Page:Popular Science Monthly Volume 23.djvu/445

Rh seaward at high water, and will swing back landward at the ebb. At sixty miles away from the Atlantic coast the deflection due to tidal action of this kind is probably quite as great as the greatest deflection due directly to the moon. These flexures are not confined to the surface, but must extend in almost equal degree beyond the depth of the deepest mine. Until the atmospheric pressure and the state of the tides at each moment can be accurately known for a great distance around any given spot, the experimental determination of the lunar disturbance of gravity is out of our reach; our instrument, even in the most favorable site, must needs record incessant variations of which no satisfactory account can be given. A gravitational observatory must, therefore, for the present content itself with registering the more or less irregular tremors of the earth that are allied with earthquake-movements.

English and Metric Measures.—The following comparative summary of English and French (or metric) standards of measures and weights will be useful: The English inch is equivalent to 2·54 centimetres; the foot to 30·48 centimetres; the yard, to 94·44 centimetres; the mile to 1609·33 metres; the nautical mile, to 1852·30 metres.

The square inch is equivalent to 6·45 square centimetres; the square foot, to 929·01 square centimetres; the square yard, to 8364∙13 square centimetres.

The cubic inch is equivalent to 16∙387 cubic centimetres; the cubic foot, to 28·516 cubic decimetres; the cubic yard, to 764·535 cubic decimetres; the pint, to 0·567 cubic decimetre; the gallon, to 45·410 cubic decimetres.

The English grain is equivalent to 64·799 milligrammes; the ounce avoirdupois, to 28·349 grammes; the pound, to 453·590 grammes; the ton, to 1016·050 kilogrammes.

The foot-pound is equivalent to 0·13825 kilogrammetre; the French horse-power, to 75 kilogrammetres a second; and the English horse-power (550 foot-pounds a second, or 33,000 foot-pounds a minute), to 76 kilogrammetres a second.

The metrical unit of pressure is the kilogramme per square centimetre. We may also count by centimetres of mercury or by atmospheres. A pressure of one atmosphere corresponds to a column of mercury 76 centimetres high, or to 1·033 kilogramme per square centimetre. The English pound per square inch corresponds with 70·3 grammes per square centimetre.

False Fangs in Harmless Snakes.—The blowing-viper or puffing-adder of the United States, although it is timid and unaggressive, and can rarely be provoked to bite, and then does no particular harm, has been reputed a dangerous and venomous reptile because it has long, fang-like posterior teeth. It belongs to the genus Heterodon, which is so called on account of this unusual and irregular dentition. It is of the same group as the Xenodons, or strange-tooths, of tropical America, which are also supposed there to be poisonous, possibly for no better reason than our North American genera are. Dr. Stradling allowed one to bite him, and suffered no ill effects from the wound. Mrs. Catharine C. Hopley states, in "Land and Water," that she has lately examined one of these strange-toothed snakes for the purpose of seeing the fangs, when she observed that they are mobile, or what are called in the viperine snakes hinged teeth—that is, they are so set that the snake can erect or depress them at pleasure by a volitional movement of the jaw. It is known, she says, that no poison-gland exists in this snake, "and it was only on venturing to feel the teeth, in order to judge of their relative sizes, that the reptile let me know it had a larger pair safely tucked away in case I took too many liberties."



toward fixing a system of uniform standards of time was made by the convention of railroad managers which was held at St. Louis, April 11th. Hour-lines were agreed upon at fifteen degrees of longitude apart, to which the clocks in the districts respectively appertaining to them shall be made to conform. Four standards were made: Eastern time, to agree with that of the 75th meridian; central time, one hour slower, to be regulated by the 90th meridian; and two other standards, two and three hours slower than Eastern time, to be fixed by the 105th and 120th meridians. All changes in time will be made at the termini of roads or at the ends of divisions. 