Page:Popular Science Monthly Volume 37.djvu/554

538 sense was brought into play in courtship, and that colors and pattern have been gradually modified by the preference of the females for the most beautiful males; he believed that such sexual selection accounts for many of the most beautiful features possessed by animals, viz., those which are especially displayed during courtship.



HUNDER-STORMS naturally attract universal attention when they occur, and it may perhaps be of interest to point out some particulars that have been ascertained about them.

The most obvious facts are that a heavy cloud passes over the observer, and that from it lightning appears, followed, after a greater or less interval, by thunder; and that usually heavy rain or hail falls from the cloud. The damage wrought by these occurrences, whether by lightning-strokes or by the hail, is so serious that, in countries especially liable to such visitations, hail insurance forms an important item in the farmer's calculations. In many countries such insurance is in the hands of the Government, and accordingly statistics as to the amount of losses are to be obtained; whereas where insurance is in the hands of private companies, information as to the expenditure of these companies is naturally not published.

As regards the liability of certain districts to suffer damage from thunder-storms, it has been maintained by several authorities that these visitations are steadily increasing in frequency. A most elaborate inquiry into the records of such occurrences was printed in the Journal of the Statistical Office of Berlin for 1886. From this it appears that the evidence indicated no general increase in the frequency of lightning-strokes, but, on the contrary, rather a decrease. Houses with soft or, in other words, thatched roofs are struck about seven or eight times more frequently than ordinary slated dwelling-houses. Houses in towns are much less frequently affected than those in the country.

The geological character of the soil has a very great influence on the risk. If this for a limestone soil be taken as one, that for a sandy soil is nine, and for swampy land twenty-two. As regards the different classes of trees, if the risk to a beech be taken as one, that to a conifer (fir or spruce) is fifteen, to an oak fifty-four, and to other deciduous trees forty. Another investigator accounts for the comparative immunity of the beech by the fact that its leaves are edged with short hairs, which allow the electricity collected in the leaves to escape quietly.

As to the actual origin of atmospheric electricity, authorities 