166 



METEOROLOGY. 



Their bene- 

 ficial ef- 

 fects. 



Wetcorolo- rents are themselves produced, chiefly by the unequal 

 v ? distribution of heat, which disturbs the equilibrium of 

 ""* ~ the atmosphere. When a column of air is heated above 

 the temperature of an adjacent column, it has a ten- 

 dency to rise in consequence of its diminished specific 

 gravity, while the colder air flows in to supply its place. 

 Upon this principle, it might be expected, what is 

 found actually to take place, that the colder air, from 

 the Poles, is continually displacing the warmer and 

 lighter air of the equatorial regions, thus producing a 

 .constant current near the surface of the earth from the 

 Causes of pole towards the equator. To supply the place of the 

 these cur- air thus carried away from the poles, an opposite cur- 

 rents. ren t nmst flow in the higher regions of the atmosphere, 

 from the equator towards the pole, where the air has 

 its temperature reduced, at the same time that the air, 

 by which it was displaced, has its temperature elevated, 

 so that the circulation is constantly kept up. This ten- 

 dency to an interchange of air between the warmer and 

 colder regions of the globe, combined with the diurnal 

 revolution of the earth, produces that constant current 

 in the neighbourhood of the equator, which is called 

 the trade wind, and which will be considered at greater 

 length under the article PHYSICAL GEOGRAPHY. 



The wisdom and beneficence which this constitution 

 of things displays, appear in the provision which is 

 hereby made for moderating the extremes of heat and 

 cold in different climates, as well as producing the rain 

 that is necessary for the purposes of vegetation. Were 

 the surface of the globe uniformly smooth, and every 

 where equally susceptible of the impressions of heat 

 and cold, the circulation in the atmosphere would pro- 

 bably suffer no interruption, nor be liable to any irregu- 

 larities. The variations of the barometer would, in that 

 case, be very small, as they are found to be in the equa- 

 torial regions, where the circulation of the atmospheri- 

 cal currents is most steady ; and rain would, in all pro- 

 bability, fall only at certain periods, and after long 

 intervals. As the globe, however, is actually consti- 

 tuted, the inequalities of its surface, the different capa- 

 cities which different parts of that surface have for ab- 

 sorbing or communicating heat, and other disturbing 

 causes, produce frequent and considerable irregularities 

 in the direction and force of the great atmospherical 

 current, and thus give rise to the variety of winds 

 which is found to take place above the 30 parallel of 

 latitude. As the upper current, or that which flows 

 from the equator, is all directed nearly to one point, 

 viz. the pole, its velocity must be increased as it ap- 

 proaches that point, and consequently the effects of the 

 disturbing causes must be greater in the higher lati- 

 tudes than nearer the equator; an inference which is 

 fully confirmed by observation. The range of the ba- 

 rometer, which may be considered as expressing the 

 amount of the disturbing force, is found to increase 

 with .the distance from the equator; but that the quan- 

 tities of air transported by the two currents are, upon 

 the whole, equal, is obvious from the remarkable fact, 

 that the mean height of the barometrical column, mak- 

 ing allowance for difference of temperature and eleva- 

 tion, is nearly the same in all latitudes, where it nas yet 

 been observed. It may be supposed, therefore, that 

 the plane which separates the two currents, will be 

 nearly at that elevation above the level of the sea, 

 where the barometer would stand at half its mean 

 height, and that here clouds are most likely to be form- 

 ed, from the contact of two portions of air of different 

 .temperatures. This also is confirmed by observation ; 

 the height at which the barometer would stand at 15 



inches, being about 3.4 miles, or nearly the average Mctcorolo- 

 height of the clouds. gy. 



The influence of different currents of air in produc- 

 ing rain, is demonstrated by a great variety of pheno- e 



mcma. In this climate, dry weather generally prevails ra j] y Je- 

 either when the atmosphere is in a settled state, orcompanied 

 when the wind blows steadily from the same point, with rain. 

 On the other hand, a change of wind is almost always 

 accompanied with rain ; and if these changes be fre- 

 quent and rapid, heavy showers are the consequence. 

 In the tropical climates, the rainy seasons may gene. 

 rally be predicted with the greatest precision, as they 

 are found uniformly to set in with the sun's approach 

 to the zenith, when the winds become variable, or with 

 the change of the monsoon, at which time the heaviest 

 rains fall. In extensive inland plains, where there is 

 nothing to promote the mixture of different currents of 

 air, and where evaporation does not furnish moisture 

 enough to bring the atmosphere to the point of satura- 

 tion, it seldom rains ; while in mountainous countries, 

 especially in the neighbourhood of the sea, it rains fre- 

 quently. 



Where there are so many irregularities to be taken Relation 

 into the account, it seems impossible to calculate with between 

 any thing like precision, the quantity of rain that may 'he annual 

 be expected to fall at any given place in the course of quant'7 

 a year. That quantity indeed, must obviously bear of ram and 

 some relation to the moisture which, at the average t^f '*" 

 temperature of the year, can be held in solution in the 

 atmosphere, compared with the yearly evaporation ; 

 and, upon this principle, an approximation to the depth 

 of rain for every 5th parallel of latitude, has been given 

 under the article HYGHOMETRY. The results in that 

 table, however, must be considered as excluding the 

 irregularities arising from local circumstances, and can- 

 not, therefore, be expected to give a very near ap- 

 proximation for particular places ; but it may be inte- 

 resting to inquire, how far it coincides with actual ob- 

 servation in our own climate. 



From a very extensive collection of facts, Mr. Dalton Mr. Dal- 

 found that the average annual quantity of rain, for 30 ton's esci- 

 places in England and Wales, amounts to 35.2 inches, mate of 

 the greatest being 67-5 inches, at Keswick in Cumber- ye }/ rain 

 land ; and the least jg.5 at Upminster in Essex. Thhj 

 average, however, is, on various accounts, greatly 

 above the truth. Of the 30 places where the observa- 

 tions were made, 2 are in Cumberland, 3 in Westmore- 

 land, and 5 in Lancashire ; the counties where, of all 

 others, the quantity of rain might be expected, and is 

 actually found to be the greatest. Mr. Dalton indeed 

 has made an allowance for the excess arising from this 

 circumstance, by taking a mean of the different places 

 in the same county, and then an average for all the 

 counties, which reduces the general mean from 35.2 to 

 31.3 inches ; but we are inclined to think that even 

 this is too high an average for the whole of England 

 and Wales. Of the 20 counties where the places re- 

 ferred to by Mr. Dalton are situated, 13 are maritime; 

 yet notwithstanding this great proportion of counties 

 where the atmosphere is most likely to be humid, there 

 are 16 of the whole 30 places, where the quantity of 

 rain is below 30 inches. Had the observations been Probably 

 made at equal distances over the whole country, it is too hign. 

 probable that the general mean would be considerably 

 below this last quantity. Taking the mean latitude of 

 England and Wales at 52^, the average yearly rain by 

 the table alluded to above, should be 23.5' inches. 



We have found from our own observations, that the 

 annual quantity of rain, in latitude 56 u , and in a situ- 



d 



