December 15, 1917 SCIENTIFIC AMERICAN SUPPLEMENT No. 2 189 373 place in the General Staff along with the tacticians. Technical and industrial problems arising for solution become daily more numerous as the armies progress, and more closely allied with the military operations. Other reconstruction work back of the lines of almost equal importance to the above is that of remaking by modern magic the cast-off wreckage of war. Few persons in this country have any idea of the tre mendous restoration

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... of the wreckage of battle that is going on behind the allied lines in France, work that is saving millions of dollars a year; and yet there are few things in the conduct of the war more wonderful. To the military workers in charge of this hive of industry is brought all the jetsam of battle, from a broken rifle or bicycle to tattered tunics and derelict shoes and boots-all to be renovated and made serv iceable again. There are shed after shed piled to the roof with dis carded shoes, most of them in such a condition that is deemed nothing short of a miracle could restore them. In former years they would all have been consigned to the rubbish heap as worthless. Not so now. Stage after stage shoes broken in war are again, by military workers, quickly converted into boots, soft and strong. First they are soaked in a mixture which renders the leather pliable as ever. They are then scrubbed and rubbed and patched and soles or heels put on where required. Over 2,000 pairs are renovated in this way every day. In other field workshops khaki uniforms tattered, torn and mud soiled are made good as new. After passing through a disinfecting bath they are carefully dried and then taken in hand by a small army of tailors who with wonderfuJ cleverness and ingenuity remake them. Then they are returned to the army ready for service again. There are also hospitals for wounded cannon, machine guns and rifles where furnaces blaze nig)J.t and day, and the vulcans work amid a thunderous crash and roar. Here are broken rifles to which a new lease of life is given. The butts and wooden parts are repaired or renewed; the damaged metal part� are replaced by new ones, rusty barrels are freshened up, and so on, and in a day or two these heaps of old iron are, for all practical pur poses, so many brand new rifles doing deadly execution in the trenches. Close by you will see thousands of broken bicycles so much scrap iron to look at, beyond all hope of repair. Here again the magician is at work. With amazing skill and rapidity the damage is repaired, broken parts are replaced and the machines, restored to usefulness again, are returned to their units. WE have receiv�d repeated requests from some of our subscribers for information as to the remedy for the fish louse which invades their sheets of water, destroying their carp and their salmonides. The following article is a response to these requests. The fish louse, or carp louse, whose scientific name is Argulus foliaceus L. is a small crustacean belonging to the group of the branchiurae which sometimes pullulate in sluggish waters, and are then found, sometimes in very large numbers, on the skin of fishes. Its most frequent victims are carp, less often tench and bream, sometimes trout. It has an oval form and its size varies from 1 to 5 millimeters according to the age and sex. The back resembles a shield or "buckler" in shape, while its ventral surface presents numerous appendices. It is as thin as a sheet of paper and appears upon the fish under the aspect of a yallowish green spot. It fast3ns itself rapidly and firmly by means of two powerful " suckers" situated below the eyes, and by two hooks borne upon its anterior antennae. The mouth is pr0vided with a proboscio of fine-toothed mandibles and exceedingly sharp edged jaws. It is surmounted by a keen retractile stylet, piarced by a channel wherein flows the venom of the dijective glands. The alimentary canal is widely ramified and its numerous diverticles permit the storing of a sufficient quantity of nourish ment to enable the argule to go without eating easily for more than a week. Thus armed, the fish louse has all the needful tools for making a good livelihood and his life is not as monotonous as it would seem. Sometimes he hooks onto a fish, especially where the skin is delicate, chiefly at the junction of the fins, which the fish cannot scrape against water plants or on the bottom. Sometimes, having fed abundantly, he quits his prey and swims away, either for mOUlting or for reproductive purposes. These duties accomplished he seeks a new host. Generally the argulus are present in numbers ��o La Nature (Paris). small to do much damage. But, at times, when the conditions are favorable, they increase rapidly, and the fish, pricked and sucked from by hundreds of the lice become anemic and ulcerated and finally die. The males are smaller than the females and swim more rapidly; they are also much less numerous; they can be recognized by two black points situated at the base of the abdomen. The females lay their eggs, num bering hundreds and glue them to stones and water plants. About a month later the larvae ara hatched, already provided with hooks, but without suckers. A few days later the first moult takes place, soon followed by others, and six weeks later the argulus begin to re produce themselves. Since each female usually lays three times during the summer it is obvious how rapidly they can infest a sheet of water and becoma dangerous to fish breeding. But generally this does not occur. Cold water arrests their multiplication, small surface fish devour a large number of the larvae, and finally healthy fish, "seem to be no more incommoded by them than a dog by his fleas," according to Wilson, a naturalist in the Amarican Bureau of Fisheries. But when the water warms up for a considerable period of time, when the fishes lan guish and become less resistant, the argulus soon become a veritable calamity. Heat, in fact, provokes a rapid increase of the parasites at the.same time that it enfeebles the host. The latter soon becomes covered with the lice; his fins are gnawed his tail slashed, his skin becomes punctured, torn, bleeding. The pond is soon covered with corpses, and the evil grows worse and worse. The lice quit their victim as soon as it is dead and precipitate themselves upon another. Sometimes sheets of water are entirely depopulated in a few days. Prophylactic measures against this brood ar'e easier to take than therapeutic ones. Aquatic plants, furnish ing shade, prevent the water from becoming heated, and an abundance of surface fish destroys great numbars of the argulus and arrests their increase. Deep bodies of water are a'ways much less infested, as also those which are fed by springs at the bottom. The water supply can also be filtered by means of a sand bar. When the argulus have already made their apperance remedial measures are much less efficacious. Branches of trees should be thrown into the water against which the fish can scrape themselves to remove their parasites. An especially excellent means of ridding a pond of such invasion is to take advantage of the winter cold to drain the pond and leave it dry for some time. But as in so many other cases prevention is better than cure. Magnetization by Rotation THE idea that a rotating magnet should behave like a gyros tat if the ampere currents consist in the motion of actual matter is an old one, and Maxwell constructed an apparatus for testing it as early as 1861. The sug gestion does not imply, however, that the magnetization generated by rotation of a magnet should simply be pro portional to the angular velocity. For if that were so, laboratory experiments on small magnets at a few revolu tions per second should give rise to magnetic forces vastly greater than the terrestrial magnetism and greater than any magnetic forces obtainable by other means. Professor A. Schuster discussed these problems, which had long been occupying him, in his presidential address to the Physical Society in 1912, with special regard to Causes of Terrestrial Magnetism. Since 19 09 Prof. S. J. Barnett, of the Ohio State University, has been experimentally investigating tha subject, and he has given an account of his researches in two long papers published in The Physical Review of October, 1915, vol. vi. pages 239 to 270, and of July, 1917, vol. x. pages 7 to 21. Like Schuster and Langevin, he starts from the assumption that rotation only sets up a magnetic force, which may © 1917 SCIENTIFIC AMERICAN, INC. or may not produce magnetization, and will only do so if the substance be magnetic and therefore constituted of atomic or molecular orbital systems with individual magnetic moments fixed in magnitude. Barnett's orbital systems consist of a number of electrons revolv ing at constant average velocity in fixed orbits about an oppositely-charged nucleus, and he further considers that the electrons would preponderantly, if not alto gether, be negative, as is generally assumed for various reasons. On the electron theory he then shows that a body of any substance set into rotation becomes, by a sort of molecular gyroscopic process, the seat of a uni form intrinsic magnetic intensity parallel to the axis of rotation and proportional to the angular velocity, the intensity being directed-like the magnetization of the earth and of the sun-oppositely to the intensity which would be produced by an electric current flowing round the body in the sense of the rotation. The in trinsic intensity per unit angular velocity would be Hln=4rrmle=-7.1XlO-7 gauss, where H is the in tensity and n the number of revolutions per second, and m and e are mass and charge of the negative electron. Barnett also discusses the theory of the converse effect, rotation caused by magnetization, which was established by A. Einstein and W. J. de Haas in their "Demonstra tion of Ampere Currents" (see Engineering, August 27, 1915, page 224), and this has sinbe been further con firmed by de Haas in a communication on "Moment of Momentum existing in a Magnet," presented to the Amsterdam Academy, 1916. In his experiments Bar nett made use of steel rods, from 2 cm. to 7 cm, in dia meter, nearly a metre in length, turned about their longitudinal axes at from 20 revolutions to 90 revolu tions (generally about 50 revolutions) per second by an electric motor. In each case two similar rods were observed, the one rotating, the other stationary and very elaborate precautions were adopted to reduce the effects of the numerous sources of error; there was always e.g., a slight fluxmeter deflection independent of the direction of rotation. The recent experiments were made with the aid of a magnetometer-not a fluxmeter -on two cold-rolled steel rods about 30 cm. long, 2 cm. or 3 cm. in diameter, and on rods of nickel and cobalt of similar dimensions. Again the results were not very concordant. But while the former series of experiments had yielded the average value HI n = -3 X 10-7 gauss per revolution per minute, the new series yielded the co efficient 5 instead of 3, against the theoretical value -7.1. In this theoretical estimate it is assumed that only the negative electrons are concerned in the phenom enon; if the positive electrons also had a part the ex perimental value should be smaller than the theoretical. A smaller value has been found. Barnett considers however, that the actually observed effect is all due t� negative electrons, and that it is too small owing to ex perimental difficulties. That there is no such effect in non-magnetic metals had already been shown by Lebe dew.-Engineering.