The Atlantic Monthly, Volume 06, No. 33, July, 1860. Various

Чтение книги онлайн.

СКАЧАТЬ grows colder than the atmosphere, and so condenses it.

      What is thermometrically called the dew-point is that degree at which the moisture present in the atmosphere, on being subjected to a decrease of temperature, begins to be precipitated or condensed. It is the same as the point of saturation. Daniell calls it "the constituent temperature of atmospheric vapor." It is our criterion for ascertaining how much moisture there is in the air, and at what degree of heat or cold it would be precipitated. When the air is saturated, a dry bulb and a wet bulb will read alike.

      The dew-point has been a puzzle to most persons. Very few treatises explain it satisfactorily. The definition just given, though explicit, is not quite enough. For it will be perceived that an ordinary subtraction of the degrees of temperature on a wet thermometer, which had cooled down by evaporation, from the actual temperature indicated by a dry thermometer, will not give us the dew- point.

      For example,–if a free or dry thermometer indicates 63°, and the one with the wet bulb has by evaporation cooled down to 54°, the difference would be 9°. The dew-point would not be 54°, but that degree to which the mercury would fall in the free thermometer, for the atmosphere to become saturated with the quantity of moisture then actually existing in it. It would be 46.8°.

      This dew-point, which figures so largely in all well-kept meteorological reports, is the key to many important conditions of the atmosphere, affecting health, vegetation, and climate.

      It is found that the air at different degrees of heat has different degrees of elasticity, different degrees of tension, and different degrees of capacity to hold vapor. Dalton, by a series of experiments with barometer-tubes, into which he introduced air and vapor at certain temperatures, found what its force was upon the mercurial column from degree to degree. He also experimentally determined the ratio of the weight of moisture and of air, the former being five-eights of the latter,–in other words, how many grains of moisture additional could be held by the air, advancing from degree to degree of temperature. This being ascertained, a table of factors was constructed, in other words, a set of figures contrived, which should, by a multiplication of the subtracted difference between the range of the dry bulb and the wet bulb of the thermometers, furnish the amount of deduction from the former which would indicate the dew-point, or the point to which the mercury in the dry thermometer must fall to show how much more moisture the air could hold without its condensation. These tables of factors have been constructed at the Greenwich Observatory, and are generally used.

      The Hygrometer, invented by Mr. Daniell, gives the dew-point by inspection.

      It is an error to suppose that dew falls like rain from the air; it forms on the body which is cooled down below the temperature of the air. It differs in quantity with the radiating or cooling surface; that which has absorbed and retained the most heat during the day radiates the most at night and furnishes the most cold in return.

      Hoar-frost, such as we find on our window-panes, or on the grass, is the moisture of the warm air cooled down and frozen, and is produced when the cold at the surface is below the freezing-point. What we in common parlance call the action of frost, and which in this climate is well known to be very powerful, is not particularly injurious to organized bodies.

      Mists are the vapor near the ground rendered visible by the temperature of the air falling below that of the vapor. When we see our breath in a cold morning, we see a mist. Where the surface is comparatively warm and damp, and the air is cooler, we have mists, which, if dense, are called fogs. These are found plentifully on the banks of Newfoundland; and with icebergs on the one hand and the Gulf Stream on the other, we must always expect to have them.

      The distribution of rain, which is one of the offices of the clouds, is another of the more important features of Meteorology. The amount of water taken up by evaporation into the atmosphere is almost incredible. It is calculated by Lieutenant Maury that there is annually taken up in the torrid zone a belt of water three thousand miles in breadth and sixteen feet deep. Rain occurs regularly and irregularly in different parts of the earth. In some places it may be calculated upon to a day; in others it is quite unknown. Latitude and longitude may indicate the points of distribution, but the causes are dependent on temperature, winds, locality, and, what may seem a strange assertion, upon the conduct of man himself. The greatest quantity falls near the equator, diminishing towards the poles. Much more falls on islands and coasts than in the interior of continents,–more in the region of the variables and less in that of the trades. There are, however, tropical countries of great extent where rain is scarcely ever seen.

      The influence of man upon rain is seen in the progress of civilization, the destruction of forests, and the drying-up of meres, swamps, and water- courses.

      Forests undoubtedly affect the distribution of rain, and the supplies of streams and springs. Their cooling influence precipitates the vapor passing over them, and the ground beneath them not getting heated does not readily evaporate moisture. Lands, on the contrary, which are cleared of forests become sooner heated, give off larger quantities of rarefied air, and the passing clouds are borne away to localities of greater atmospheric density.

      The Canary Islands, when first discovered, were thickly clothed with forests. Since these have been destroyed, the climate has been dry. In Fuerteventura the inhabitants are sometimes obliged to flee to other islands to avoid perishing from thirst. Similar instances occur in the Cape Verdes. Parts of Egypt, Syria, and Persia, that once were wooded, are now arid and sterile deserts.

      In the temperate zones these results are not so immediately apparent. It is now much in doubt whether the climate of our country has changed its character within the last two hundred years. Jefferson and Dr. Rush both contended that it had. Our oldest inhabitants assert that in their day our winters began nearly two months earlier than they do now.

      The general laws laid down in relation to rain are these:–

      1. It decreases in quantity as we approach the poles.

      2. It decreases as we pass from maritime to inland countries.

      3. It decreases in the temperate zones on eastern coasts as compared with western coasts, but within the tropics it is the reverse.

      4. More rain falls in mountainous than in level countries.

      5. Most rain falls within the tropics.

      The rainless regions, not deserts, are parts of Guatemala, the table-land of Mexico, the Peruvian coast, parts of Morocco, Egypt, Arabia, Persia, etc.

      The electric character of the air is another subject of interest, and a leading one in Meteorology. What can be more magnificent, what more awful, than those storms of lightning and thunder which are witnessed sometimes even in our own latitudes?

      Faraday, who as a chemist and philosophical writer is of the highest authority, professes to have demonstrated that one single gram of water contains as much electricity as can be accumulated in eight hundred thousand Leyden jars, each requiring to charge it thirty turns of the large machine at the Royal Institution.

      It is not intended that this astounding statement should be received without some grains of allowance; but a very elegant and scientific writer, who adopts it without hesitation, adds, "We can from this crystal sphere [of water] evoke heat, light, electricity in enormous quantities, and beyond these we can see powers or forces for which, in the poverty of our ideas and our words, we have not names."

      Flashes of electricity have been detected, during warm, close weather, issuing from some species of plants. The Tuberose and African Marigold have been seen to emit these mimic lightnings. (Goethe is the authority for this.) To atmospheric electricity we doubtless owe the coruscations of the Aurora, one of the most beautiful of our meteors.

      The usual forms of lightning are the zigzag СКАЧАТЬ