M E T E O R L O G Y. 



167 



OtMCTTa- 



Uoat by 

 itie writer 

 atihitu- 

 title. 



C ,"; par a- 

 W quan- 

 Ui of 

 rain on 

 mountaini 

 od plain*. 



Ml* on 



i3.-r.ti.ru. 



atlon not likely to be much affected by local circum- 

 stances, is, on an average of 6 years, commencing with 

 1813, 23.7 inches ; and leaving out the years 1817 and 

 1818, when the quantity of rain wan unusually great, 

 the average is 21..X Hy'the table it should be 21. At 

 another place, under the name latitude, but in the im- 

 mediate vicinity of a hill tj or 7 hundred feet in height, 

 the average depth of rain for the same six years, is 

 about 25.5 inches. 



A question has sometimes been agitated respecting 

 the comparative quantities of rain, that fall at the sum- 

 mit and base of a hill. It aptiears from experiment, 

 that a rain gage, placed near tlie surface of the earth, 

 collects a greater quantity of rain than one elevated 

 above it, and the difference has been generally ascribed 

 to the influence of the wind. It has also been sup- 

 posed, that indqxmdent of this cause, the quantity 

 must be greater below than above, owing to the gradu- 

 al accumulation, and consequent enlargement of the 

 drops in their descent through the atmosphere. Now 

 there are only two cases in which it appears that this 

 can take place ; either when the atmosphere through- 

 out is saturated with moisture, or when the tempera- 

 ture of the rain is so low, as to reduce the air with 

 which it comes in contact in its descent, to the point of 

 deposition. There is reason to doubt, however, whe- 

 ther these case* occur generally or even frequently. 

 We have often observed the hygrometer during a 

 heavy and even long continued rain, indicate from 5 

 to 10 degrees ot'drynesn, so that instead of increasing, 

 there is reason to believe that the drops sometime* di- 

 minish on their approach to the earth. Bat whatever 

 may be the cause of the difference between the indica- 

 tion* of two rain gage* placed at different altitudes 

 above the same plane, there can be little dmilit, that 

 the quantity of rain which falls on the summit of a 

 hill, is greater than what fall* at the base, and that the 

 difference is easily accounted for from the well-known 

 fact, that the rumbas or rain-cloud, as well a* fog* and 

 mists, is often attracted by mountains, and is seen to 

 rest upon them, while there is little or no rain below. 

 This, of course, suppose*, that the hill is of consider- 

 able height and magnitude, and that the observation in 

 this case holds true, is fully established by the follow- 

 ing fact*. 



In the neighbourhood of Kinfaun's Castle, a rain- 

 gage is placed on the summit of a hill, 600 feet above 

 the level of the sea, and another in the garden at the 

 base of the hill, about SO feet above the sea. The fol- 

 lowing are the annual results of the two for five years : 



pnrt of the above difference can be ascribed to 

 any thing in the situation of the garden gage but its 

 being lower down. It it fully exposed to the rain ; 

 and, as the best proof of this, it is only necessary to re- 

 mark, that its average amount for six years is about two 



Uncertain. 



inches greater than that of another, situated in an open Meteorolo- 

 country, at the distance of about a mile. <_'*'-_- 



When clouds are formed at an elevation, where the p^^ ti ~ 

 temperature is below 32, the particles of" moisture be- of mow 

 come congealed, and fall down in the form of snow or an( ; na u. 

 hail. If the temperature of the lower regions of the'at- 

 mosphere be much above 32, it will sometimes hap- 

 pen that the snow is again dissolved before it reaches 

 the earth ; but hail, from its greater solidity, as well as 

 more rapid descent, frequently arrives at the ground in 

 its congealed state even during the summer months. 

 There can be no doubt that the moisture, of which h.iil 

 consists, had been collected into Urge drops of rain 

 previous to its being congealed ; and Mr. Leslie has 

 proposed an explanation of the manner in which the 

 congelation takes place. " If we examine," says he, 

 " the structure of a hail-stone, we shall perceive a 

 snowy kernel incased by a harder crust. It has 

 very nearly the appearance of a drop of water sud- 

 denly frozen, the particles of air being driven from the 

 surface towards the centre, where they form a snongy 

 texture. This circumstance suggest* the probable ori- 

 gin of hail, which is perhaps occasioned by rain falling 

 through a dry and very cold stratum of air." He then 

 proceeds to point out a method of putting this theory 

 to the test of experiment ; but, in tact, it has been al- 

 ready confirmed, by the process of freeing water un- 

 der the receiver of an air-pump. 



From all that has been stated on the formation of p ro gn<Mt!- 

 douds, and the production of rain, we are naturally led catiom of 

 to remark, that however important may be the facts rain " 



which l 



disclose, 



able to predict, 



case, the approach of the mosf common and familiar of 



all atmospherical phenomena. By the extended use of 



the hygrometer, we may discover a great deal more than 



we yet know of the state of the atmosphere near the 



surface of the earth ; but we cannot from this infer any 



thing with certainty regarding the processes that are 



going on in the region of the clouds, or anticipate the 



nature and extent of future changes in the weather. 



We have repeatedly remarked, in the course of cur ob- 



servations, a heavy and long continued fall of rain, 



commencing a few hours after the hygrometer had in- 



dicated an unusual degree of dry nets in the lower at- 



mosphere ; and, on the contrary, the air passing from a 



state of almost complete saturation to great clryness 



without any rain at all. The barometer is the only in- 



strument by which distant changes in the atmosphere 



can be detected ; but as its fluctuations merely give us 



notice of variations in the weight of that fluid, they do 



not necessarily indicate the approach of rain. In fact, 



the only information that this instrument gives us, is 



the amount of the disturbance that has taken place in 



the equilibrium of the atmosphere. When the mercu- e 



rial column, therefore, sinks below its mean height, all i (c iwn 



that we can directly infer is, that some change has h.ip- the fluetu- 



pened, or will soon happen, in the force or direction of tion of ifc 



the wind ; but as this is likely to bring different or op- baromewr 



posite currents into contact, it may naturally be expect- a " d ,^ h 



ed that rain will be the consequence. Hi", in this way h(r ' 



only that the fall of the barometer it an indication of 



rain. It may be observed, too, that the intermixture 



of different portions of the atmosphere, in such circum- 



stances as to produce rain, sometimes takes place, not at 



the first disturbance, but at the subsequent restoration 



of the equilibrium ; and we find accordingly, that, on 



lark, Uiat however important may DC i 

 meteorological researches arc in time likely to 

 c, there is out little probability of our ever being 

 > predict, with any thing like certainty in every 



