BOOK II. OF THE ATMOSPHERE. -267 



1288. The study of the weather from precedent affords useful hints as to the character of 

 approaching seasons. From observing the general character of seasons for a long period, 

 certain general results may be deduced. On this principle, Kirwan, on comparing 

 a number of observations taken in England from 1677 (Trans. Ir. Acad. v. 20.) to 

 1789, a period of 1 12 years, found : 



That when there has been no storm before or after the vernal equinox, the ensuing summer is generally 

 dry, at least five times in six. 



That when a storm happens from an easterly point, either on the 19th, 20th, or 21st of May, the sue. 

 ceeding summer is generally dry, at least four times in five. 



That when a storm arises on the 25th, 26th, or 21th of March, and not before in any point, the succeed- 

 ing summer is generally dry, four times in five. 



If there be a storm at S. W. or W. S. W. on the 19th, 20th, 21st, or 22d of March, the succeeding- sum- 

 mer is generally wet, five times in six. 



In this country winters and springs, if dry, are most commonly cold; if moist, warm : on the contrary, 

 dry summers and autumns are usually hot, and moist summers cold ; so that, if we know the moistness 

 or dryness of a season, we can form a tolerably accurate judgment of its temperature. In this country 

 also, it generally rains less in March than in November, in the proportion at a medium of 7 to 12. It 

 generally rains less in April than October, in the proportion of 1 to 2, nearly at a medium. It generally 

 rains less in May than September ; the chances that it does so, are, at least, 4 to 3 ; but, when it rains 

 plentifully in May, as T8 inches or more, it generally rains tut little in September ; and when it rains one 

 inch, or less, in May, it rains plentifully in September. 



1289. The probabilities of particular seasons being followed by others, has been calculated 

 by Kirwan, and although his rules chiefly relate to the climate of Ireland, yet as there 

 exists but little difference between that island and Great Britain, in the general appear- 

 ance of the seasons, we shall mention some of his conclusions. 



In forty-one years there were six wet springs, 22 dry, and 13 variable ; 20 wet summers, 16! dry, and 

 5 variable ; 11 wet autumns, 11 dry, and 19 variable. 



A season is accounted wet, when it contains two wet months. In general, the quantity of rain, which 

 falls in dry seasons, is less than five inches, in wet seasons more ; variable seasons are those, in which 

 there falls between SOlbs. and 361bs., a Ib. being equal to '157039 of an inch. 



January is the coldest month in every latitude ; and July is the warmest month in all latitudes 



above 48 degrees : in lower latitudes, August is generally the wannest. The difference between the 



hottest and coldest months increases in proportion to the distance from the equator. Every habitable 



latitude enjoys a mean heat of 60 degrees for at least two months j which heat is necessary for the pro- 



' duction of corn. 



SECT. III. Of the Climate of Britain. 



1290. The climate of the British isles, relatively to others in the same latitude, is tem- 

 perate, humid, and variable. The moderation of its temperature and its humidity are 

 owing to our being surrounded by water, which being less affected by the sun than the 

 earth, imbibes less heat in summer, and from its fluidity is less easily cooled in winter. 

 As the sea on our coasts never freezes, its temperature must always be above 33 or 

 34 ; and hence, when air from the polar regions at a much lower temperature passes 

 over it, that air must be in some degree heated by the radiation of the water. On the 

 other hand, in summer, the warm currents of air from the south, necessarily give out 

 part of their heat in passing over a surface so much lower in temperature. The vari- 

 able nature of our climate is chiefly owing to the unequal breadths of watery surface 

 which surround us ; on one side, a channel of a few leagues in breadth ; on the other, the 

 Atlantic ocean. 



1291. The British climate varies materially within itself: some districts are dry, as the 

 east ; others moist, as the west coast ; in the northern extremity, dry, cold, and windy ; 

 in the south, warm and moist. Even in moist districts some spots are excessively dry, 

 as part of Wigtonshire, from the influence of the Isle of Man, in warding off the watery 

 clouds of the Atlantic ; and, in dry districts, some spots are moist, from the influence of 

 high mountains in attracting and condensing clouds charged with watery vapor. 



1292. The deterioration of the British climate is an idea entertained by some ; but whether 

 in regard to general regularity, temperature, moisture, or wind, the alleged changes are 

 unsupported by satisfactory proofs. It is not improbable but the humidity of our climate, 

 as Williams alleges (Climate of Britain, &c. 1816), has of late years been increased by 

 the increase of evaporating surface, produced by the multiplicity of hedges and plant- 

 ations ; a surface covered with leaves being found to evaporate considerably more than a 

 naked surface. If the humidity of the climate was greater before the drainage of mo- 

 rasses and the eradication of forests for agricultural purposes, a comparative return to 

 the same state by artificial planting and irrigation, must have a tendency to produce 

 the same results. However, it will be long before the irrigation of lands is carried to 

 such a degree as to produce the insalubrious effects of untrained morasses ; and as to 

 our woods and hedges, we must console ourselves with the beauty and the shelter which 

 they produce, for the increase of vapor supposed to proceed from them. 



