Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/134

Rh 124 METEOEOLOGY [BAROMETRIC OSCILLATIONS. the temperature of the sea. The smallness of this range over the North Atlantic, which is less than occurs in any other ocean in the same latitudes, is to a large extent caused by the small dip in the diurnal curve of the after noon minimum. FIG. 3. Oscillations of Barometer for June. If the map of the distribution of pressure over the globe for July be examined (fig. 17) it is seen that this part of the Atlantic is occupied by a well-defined area of high mean pressure, higher indeed than occurs at any season over any ocean ; and it is shown below that out of this area the surface winds blow in all directions. But, since air is constantly being drained out of this region by the wind without diminishing the pressure, it follows of necessity that the high pressure must be maintained by accessions of air received from above through the upper currents. Now the regions whence such accessions can come are the upper currents which have their origin in the ascending currents that rise from the heated plains of Africa, Europe, the belt of calms, and the two Americas sur rounding the North Atlantic. It is evident that the major portion of each day s overflow of air from the continents through the upper regions of the air upon the Atlantic, whether this overflow takes place by convection currents or from a tidal movement similar to what has been already described, will take place during mid afternoon. In other words, the overflow will occur about the time of the after noon minimum of the Atlantic, thus diminishing the dip of this minimum, and so producing the abnormally small range now under examination. It is in favour of this view that the abnormality follows the sun s course and is not cumula tive, and is felt also on both sides of the Atlantic, even although the weather on the east side is dry and all but rainless, and on the west moderately moist and characterized by a rather copious rainfall. It is also full of significance that the peculiarity is most strikingly seen in that part of the ocean of the globe which is closely hemmed in by large masses of land. Influence of the Moon on Atmospheric Pressure. Fifteen years hourly observations have been made at Batavia and discussed by the late Dr Bergsma in their relation to the lunar day, which was assumed in the calculations to com mence with the time of the upper transit of the moon. The result of the inquiry is that atmospheric pressure at Batavia has a lunar tide quite as distinctly marked as the ordinary diurnal barometric tide, except that its amplitude is much less. The four phases are these : 1st max. + 0022 inch at lunar hour 1 1st min. -0-0021 ,, ,, ,, 7 2il max. +0-0025 ,, ,, ,, 13 2d min. -0-0024 ,, ,, ,, 19 The lunar tide has the important difference that its phases follow the moon s apparent course much more&quot; closely than the ordinary diurnal fluctuations of the barometer follow that of the sun. The two maxima occur about the 1st and 13th, and the two minima about the 7th and 19th, whereas these four daily phases of the diurnal barometric fluctua tion occur with respect to the sun s apparent course from one to six hours later. It is interesting to note that in the higher latitudes in inland situations during winter, or at times and in situations where the disturbing influences of temperature and humidity tend towards a minimum, the times of occurrence of the four phases of the daily oscilla tion of the barometer approximate to those of the daily lunar atmospheric tide. Since a distinct lunar tide is traced to the attractive influence of the moon, it follows that the- attractive influence of the sun will enter as one of the several causes which determine the phases and amplitude of the diurnal barometric curve. It also follows from the much less attractive influence of the sun than that of the moon on the earth s atmosphere that the effects of the sun s attrac tion on the pressure will be wholly concealed by the much larger effects of the other forces concerned in determining the diurnal oscillation, except in the case or cases where the variation in the fluctuation is small at 1 and 7 A.M. and 1 and 7 P.M. Now at places north of lat. 45 N. the variation at 1 A.M. is small during the winter, and it is a singular fact that some years ago Rykatchew of St Petersburg drew the attention of meteorologists to the existence at these northern stations of a faintly marked third maximum ; and it is further of importance to remark that, at many places where on the mean of years the third maximum is scarcely or not at all marked, it appears in the mean of some of the separate years. Thus, though it does not appear in the mean of the twenty years ending 1873 at Greenwich for January, it appears in nine of the individual years. It is highly probable that this maxi mum, which may be named Rykatchew s maximum from its discoverer, is due to the attractive influence of the sun, its amplitude and time of occurrence being in accordance with such a supposition. Diurnal Variation of the Force of the Wind. During the three and a half years cruise of the &quot; Challenger,&quot; ending with May 1876, observations of the force and direction of the wind were made on 1202 days, at least AJV1 PM 187 FIG. 4. Diurnal Force of Wind at Sea and near Land. twelve times each day, 650 of the days being on the open sea and 552 near land. The observations of force were made on Beaufort s scale 0-12, being the scale of wind- force observed at sea. The mean diurnal force of the wind on the open sea and near land respectively is shown