BAKOMETEK NOEMALS. , 109 



The accompanying diagrams are taken from the " Charts showing the 

 Mean Barometric Pressure over the Atlantic, Indian, and Pacific Oceans," 

 pubhshed by the Meteorological Council, in 1887. On these charts the 

 observations are considered as taken with the cistern at 11 feet above sea- 

 level, reduced to 32° F. The months selected are February, May, August, 

 and November, as representing the Winter, Spring, Summer, and Autumn 

 seasons of the Northern Hemisphere. The probable pressure for the inter- 

 vening months can be easily deduced with fair accuracy. 



We may also refer here to the useful Table of Barometer Normals appear- 

 ing on the United States monthly "Pilot Chart of the North Atlantic 

 Ocean," published at the Hydrographic Office, Washington. These differ 

 shghtly from the figures shown on the above charts. 



As stated in (21), unless the exact error (if any) of the ship's Barometer 

 is known, no practical use can be made of these Charts or Tables. 



(24.) The lines drawn on the diagrams show where certain barometric 

 pressureis, indicated by the figures upon them, are observed. These lines 

 are termed isobaric lines, or Isobars, because they pass over places having 

 equal barometrical pressure. It will be seen that, speaking generally, in 

 winter the barometer is highest over the land, which is then colder than 

 the sea ; and lowest over the sea, which is then warmer than the land. 

 In summer the barometer is lowest over the great continents, which are 

 then relatively hot, and highest over the sea, which is then relatively cool. 



Over the Equator, and between the Tropics, where the temperature is 

 always comparatively high, the barometer is low relatively to the neigh- 

 bouring zones just beyond the Tropics, where the temperature is relatively 

 low, and the barometer is high.* 



(25.) The main features brought out by these Charts are that over the 

 sea the pressure is more uniform throughout the year than over the con- 

 tinents, and that, roughly speaking, the Ocean, that part of the earth's 

 surface which more immediately concerns the seaman, may be divided 

 in respect of barometric pressure into five great areas. 



First, a belt of moderately low pressure, over the Calm belt of the Equa- 

 torial region. Then, North and South of the Equatorial region, are two 

 belts of high pressure, over the Trade Wind regions. Proceeding to the 

 higher latitudes, and towards the Poles, we again find, in each Hemisphere, 

 an area of low and diminishing pressure. 



In the Equatorial region the barometer stands at, or close to, 30'0. 

 From the Tropic to about lat. 40° N. and S., the readings are above 30'0, 

 and in the central parts of the oceans readings as high as 30*2 are found 

 over large areas. ' North and South of the parallels of 40°, the pressure 

 diminishes, and in mid-winter. South of Greenland and Iceland, an area 

 of low pressure, 29-4, exists, which disappears in mid-summer, the baro- 

 meter then standing half an inch higher.* & 



Captain Toynbee also remarked in a lecture at the United Service 

 Institution, May 19th, 1871 : — " Having treated of the more permanent 

 areas of high and low pressure in the North Atlantic, here let me add, that 



* " A Barometer Manual for the Use of Seamen," published by the Meteorological 

 OoTinou. 



