Page:Science vol. 5.djvu/520

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��duced, continued to exceed in amplitude almost any that I have recorded.

Fortunately, however, thU earthquake was pre- vented from being exceMivel; deBtructlve by the uo- Uflual slowness of the oscillations, The psriod of the principal movements appears to have been not far aliort of two seconds, For a rough estimate of llie greatest velocity and acceleration, we may treat the 4.3 centimetres movement as simply harmonic; and we &id for the greatest veioclty 6.8 centimetres per second, and, for the greatest acceleration, 21 centi- metres per second, or jV of j/. If the amplitude of motion which was recorded here bad occurred in con- junction with the more usual period of three-quarters of a second or so. the destruetiou would liave been immense. The earthquake appears to have been felt over an area of about twenty thousand square miles.

Mr, Seklya writes, " We are going to exhibit your seismograph in the exhibition in London, to be held next May. I am sure we will gets first-priEe medal."

Whether Mr. Sekiya and the Tokio university au- thorities get their medaJ or not, they should at least excite admiration for the xeal and success with which they are pursuing the study of seismology.

J. A. EWINO. UnlTBnllf college, DudJm.

��ELECTRIC LIGHTING ON SHIPBOARD.

A PAPEB recently presented to the British institu- tion of civil engineers by Mr. .\ndrcw JamieBon gftve rise to anexceedingly Interesting and instructive discussion. The author of the paper considered the advantages of the electric light on board ship to be summed up in the following points: Its healtliful- ness; freedom from heat, odor, or gaseous products; its general agreeable n ess ; Its freedom from danger of setting fire to combustible material; removal of the danger of storage of Inflammable illurainants; avoidance of the nuisance of cleaning and refilling lamps; reduction of space occupied by total plant; and a fair competition in cost of Illumination.

The dynamo should be placed with its axis in the fore and aft line, in. order to reduce the gyrostatic effect caused by rolling, and thus to lessen the heat- ing of Its bearings. It should be capable of develop- ing the required electromotive force at Its regular speed; should be self-regulating; should not 'spark;' should not heat the conductors when running light; should contain, either in its own colls or lu the con- ducting system, not less than ninety-six per cent pure copper; and the system should have an Insulation resistance of not less than ten thousand ohms per volt, generated at the regular speed of working. The speed Is generally preferred to be under six hundred or sis hundred and fifty revolutions per minute. Higher speeds demand more careful supervision, give rise to danger of heated bearings and sometimes of bursting the armature, cause objectionable gyrostatic action in uneasy ships, and make it difficult to drive by direct connection.

��The engine should be capable of driving eontinu- ously and indeSnitely as to time, without dxnger of heating or break-down. Its governor should control the speed within five per cent,' with variatioa of s team-pressure ot ten pounds or more per sqtiare inch, and a variation of load of ninety per cent. i.e.. with full load, or with nothing on but the dyiuuxio. A tachometer, or continuous speed-indicator, is a valuable adjunct to the engine as ezhlbltiDg all varia- tions of speed. An electrical governor acting upon the throttle- valve is thought to be a desirable

��When not driven directly, the dynam connected to its engine by cotton rope, the ateel-i coiled belting coming into use in the United Stat«B not apparently having been introduced into Gre*l Britain. The Westlnghouse engine Is reported to be doing excellent work. Brotherhood's ' three-cylinder engines,' and the Tower ' spherical engine,' are also working satisfactorily. Friction pulleys have Iteeu used, In some cases. Instead of belting, for indirect connection.

The system of distribution h usually one of two principal kinds : in the one method, a set of return wires is used; in the other, the hull of the ship takes the return cnrreuts. The latler system is the laaa costly and more easily fitted, and gives rise to leM re- sistance: but it has the disadvantages thltt a fault In the leading wire has more effect than In the oUier, « contact with the bull short-circuiting the current; it Is more likely to be Injured by leakage of salt water upon the conductor. In which event corrosion gooson with serious rapidity; but care In protecting the wirea, and in placing Uiem. n^ucea the danger from the*a causes to a very small rea, according to the rule ol Sir Wi Thomson. But the author of the paper would ad< the rule: Make the conductivity of the wire than ninety-five per cent that of pure copper, give it a cross-section of a square millimetre for amp5re and a half of current, or about a square Idi to a thousand ampj^res; the insulaiion resiauiuce the whole circuit, Including switches, etc, ta be leas than a thousand ohms per volt of eleclromoUl force of the dynamo. Failures are usually due neglect of the precaution of testing the Inaulatli when the plant is put in place. Satety-vrirea, prevent the overheating of any part in case of wl crossing, should always be Introduced.

The size of lamp tdiould be ten-candle power ft staterooms or 'cabins,' tweuty-candle power for saloons and larger room*, and flfty to a b candle power for above-deck illumination. lamps of ten thousand to twenty thousand power are used on men-of-war tor llluminBtiog surroundings of the ship, and for protection the unobserved approach of torpedoes.

■ In thB UnHed flnWs, n vsrinllon of Iwo per mil tt

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