Page:Popular Science Monthly Volume 13.djvu/525

Rh and within the same months, 928 auroras were seen in Europe or America which were invisible in Finland. These statistics further serve to show the geographical distribution of auroras, which is as follows: The zone of greatest frequency and intensity begins near Barrow Point (latitude 72° north), on the northern coast of America; thence it passes across the Great Bear Lake toward Hudson Bay, which it crosses at latitude 60° north, passing over Nain on the coast of Labrador, keeping south of Cape Farewell. Its further course is between Iceland and the Faroe Isles, to the vicinity of North Cape in Norway, and thence into the Arctic Sea. Thence it probably passes round Nova Zembla and Cape Isheljuskin, approaches the north coast of Asia in the eastern part of Siberia, in the longitude of Nizhni Kolymsk, and thence returns to Barrow Point.

Analogies of Plant and Animal Life.—Some very interesting analogies of plant and animal life are pointed out by Mr. Francis Darwin in a recent lecture. In the first stage of individual existence—the egg or the germ—the analogy is perfect; it is no less so in the stage just succeeding. Among animals there are great differences as to the degree of development attained by the young ones before they enter the world. For instance, a young kangaroo is born in a comparatively early stage of development, and is merely capable of passive existence in its mother's pouch, while a young calf or lamb soon leads an active existence. Or compare a human child, which passes through so prolonged a condition of helplessness, with a young chicken, which runs about and picks up grain as soon as it quits the shell. As analogous cases among plants Mr. Darwin cites the mangrove and the tobacco-plant. The ripe seed of the mangrove is not scattered about, but remains attached to the capsule still hanging on the mother-plant. In this state the seeds germinate and the roots grow out and down to the sea-level, and the plant is not deserted by its mother until it has got well established in the mud. For the conditions of life against which the young mangrove has to make a start are unfavorable enough. The most intrepid seedling might well cling to its parent on finding that it was expected to germinate in soft mud daily flooded by the tide. Perhaps, adds the author, the same excuse may be offered for the helplessness of babies—the more complicated the conditions of life, the greater dependence must there be of offspring on parent. Compare a young tobacco-plant with a young mangrove. All the help the seedling tobacco receives from its parent is a very small supply of food. This it uses up in forming its first pair of leaves, and it has then nothing left by way of reserve, but must depend on its own exertions for subsistence—that is to say, it must itself manufacture starch (which is the great article of food required by plants) from the carbonic acid in the air. In this respect it is like a caterpillar which is formed from the contents of the egg, but has to get its own living as soon as it is born.

The Construction and Erection of Lightning-Rods.—Dr. R. J. Mann lately summed up in a series of aphorisms the fundamental conditions to be observed in the construction of lightning-rods. We have not room here for the whole series, but select a few of the most important. The main stem of a copper lightning-conductor should never be less than of an inch diameter; this dimension is not sufficient for a building more than eighty feet high. Galvanized iron may be used instead of copper, but then the diameter must be greater in the ratio of 6. 7 to 2.5, the conducting capacity of iron being to that of copper as 14 to 77. A galvanized iron-rope conductor should never be less than of an inch diameter; a galvanized iron strip should be 4 inches wide and  of an inch thick. A lightning-rod must be continuous and unbroken from end to end. A rod need not be attached to a building by insulated fastenings; metal clamps may be safely employed, provided the rod be of good conducting capacity and otherwise efficient. Above, the rod must terminate in metal points, well projected into the air; these points should be multiple and perfectly sharp. The bottom of the conductor must be carried down into the earth and be connected with it by a surface-contact of large extent. All large masses of metal in a building should be metallically connected with the lightning-rod, except when they