THE FARMER'S MAGAZINE. 



Ul 



perature is reduced to 45*, it can then only retain 

 between three or four grains ; but then a warm 

 stratum of air can only, as far as we see, be cooled 

 by admixture with a current of air of a lower 

 temperature, and if so, then what the one loses in 

 temperature, and in dissolvinpr power, the cooler 

 current would seem to gain. Dr. Hutton was the 

 first to throw light upon this difficult enquiry. lie 

 traced the result of mixing two currents of air at dif- 

 ferent temperatures, and reasoned upon the fact 

 that the quantity of moisture which air can hold 

 increases in a much faster ratio than its tempera- 

 ture. He supposed the case of equal bulks of air, 

 in a state of saturation, and at the different tem- 

 peratures of 15 and i5 centisimal degrees, and that 

 these were intermixed, the mixture will evidently 

 have the mean temperature of 30'^. But since at 

 these temperatures, the one portion held 200 parts 

 of humidity, and the other 800, the aggregate must 

 contain 1000 parts, or either half of it 500; at the 

 mean or resulting temperature, however, this por- 

 tion could only retain 400 parts of water, and con- 

 sequently the difference, or 100 parts, amounting 

 to the 200th part of the whole weight of air, must 

 be precipitated. Thus separating, it must appa- 

 rently appear to us as steam floating in air saturated 

 with moisture, or it must descend in rain. 



The explanation is not, however, unattended with 

 difficulties ; and the origin of rain has been hitherto 

 60 little understood, that, as usual with us in our 

 want of knowledge, we have recourse to other mys- 

 teries, and employing these, call them explanations 

 of the one immediately engaging our attention. 

 Thus we suppose that electricity operates upon the 

 clouds, and then the electrified clouds upon each 

 other : hence producing rain, lightning, and thun- 

 der. That clouds are formed, besides, of water 

 heavier than the atmosphere in which they float, is 

 well known. The reason of the lighter body sup- 

 porting the heavier does not at first appear so 

 clearly. Von Kaemtz, in his work on meteoi'o- 

 logy, however, is of opinion that a cloud is not a 

 motionless mass, but that, on the contrary, it is in 

 continual motion ; when its vesicles, as they are 

 carried along by the wind, arrive in a dry air, they 

 dissolve, whilst on the side of the wind the vapour 

 is precipitated in the vesicular state. Thus a cloud 

 apparently motionless, often slowly descends, and 

 its lower part is continually dissolved, while the 

 upper portion increases by the addition of new ve- 

 sicles. There exists a force directly opposed to the 

 fall of clouds in that of ascending currents of air. 

 On this account it is, that the cumuli are more ele- 

 vated at mid-day than the morning ; since the 

 currents of air become lighter as they become 

 warmer. Towards evening, on the contrary, as soon 

 as this current becomes weaker, the clouds actually 

 descend, and dissolve when they arrive at the 

 lower and warmer regions of the atmosphere. The 

 horizontal currents also oppose the fall of clouds. 

 There are other bodies, besides the cloud vesicles, 

 which are continually supported by currents of air: 

 the fine particles of the dust of our rooms, rendered 

 visible to us in a sunbeam ; the clouds of fine soot 

 dust falling twenty miles from London ; the impal- 

 pable dust of the desert, or of volcanic eruptions 

 falling on vessels at sea ; all indicate the same thing. 



Let us not, then, observes Kaemtz, seek to explain 

 the suspension of clouds by extraordinary causes : 

 it is as easy of comprehension as that of dust. 



Fogs hardly differ from the clouds in composi- 

 tion. When the aqueous vapour separates from 

 the atmosphere, and becomes visible at considera- 

 ble heights from the earth, we call it cloud. 

 When the same separated vapour rests on the earth, 

 we call it fog. A traveller sees the clouds hanging 

 half way up the mountain side : he tells you that 

 the summit is hidden by the cloud from his sight ; 

 when he has ascended that mountain, the same mist 

 hides the earth from his sight; or, as he says, there 

 is a fog beneath him. These fogs, are, from their 

 vicinity to the earth, probably more impure than 

 the clouds ; they mix with the smoke of our fires 

 and the emanations of animal life. The amount 

 of some of the impurities in fog water naturally 

 increases as the population of the district in which 

 it is collected becomes mora dense. For instance, 

 in a gallon of water of a fog, there has been de- 

 tected — Grains of Ammonia. 



At Liebfraunberg, in Germany 0.1790 



At Paris « 9.600 



In the same amount of dew water at I * ^^ 

 Liebfraunberg J 0.4340 



In rain water at Paris 0.2 100 



„ „ Liebfraunberg...*.... 0.0350 



The ammonia of the fog— as well as another 

 soluble matter— nitric acid, found occasionally in 

 it, are both in a state of solution in the vapour ; 

 other matters are frequently detected, that are 

 merely mechanically suspended among its vesicles. 

 "Seamen," says Maury (266), "tells us of 'red 

 fogs,' which they sometimes encounter especially 

 in the neighbourhood of the Cape de Verde Islands." 

 In other parts of the sea, they meet showers of 

 dust. What these showers precipitate in the 

 Mediterranean is called, " Sirocco dust," and in 

 other parts "African dust," because the winds 

 which accompany them are supposed to come from 

 the Sirocco desert, or the parched land of the 

 African Continent. It is of a brick-red or cinna- 

 mon colour, and it sometimes comes down in such 

 quantities, as to cover the sails and rigging, though 

 the vessel may be hundreds of miles from the 

 land. 



Late microscopical observations have shown the 

 very curious fact that this dust consists of infusoria 

 and organisms whose habitat is not Africa, but 

 South America, and in the south-east trade-wind 

 region of South America. Professor Ehrenberg 

 has examined specimens of sea-dust from the Cape 

 de Verde Islands, Malta, Genoa, Lyons, and from 

 the Tyrol, and he has found a similarity among 

 them as striking as it would have been had these 

 specimens been all taken from the same pile. In 

 all. South American were the prevailing forms. " It 

 may now be regarded as an established fact," adds 

 Maury, " that there is a perpetual upper-current of 

 air flowing from South America to North Africa, 

 and that this volume of air is nearly equal to that 

 which streams to the southward with the north- 

 east trade-winds." 



The mystic movements oJ the winds early at- 



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