Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/43

Rh temperature and greatest chyness combined. The regions of largest amplitude include the East India Islands, Eastern Peninsula, India, Arabia, tropical Africa, and tropical South and Central America, where it either closely approaches or exceeds O lOO inch. At Silchar, in Assam, it is O133 inch. In the tropical parts of the ocean the oscillation is from 0-020 to O OSO inch less than on land. The influence of the Mediterranean Sea in lessening the amount over all regions bordering it is very strongly marked. The line showing an oscillation of 050 inch crosses North America about lat. 44, curves southward at some distance from the east coast to lat. 23, then north-eastward along the c jast of Africa, passes eastwards near the north coast of that continent, thence strikes northwards, cutting the eastern part of the Black Sea, and eastward across the Caspian to a point to northward of Peking, and then bends southward to the Loo Choo Islands. The line of 0-020 inch cuts the N.W. of Spain and N.W. of France, and runs northward through Great Britain as far as the Tweed, thence to Christiania, then southwards to Copenhagen and to Cracow, the latitude of which it follows eastward through Asia. The more marked seasonal changes are these: In India the oscillations during the dry and wet seasons, or in January and July, respectively, are Bombay, 120 and 0-067 inch; Poonah, 0-133 and G 059 inch ; and Calcutta, 132 and 0-091 inch. At Madras, where the rain-bringing characters of the monsoons are reversed, the numbers are 0*114 and 0*1 15 inch, and at Roorkee, where rain falls all the year round, O OSS and 0-079. Again, at Aden, in Arabia, where the weather of July is peculiarly hot and dry, the oscillation in December is - 106, but in July it rises to 137 inch. The point to be insisted on here is, that, whatever be the cause or causes to which the daily barometric oscillation is due, the absolute amount is largely dependent on comparatively local influences. While illustrations similar to the above may be adduced from many other parts of the globe, showing the influence in the same direction of prevailing dry or wet, hot or cold seasons on the amplitude of the oscillation, the North Atlantic and regions adjoining present an apparent excep tion to the law which seems to be indicated by these results. The whole of the North Atlantic, particularly north of lat. 20, and the sea-boards which bound it, to which the Mediterranean and its immediate sea-board may be added, are strikingly characterised by a small summer oscillation ; and this diminution is most strongly marked along the eastern part of the ocean. Thus, in July, at Ponta Delgada, in the Azores, the oscillation is only 06 inch; at Angra do Heroisma, also in the Azores, O OIO inch; at Funchal, Madeira, O Oll inch; at Oporto, O OIS; Lisbon, 0-030; and Lagos, 0-021 ; at Naples and Palermo, O OOS ; and at Malta, 020 inch. Now, with reference to this extensive region, it is to be noted -that the rainfall of July is either zero or very small ; and yet with this dry state of the atmosphere and high temperature (the annual maxi mum occurring at the time), this oscillation is extraor dinarily diminished, being exactly the reverse of what takes place during the dry and wet seasons in India. The diminution on the western half of the Atlantic, though not so great, is also striking, the January and July oscilla tions being O OoG and 036 inch in Barbadoes, O OSO and 056 at Jamaica, 082 and 054 at Havanna, 0-053 and 024 in the Bahamas, and 0-054 and 022 in Bermuda. Over the whole of the region here indicated the rainfall of July is largely in excess of that of January. The apparently exceptional character of this region is pro bably due to the circumstance, that at this time of the year the sun s rays fall perpendicularly over a more diversified surface of the earth, that is, on a greater extent of land, than at any other season. At this time the Mediterranean, which is completely shut in by land, and the Atlantic, which is bounded by two great continents, show a much smaller oscilktion than prevails over the laud adjoining them, and the lines of equal oscillation now attain their annual maximum. On the other hand, in January, when the sun s rays fall perpendicularly over the most uniform surface, or over the maximum extent of ocean, the lines are almost everywhere parallel with the parallels cf latitude. Again, on advancing inland from the Atlantic, the effects of comparatively local influences are very striking, as the following mean July oscillations, from places situated in lines running in different directions, show : Dublin, 0-012; Oxford, 022 ; Osteud, 0-009; Brussels, 0-019; Vienna, 049 ; Odessa, 024; and Tiflis, 077 : Limerick, O OIO; Helston, 007 ; Paris, 020; Geneva, 0-045 ; Turin, 052 ; Rome, 036 ; Palermo, O OOS ; and Malta, 020. But the most remarkable illustration is the following, the places being all situated between 38 and 42 N. lat, : San Francisco, 068; Fort Churchhill, 0-091 ; Washington, 063; Angra do Heroisma, 006; Lisbon, 0-030; Campo Maior, 054; Palermo, O OOS; Tiflis, 0-077; and Peking, 060. It follows from what has been stated that much which has been written regarding these fluctuations, and in ex planation of them, does not rest on facts; and nearly everything yet requires to be done in the way of collect ing data towards the representation and explanation of the daily oscillations of atmospheric pressure which are, as regards two-thirds of the globe, perhaps, as already stated, the most regular of recurring phenomena, and an explanation of which cannot but throw much light on many of the more important and difficult problems of the atmosphere. The data chiefly required are barometric data from which the amplitude of the four daily oscillations can be represented in their distribution and tunes of occurrence for each of the months ; temperature data, com parable inter se, from which the diurnal march of tempera ture for each month can be ascertained ; hygrometric data for hourly values ; rain data also for the hours ; wind observations conducted on a satisfactory and uniform plan ; together with magnetic and electrical observations. It is singularly unfortunate that the disposition of meteorologists of recent years has been to recommend as hours of observa tions for places which observe only twice or thrice daily, hours which do not correspond with the tunes when the great barometric and thermometric daily phases occur ; hence these phases cannot be noted except at the great observatories, which are too few and far apart to give sufficient data for the proper discussion of many of those questions. Since the two maxima of daily pressure occur when the temperature is about the mean of the day, and the two minima when it is at its highest and lowest respectively, there is thus suggested a connection between the daily barometric oscillations and the daily march of temperature; and similarly a connection with the daily march of the amount of vapour and humidity of the air. The view enter tained by many of the causes of the daily oscillations may be thus stated : The forenoon maximum is conceived to be due to the rapidly increasing temperature, and the rapid evaporation owing to the great dryness of the air at this time of the day, and to the increased elasticity of the lowermost stratum of air which results therefrom, until a steady ascending current has set in. As the day advances, the vapour becomes more equally diffused upwards through the air, an ascending current, more or less strong and steady, is set in motion, a diminution of elasticity follows, and the pressure falls to the afternoon minimum. From this point the temperature declines, a system of descending currents set in, and the air of the lowermost stratum 