Page:Popular Science Monthly Volume 47.djvu/729

Rh in favor of the girls. Voluntary motor ability is measured by the number of taps the child can make in five seconds. The average child at six years taps 20·8 times in that interval. From this there is a gradual increase till the age of twelve, when the rate is 29·9 taps. This is lowered one tap at thirteen after which the increase is resumed and reaches a maximum at seventeen, when the rate of tapping is 33·8 in five seconds. The rate is higher for boys than for girls. After tapping for forty-five seconds fatigue enters into the results very noticeably. The fatigue is most marked at the age of eight and least marked at fifteen. Boys tire more quickly throughout in voluntary movement than girls, but as they act more vigorously it can hardly be said that they tire more easily. Boys have a larger lung capacity than girls throughout. Girls become nearly stationary in it at twelve, but boys do not begin their most rapid growth till they are fourteen years of age. The time of simple reaction decreases with age. The results, when considered for girls and boys separately, show marked differences in sex. The bright children react more quickly than the dull. But all react in about the same time just before those ages—eleven and sixteen—in which changes of growth manifest themselves. In the test for reaction with discrimination and choice, ability increased and the length of time required decreased with advance in age. This test implies more complicated mental activity, and the influences that affect mental life show themselves more plainly in the curve representing such development.

Uses of the Sand Blast.—It appears from an account of the applications of the sand blast given by Mr. J. J. Holtzapffel, in the English Society of Arts, that glass is almost immediately depolished by the blasts now in use, and only a little time is required to pierce and cut holes through sheet and plate glass. Stone, marble, slate, and granite are equally amenable to its action. Iron, steel, and other metals have their surfaces easily reduced and smoothly or coarsely granulated, according to the force and abrasive powder used. The abrasive need not be harder than the metal to which it is applied. The blast is used for frosting and decorating glass, the labeling of graduated measures, for removing hard scale from castings and forgings, for carvings and inscriptions in intaglio or relief on stone, slate, and granite, for delicate drawings for lithography, for removing fur and deposits in tubs and tanks, for cleaning off accumulations of paint and dirt within iron ships, for decorating buttons, for piercing the holes in glass ventilators, for marking pottery and ornamental tiles, for refacing grindstones, emery and corundum wheels, for granulating celluloid films for photography, and on wood to bring out the grain in relief, and, latterly, for blocks for printing.

Tuberculosis in Meat.—The Royal Commission appointed in July, 1890, to inquire into the effect of food derived from tuberculous animals on human health has reported, as the result of its five years' investigations, that it has obtained ample evidence that "food derived from tuberculous animals can produce tuberculosis in healthy animals. The proportion of animals contracting tuberculosis after experimental use of such food is different in one and another class of animals; both carnivora and herbivora are susceptible, and the proportion is high in pigs. In the absence of direct experiments on human subjects we infer that man also can acquire tuberculosis by feeding upon materials derived from tuberculous food animals. The actual amount of tuberculous disease among certain classes of food animals is so large as to afford to man frequent occasions for contracting tuberculous disease through his food." The commission thinks it probable that an appreciable part of the tuberculosis that affects man is obtained through the food. Tuberculous disease is observed most frequently in cattle and in swine. It is found far more frequently in full-grown cattle than in calves, and with much greater frequency in cows kept in town cowhouses than in cattle bred for the express purpose of slaughter. It is but seldom found in the meat substance, but principally in the organs, membranes, and glands. It is found in the milk of cows when the udder has been attacked by tuberculous disease, and seldom or never when the udder is not diseased. In the milk it is exceptionally active in its operation upon animals fed either with the milk or with dairy produce derived from it. Provided every part that is the seat of