110 MOTIONS AND PEESSUEE OF THE ATMOSPHEEE. 



similar areas of high and low pressure are found to exist in the other great 

 oceans. If these areas were quite fixed, and the only disturbances to which 

 the air of the North Atlantic was subjected, the direction and force of the 

 wind might be expected to be equally fixed. But, besides the ai?ove, local 

 and temporary disturbances are very common." 



(26.) The diagrams show the mean barometrical pressure, but this is 

 liable to variation from several causes, periodical and non-periodical. 



The Diurnal Variation (mainly due to temperature) is one of the most 

 regular of recurring phenomena in the Tropical and adjacent seas, gradually 

 diminishing in higher latitudes, and is hardly perceptible within the Arctic 

 and Antarctic Circles. This diurnal variation of pressure consists of a 

 double oscillation, there being two periods of increase and two of decx-ease ; 

 the barometer rising from about 4 a.m. to about 10 a.m., then falling to 

 about 4 p.m., and again rising till about 10 p.m., when it once more falls 

 to 4 a.m. In Tropical seas the daily range between the highest and 

 lowest may be taken at about -07 or -08. In the British Isles the changes 

 of pressure due to this cause are hardly more than one-fourth of those 

 observed in the Tropics, amounting on the average to about -02. 



The Annual Variation of pressure is also a well-marked phenomenon 

 within the Tropics, and in these seas it amounts to about -10 ; on approach- 

 ing the land it becomes mueh greater. As, however, the annual variation 

 takes place very gradually, it calls for no further comment here. 



The non-periodical changes of pressure are those immediately associated 

 with changes of weather. The extent of these changes, under ordinary 

 conditions, and taking the average of the various seasons of the year, varies 

 with the latitude, being smallest near the Equator and increasing as we 

 recede from it. "Within the Tropics, the ordinary fluctuations of the 

 barometer, including the diurnal variation, seldom exceed -3 or -4, except 

 in the event of one of those furious and dreaded revolving storms commonly 

 known as Hurricanes, Cyclones, or Typhoons (according to the part of the 

 globe in which they occur), when the barometer may fall much more, and 

 in the dangerous part of the storm-field may fall to the extent of 2 inches 

 or more. The magnitude of the range in the higher latitudes, as compared 

 to the Tropics, is exemphfied in the British Islands, where the average 

 range in the course of a month is about 1-7 for January, and 0-9 for July. 



(27.) Range. — The following Table is given in the "Barometer Manual" 



as the approximate range of the Barometer, under ordinary conditions of 

 weather. In severe storms the range is naturally much greater ; as an 



