144 The Realm of Nature CHAP. 



world. From January onward the atmospheric pressure 

 increases rapidly in the north and much more slowly in the 

 south, so that in the month of April the gradient, though 

 still for westerly winds, is very slight. A small temporary 

 rise of pressure in the north may thus reverse the gradient, 

 and as soon as the pressure in the north becomes higher 

 than that in the south, east wind sets in. A similar state 

 of matters occurs again in November, on account of the 

 pressure in the south falling more rapidly than that in the 

 north, and the months of April and November are famed 

 for bitter east winds in all parts of Britain. 2 



203. Temperature of the British Islands. The 

 temperature of the British Islands on the average for the 

 year is about 48, increasing from 45 in Shetland to 53 in 

 Scilly, or an average rise of temperature of i for every i oo 

 miles toward the south. In winter the temperature has no 

 relation to the latitude, the islands grow colder from 

 west to east. The isotherms of January (Plate IX.) run 

 from north-west to south-east. A broad strip of country from 

 Caithness to Lincoln has an air temperature of 38 or less. 

 Shetland, Orkney, Ayr, Liverpool, Oxford, and London 

 are traversed by the isotherm of 39. The points of Kerry, 

 Cork, and Scilly are at 45. The south of England is mild 

 in winter, not because it is the south, but because it runs 

 so far to the west. By the month of April the isotherms 

 run nearly east and west ; the temperature is 42 in 

 Shetland, 45 from Skye to Aberdeen, and 48 from 

 Erris Head through Dublin and Liverpool to Harwich. 

 In this month land and sea have practically the same 

 temperature. In July the land has heated up more than 

 the sea, so that the south-west wind now has a cooling 

 effect, and the isotherms (Plate X.) run roughly from S.W. 

 to N.E. Shetland is at 54, the line of 58 runs from 

 Malin Head, near Rothesay and Inverness, to Peterhead, and 

 that of 60 from Killarney across Ireland, through Lough 

 Neagh southward, north through Whitehaven to Selkirk, 

 and then south to Newcastle. The hottest region is round 

 London, where the temperature averages 64. As autumn 

 advances the air cools down most rapidly on the east coast, 



