maps issued by the United States Signal Service. Of these that for December 1878 is a striking example. This month was characterized over the globe by unusually abnormal weather. A line drawn from Texas to Newfoundland, across the Atlantic, the north of France, and Germany, thence round to south-east, through the Black Sea, the Caucasus, India, the East India Islands, and Australia to the South Island of New Zealand, passes through a broad and extended region where pressure was throughout con siderably below the mean of December, and this low pressure was still further deepened in various regions along the line. An other line passing from Australia, through the Philippine Islands, Japan, Manchuria, Behring s Strait, and Alaska, also marks out an extensive region where pressure was uninterruptedly below the mean. On the other hand, pressure was above the average, and generally largely so, over the United States to west of longitude 90, over Greenland, Iceland, the Faroes, Shetland, and a large portion of the Old Continent bounded by a line drawn from Lapland round by Lake Balkhash, Canton, Peking, to the upper reaches of the Lena. Another area of high pressure extended from Syria, through Egypt and East Africa, to the Cape ; and part of a third area of high pressure appeared in the North Island of New Zealand. As regards North America, the greatest excess of pressure, 196 inch above the mean, occurred in the Columbia Valley, from which it gradually fell on proceeding eastward to a defect from the average of 146 inch near Lake Champlain and to northward, rising again to near the mean on the north of Nova Scotia. To the north and north-east exceedingly high pressures for these regions and the season prevailed, being 635 inch above the mean in Iceland, 500 in the south of Greenland, and at the three stations in West Greenland, proceeding northward, 445, 402, and - 346 inch. West Greenland being thus on the west side of the region of high pressure which occupied the northern part of the Atlantic, and on the north-east side of the area of low pressure in the States and Canada, strong south winds set in over that coast, and the temperature at the four Greenland stations, proceeding from south to north, rose to l l, 8 8, 12 l, and 14 4 above the means. As the centre of lowest pressure was in the valley of the St Lawrence about Montreal, strong northerly and westerly winds predominated to westward and southward, where consequently temperature was below the average, the deficiency at Chicago and St Louis being 9 5; and, winds being easterly and northerly in California, the tem perature there was also under the mean. On the other hand, in the New England States, the greater part of the Dominion of Canada, and West Greenland temperature was above the average. Pressure was much higher at St Michael s, Alaska, than to south-westward at St Paul s, Behring s Strait, and hence, while temperature at St Paul s was 2 9 below the normal, it was 12 above it at St Michael s, where strongly southerly winds ruled. With these strong contrasts of pressure, America presented contrasts at least as striking in the distribution of the temperature. Along the south of Lake Michigan the November temperature was 13 7 above the normal, whilst the December temperature was 9 0- 5 below it, the difference there between the two consecutive months being thus 23 2. As regards Europe, Iceland was on the east side of the patch of high pressure which overspread the north of the Atlantic, and hence northerly winds prevailed there and temperature fell 7 2 below the mean, presenting thus a marked contrast to the high temperature of West Greenland at the time. In Europe, the area of lowest pressure occupied the southern shores of the North Sea, extending thence, though in a less pronounced form, to south-eastward. Hence over the whole of western Europe winds were N. E., N., and in the south-west of l.urope W. ; and hence everywhere from the North Cape to the north of Italy temperature was below the normal, in some places greatly so, the deficiency being 10 4 in the south of Norway and 12 2 in the south of Scotland. On the other hand, on the east side of this area of low pressure winds were southerly and temperature consequently high. In some localities in Russia the excess above the mean was 15 "0, and over a large proportion of European Russia the excess was not less than 9 0. This region of high temperature extended eastward into Siberia as far as the Irtish, being coterminous with the western half of the anticyclonic region of high pressure which overspread central Siberia. But over the eastern portion of the anticyclone northerly winds prevailed, with the inevitable accompaniment of low tem peratures over the whole of Eastern Asia, the deficiency at Ner- tchinsk on the upper Amur being 6 8. Here again, just as in America, Greenland, and Iceland, places with atmospheric pressure equally high presented the strongest contrasts of temperature. Thus at Bogoslovsk, on the Ural Mountains, pressure was 211 inch and at Nertchinsk 154 inch above the normals, but Bogoslovsk on the west side of the high pressure area had a temperature 15 above, whilst at Nertchinsk it was 6 8 below the average. At this time of the year the mean pressure falls to the minimum in Australia, but during December 1878 the usually low pressure was still further diminished. Pressure at this season also falls to the annual minimum in the North Pacific and North Atlantic, and it has been seen that the low pressure of these regions was likewise still further diminished. But in the case of the Atlantic it was attended with a most important difference. The centre of lowest pressure, usually located to the south-west of Iceland, was removed some hundreds of miles to the south-east, and an unwonted
development of extraordinarily high pressure appeared to the north-