Page:Popular Science Monthly Volume 6.djvu/653

Rh of the ocean appears to depend upon the depth of the superincumbent water. So universally is this the case that the observers on board of the Challenger needed only to know the depth at any locality in order to foretell the character of mud that would there be brought to the surface by the dredging apparatus. According to Prof. Thomson, "the mean maximum depth at which the globigerina-ooze occurs is about 2,250 fathoms. The mean depth at which we find the transition gray ooze is 2,400 fathoms, and the mean depth of the red-clay soundings is about 2,700 fathoms." These three sedimentary formations, however much they differ from each other, are all the result of the precipitation to the bottom of the dead shells of the globigerina and other surface animals. Why, then, do they differ so much in appearance and in chemical constitution? The globigerina-ooze is 98 per cent, carbonate of lime; the gray ooze consists of carbonate of lime with a greater or less proportion of clay; the red ooze is almost pure clay, viz., silica, alumina, and red oxide of iron. Prof. Thomson accounts for the absence of carbonate of lime from the red clay, and its partial absence from the gray ooze, by the theory that at great depths there is an excess of free carbonic acid. This would convert the carbonate of lime of the shells into a soluble compound. In that case the red clay would be "the insoluble residue, the ash, as it were, of the calcareous organisms which form the globigerina-ooze after the calcareous matter has been removed." It is worthy of note that living animals, brought up by the dredge from great depths, have their calcareous shells very rudimentary.

A Ballooning Spider.—A paper of singular interest, by Dr. Lincecum, contributed to the Smithsonian Institution and published in the American Naturalist, describes the marvelous art of the gossamer spider in the construction and navigation of her aëronautical ships. In Texas, according to the author, December is the month for these ballooning spiders to emigrate. When they intend to make an ascension, they fix themselves on some extreme point of the-branch of a tree, or weed, or corn-tassel, then carefully spin out a lock of white gossamer, five or six inches long and two inches wide in the middle, tapering toward the ends, holding it all the time in the gentle breeze by a thread two or three inhcesinches [sic] long, which, being attached to the end of the selected point, detains the balloon until it is finished. They then spin out at the bow two lines, thirty or forty feet in length, and another of twenty or thirty feet at the stern, then cut the cable and sail away on an inclined plane. There are a mother and half a dozen or more young spiders aboard every balloon, and thus the species is scattered over vast districts. These tiny aëronauts choose for starting on their voyage a clear day, temperature 60° Fahr., wind gently from the south. At about 1 they may be seen sailing with the wind. Toward 4 the spectator will observe that the balloons are beginning to descend. When the streamers strike some tall weed or grass the air-ships are made fast and the passengers instantly leap out, spinning out a thread as they fall, thus landing in safety.

A Demand of Modern Education.—In an address delivered on the occasion of the dedication of Pardee Hall, the scientific school attached to Lafayette College, Prof. Rossiter W. Raymond made some timely remarks upon the absurdity of attempting to complete a young man's education in the same time now as fifty years ago. The enormously-increased demands of modern life, said Prof. Raymond, requiring as they do that a man shall know more things, and know how to do more things, than were formerly sufficient for his reasonable success, are not to be satisfied by a mere change in a few subjects of instruction. It is not enough to substitute one study for another. The period of study must also be prolonged. In recognition of this principle, while it is for the present impracticable to make it an invariable part of a college education, by imperatively increasing the length of the college course, or by raising the standard of admission to colleges, the device of a post-graduate course has been very generally adopted; and it will not be long before experience will demonstrate that those men who have received the most thorough preparatory training are able to overtake and to outstrip in the subsequent race of life