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THE POPULAR SCIENCE MONTHLY.

The British Association.—The fifty-second meeting of the British Association will be held at Southampton, beginning August 28d, when the chair will be resigned by Sir John Lubbock and assumed by Dr. C. W. Siemens, F. R. S., president-elect, with the usual address appropriate to the occasion. The addresses at the evening general meetings will be, "On the Tides," by Sir William Thomson, F. R. S., August 25th, and on "Pelagic Life," by Professor H. N. Moseley, August 28th. The presidents of the sections will be: A, Mathematical and Physical Science, Lord Rayleigh; B, Chemical Science, Professor D. Liveing; C, Geology, R. Etheridge, F. R. S.; D, Biology, Professor A. Gamgee, with Professors Gamgee, M. A. Lawson, and W. Boyd Dawkins as presidents in the Departments of Anatomy and Physiology, Zoölogy and Botany, and Anthropology; E, Geography, Sir R. Temple, Bart.; F, Economical Science and Statistics, the Right Hon. G. Sclater-Booth; and G, Mechanical Science, John Fowler, F. G. S. Excursions to places of interest in the neighborhood of Southampton will be made on the afternoon of Saturday, August 26th, and on Thursday, August 31st.

 

Forests and Climate.—Dr. J. M. Anders, in the "American Naturalist," has carefully examined the influence of forests upon climate and rain-fall. The principal influence exerted by woods upon climate is as windbreaks, in which capacity the service they render is familiar enough. The experiment has been tried extensively in France of planting trees in belts one hundred metres apart, with marked benefit to the climate. Forests may slightly promote the condensation of moisture by inducing an upward movement of the air, as mountains are known to do on an extensive scale; but their action in this respect, on account of their low height, is not important enough to be made account of. Woods play a more important part in furnishing the air with moisture by transpiration of water through their leaves. It is computed from experimental tests that they give off in this way twelve times as much water as is evaporated directly from the soil on which they stand, twice as much as goes up from a free soil, and more than is emitted from an equal body of water. They are able to do this, and keep it up, because they are at all times supplied with an abundant store of moisture for transpiration. This is given them partly by the power which their roots have to attract moisture from every direction; partly by the retention of the rain-fall in their net-work; and partly by the property possessed by vegetable mold of absorbing moisture and holding it. This power of evaporation is shared by the humbler vegetation, and it operates nearly constantly, even during long droughts. Climate is also materially affected by this quality, for moist air during winter tends to moderate extreme cold and during summer produces a refreshing coolness. Now, since it is established that forests moisten the air over, in, and to some extent around themselves, "may we not be pardoned for concluding that warm currents sweeping over a country and striking the cool moist air in and above the forests, and mingling with it, would have a portion, at least, of the contained moisture condensed into gentle showers, extending their beneficent influence to neighboring fields? Again, let some stray current come along, of a lower temperature than the air of the forest, and the moist air of the forest would readily be condensed, since it is a well-known fact that a moist air discharges its vapor more readily in the form of rain than a drier atmosphere. We have now seen how trees can cause local rains; it will also be observed that the rain is formed chiefly above the forest, though it may be through the influence of winds that it falls to the earth for some distance around. By increasing the frequency of light rains, forests tend to obviate drought, which is of ultimate importance to the farmer's crops and vegetation in general. It will be seen that all our deductions have been drawn largely from the known facts from observations." Forests also produce abundant dews, an office not to be despised, for heavy dews are often very refreshing to vegetation.

 

Atmospheric Pressure and the Sea-Level.—One of the most interesting phenomena of the recent winter in Europe was a remarkable depression of the level of the Mediterranean Sea under the influence of the high barometric pressures of December