484 



SCIENCE 



[Vol. V., No. 123. 



duced, continued to exceed in amplitude almost any 

 that I have recorded. 



Fortunately, however, this earthquake was pre- 

 vented from being excessively destructive by the un- 

 usual slowness of the oscillations. The period of the 

 principal movements appears to have been not far 

 short of two seconds. For a rough estimate of the 

 greatest velocity and acceleration, we may treat the 

 4.3 centimetres movement as simply harmonic ; and 

 we find for the greatest velocity 6.8 centimetres per 

 second, and, for the greatest acceleration, 21 centi- 

 metres per second, or -£- T of g. If the amplitude of 

 motion which was recorded here had occurred in con- 

 junction with the more usual period of three-quarters 

 of a second or so, the destruction would have been 

 immense. The earthquake appears to have been felt 

 over an area of about twenty thousand square miles. 



Mr. Sekiya writes, " We are going to exhibit your 

 seismograph in the exhibition in London, to be held 

 next May. I am sure we will get a first-prize medal." 



Whether Mr. Sekiya and the Tokio university au- 

 thorities get their medal or not, they should at least 

 excite admiration for the zeal and success with 

 which they are pursuing the study of seismology. 



J. A. Ewing. 



University college, Dundee. 



ELECTRIC LIGHTING ON SHIPBOARD. 



A paper recently presented to the British institu- 

 tion of civil engineers by Mr. Andrew Jamieson 

 gave rise to an exceedingly 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 healthful- 

 ness ; freedom from heat, odor, or gaseous products ; 

 its general agreeableness ; its freedom from danger 

 of setting fire to combustible material; removal of 

 the danger of storage of inflammable illuminants; 

 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 coils or in 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 six 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 continu- 

 ously and indefinitely as to time, without danger of 

 heating or break-down. Its governor should control 

 the speed within five per cent, 1 with variation of 

 steam-pressure of ten pounds or more per square 

 inch, and a variation of load of ninety per cent, i.e., 

 with full load, or with nothing on but the dynamo. 

 A tachometer, or continuous speed-indicator, is a 

 valuable adjunct to the engine as exhibiting all varia- 

 tions of speed. An electrical governor acting upon 

 the throttle-valve is thought to be a desirable instru- 

 ment. 



When not driven directly, the dynamo is, as a rule, 

 connected to its engine by cotton rope, the steel-wire 

 coiled belting coming into use in the United States 

 not apparently having been introduced into Great 

 Britain. The Westinghouse 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 been 

 used, in some cases, instead of belting, for indirect 

 connection. 



The system of distribution is 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 currents. The latter system is the less 

 costly and more easily fitted, and gives rise to less re- 

 sistance : but it has the disadvantages that a fault in 

 the leading wire has more effect than in the other, a 

 contact with the hull short-circuiting the current; it 

 is more likely to be injured by leakage of salt water 

 upon the conductor, in which event corrosion goes on 

 with serious rapidity ; but care in protecting the wires, 

 and in placing them, reduces the danger from these 

 causes to a very small quantity. 



It is of great importance that the junctions of 

 wires should be very carefully and thoroughly sol- 

 dered ; and the size of wire should be such that it 

 should give at least a square centimetre area per 

 fifty amperes, according to the rule of Sir William 

 Thomson. But the author of the paper would adopt 

 the rule : Make the conductivity of the wire not less 

 than ninety-five per cent that of pure copper, and 

 give it a cross-section of a square millimetre for an 

 ampere and a half of current, or about a square inch 

 to a thousand amperes ; the insulation resistance of 

 the whole circuit, including switches, etc., to be not 

 less than a thousand ohms per volt of electromotive 

 force of the dynamo. Failures are usually due to 

 neglect of the precaution of testing the insulation 

 when the plant is put in place. Safety-wires, to 

 prevent the overheating of any part in case of wires , 

 crossing, should always be introduced. 



The size of lamp should be ten-candle power for 

 staterooms or ' cabins,' twenty-candle power for the 

 saloons and larger rooms, and fifty to a hundred 

 candle power for above-deck illumination. Arc- 

 lamps of ten thousand to twenty thousand candle 

 power are used on men-of-war for illuminating the 

 surroundings of the ship, and for protection against 

 the unobserved approach of torpedoes. 



1 In the United States, a variation of two per cent is con- 

 sidered too great. 



