168 



METEOROLOGY. 



Wtteorolo- some occasions, the barometer, after being depressed be- 

 gy- low its mean height, has begun again to rise before the 

 "\~* ra j n commences. To illustrate these remarks, we shall 

 extract from our register the greatest falls of rain that 

 have taken place since 1815, with the corresponding 

 state of the barometer, and the relative humidity of the 

 atmosphere, at 10 morning and evening, confining our- 

 selves to those cases in which the quantity of rain 

 amounted to an inch in 48 hours. 



Table illur. 

 Ir.uivc of 

 that rcla- 

 tion. 



Remarks The mean height of the barometer, from the begin- 

 on the ning of 1815 to the middle of 1819, w a 8 29.650, and 

 above table: (|j e mean relative humidity, at 10 morning and even- 

 ing, about 80. The average of the barometer in the 

 above table is 29.475, and the relative humidity 88.5. 

 The table, however, presents many anomalies from any 

 general inference that might be drawn from these facts. 

 Thus, on the 7th of July, 1816, the barometer was al- 

 most stationary at a point rather above than below the 

 mean height, not only on that but also on the preceding 

 day, though the quantity of rain amounted to an inch 

 in 24 hours. It may be alleged, indeed, that the wind 

 on that occasion blew from the east ; and as an east 

 wind, in this country, generally raises the barometer 

 two or three tenths above its mean height, the altitude 

 of the mercury on that day was as far below what it 

 ought to have been, as the average of the preceding ta- 

 ble is below the average of the whole period. There 

 are cases, however, where this explanation does not ap- 

 ply. On the 9th and 10th of June, 1817, and the 26th 

 and 27th of Ju'.y, 1818, upwards of an inch of rain fell ; 

 while the barometer, with a west wind, stood in the 

 one case very little below, and in the other considerably 

 above, the mean height. In the former case, indeed, 

 the mercury had sunk on the 8th to 29.4, but in the 

 latter it was almost stationary for several days. 



But the most remarkable fact in the above table is, 

 Che unusual degree of dryness that sometimes existed 

 in the lower atmosphere immediately before a consider- 



able fall of rain. Thus, on the 26th of May, 1817, the Metcorol , 

 relative humidity was 19, and, on the 20th of June, ._J! y ' . I _ J 

 1819, it was 38 degrees below the ordinary annual ""Y""* 

 mean, though a few hours afterwards rain commenced, 

 and in both cases exceeded an inch. It would thus ap- 

 pear, that the formation of rain sometimes takes place 

 at a great height above the surface of the earth, and is 

 altogether independent of the quantity of moisture that 

 exists in the lower strata of the atmosphere. 



But though it may be difficult to assign any satisfac- Anderson * 

 tory reason for the particular facts now stated, it seems estimate of 

 to be sufficiently clear, that in general the quantity of the a n 

 rain that falls at any given place in the course of a year ^j'"^ " 

 bears a certain relation to the moisture which can be evapora . 

 held in solution by the atmosphere, at the average tern- tion. 

 perature of that place ; and that the depth of rain, 

 therefore, must depend in some degree on the quantity 

 of evaporation. The relation between these quantities, 

 as deduced from the law of aqueous solution in the at- 

 mosphere, may be seen from the table formerly alluded 

 to in section 99 of the article HYGROMETRY, containing 

 the quantity of rain for every 5" parallel of latitude, 

 compared with another table in section 5 1 of the same 

 article, shewing the daily and yearly evaporation for 

 the same parallels. We have already seen that the 

 former of these tables gives an approximation to the 

 depth of rain as found by actual observation ; and 

 though it is difficult, perhaps impossible, to measure 

 with accuracy the quantity of evaporation from the sur- 

 face of the ground, the most satisfactory experiments 

 on the subject give results coinciding very nearly with 

 the quantities given in the table of evaporation. 



From observations made by Dr. Dobson at Liver- Obserrt- 

 pool, for four years, commencing with 1 772, it would tio 3 b y 

 appear that the annual evaporation from the surface of Dr> D 

 the water in that latitude, amounts to 36.78 inches. 30 

 There can be little doubt, that 'vhere the water in the 

 evaporating basin is freely exposed to the influence of 

 the sun and wind, or even of the latter alone, the quan- 

 tity evaporated in the course of a year may amount to 

 36 or 37 inches. This, however, is obviously a much Estimates 

 greater quantity than could be dissolved by a column evaporation 

 of the atmosphere, whose base is equal to the exposed 100 hl S h - 

 surface, because in the case of even a moderate wind, 

 the original rate of evaporation is kept up by a constant 

 succession of new columns, whose bases are every in- 

 stunt coming in contact with the surface of the water. 

 It must be greater also than what is actually evaporat- 

 ed from the earth ; for though the latter is exposed to 

 the influence of both the sun and wind, the surface in 

 a short time becomes dry, and the moisture is not drawn 

 up from below so quickly as to yield a supply equal to 

 that afforded from the surface of water. There is rea- 

 son, however, to believe, that the actual evaporation 

 from the ground in ordinary cases, approaches nearer, 

 than that from an exposed evaporating vessel, to the 

 true quantity of evaporation at any given place ; mean- 

 ing by this, the quantity which the atmosphere at that 

 place would dissolve, were the whole surface of the 

 globe covered with water. The ground indeed after a 

 time ceases to supply moisture as quickly as the air is 

 capable of dissolving it ; but were the surface continu- 

 ally wet, the dissolving power of the air, and consequent- 

 ly the rate of evaporation, would diminish, and perhaps 

 as rapidly as the supply of moisture from the earth is 

 found to do in the present state of things. But the rea- 

 diest, and perhaps, after all, the most accurate estimate 

 of evaporation, is found by placing the common evaporat- 

 ing basin, or atmometer, in a situation out of the reach of 

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