May 5, 1881] 



NATURE 



Before drawing any comparison between these two 

 systems it will be just as well to describe how each has 

 been carried out 



The Messrs. Siemens have applied to their (No. i) 

 district six powerful lights, fixed at a height of 80 feet 

 from the street level upon tall latticed iron masts similar 

 to those which are used on many railways for signalling 

 purposes. They have also twenty-eight smaller lights, 

 carried upon special iron posts, considerably higher than 

 the ordinary lamp-posts, being 20 feet from the pavement. 

 The powerful lights, fixed high up in air, are used more 

 for the illumination of open spaces, and this system — a 

 special feature of Dr. Siemens' — has been very extensively 

 adopted at the Albert Docks, Blackpool, and other places. 

 The central station of the Siemens system is in Old Swan 

 Lane. Here three engines, supplied by Messrs. Marshall 

 of Gainsborough, of their semi-portable type, and of 10- 

 horse power each, fitted with an admirable automatic 

 expansion gear(Hartneirs) specially applicable to engines 

 used for electric lighting purposes, are tixed. Two of these 

 engines are always at work during the time of lighting, 

 but one is kept in reserve, ready at a moment's notice to 

 replace either that may fail. These engines, by means of 

 beltings and a counter-shaft, apply their power to various 

 dynamo-machines of the well-known Siemens type. Each 

 of. the high lights is worked by a separate and distinct 

 dynamo-machine, with which it is connected by separate 

 conducting-wires. The wires throughout the whole of the 

 City are of thick copper, very well insulated, and laid 

 under ground after the customary manner of laying down 

 wires for telegraphic purposes. The smaller lights are 

 worked by alternating currents on the system with which 

 we are familiar on the Thames Embankment applied to 

 the Jablochkoff candle. In Swan Lane there are two 

 alternating current machines each working two circuits 

 of seven lights each, the lamps being arranged so that 

 consecutive lamps are not in the same circuit, and by 

 that means, if any accident should occur to one set of 

 lamps it would only extinguish one out of two lights in a 

 street, and not throw the whole district into darkness, 

 which would be the case if all the lights were worked on 

 one circuit. The field-magnets in all the large dynamo- 

 machines are excited by a similar dynamo-machine, 

 while the magnets of the alternating current machines 

 are fed by currents from a smaller continuous dynamo- 

 machine. Each large machine absorbs between 4- and 

 5-horse power, but the alternating machines require 

 much less. The furthest light from the generating centre 

 — Old Swan Lane — is that in front of the Guildhall, which 

 is nearly three quarters of a mile distant, involving a 

 length of wire of 2500 yards (a return wire being used), 

 whose total resistance does not exceed one Siemens' unit. 

 The illuminating power of the high lights is estimated to 

 be 6000 candles, but it is well known that this estimate of 

 the illuminating power of an electric light is a very wild 

 one. There is no doubt that the lights are very powerful, 

 and a stream of brilliant illumination is thrown all over 

 such an area as that in front of the Royal Exchange. 

 The Siemens' lamps burn for eighteen consecutive hours, 

 owing to the size and length of the carbons used. They 

 are provided with reflectors which throw a bright cone of 

 light down in a very peculiar way, giving to this experi- 

 ment a very marked feature. 



There is no doubt whatever that where it is required to 

 illuminate a large area this is very much more efficiently 

 and economically done by using one single powerful light 

 high up in air, than by distributing several smaller lights 

 over the surface. In the former case the light is more 

 evenly, uniformly, and perfectly diffused, in fact it acquires 

 the character of bright moonlight, while in the latter case 

 the light is distributed in patches of intensity and dark- 

 ness over the whole space. 



When streets are dealt with the conditions are different, 

 and it is here 'quite easy to show that economy and effi- 



ciency are provided for by properly distributing smaller 

 lights along the street. The Messrs. Siemens have set 

 to work to solve this problem in a scientific way, and 

 Mr. Alexander Siemens, under whose control and 

 management the system has been carried out, can show 

 mathematically that to distribute light uniformly and 

 properly a certain definite proportion should exist be- 

 tween the height of the posts and the distance at which 

 they are apart. That this has been carried out is abun- 

 dantly evident by the very even way in which light is 

 distributed along Queen Street, King William Street, and 

 Cheapside. Indeed it is difficult to see any break in the 

 intensity of the light along the route — a proof that the 

 practical application of the law very nearly approaches 

 its theoretical limit. The theoretical point to be aimed 

 at is that the height of the poles should be to half the dis- 

 tance between them as i is to the square root of 2. This 

 has not been absolutely obtained, but a very close ap- 

 proach to it. The small lights only give an illuminating 

 power, according to Messrs. Siemens, of 300 candles, and 

 this probably is well within the mark. Comparisons be- 

 tween lights of low intensity are very easily and accurately 

 measured ; it is only when a power equal to thousands of 

 candles is arrived at that the failure of comparison with a 

 standard candle becomes evident. 



The high lights have not been burning uniformly with 

 that steadiness that success demands. Instances of 

 failure are not numerous, though they have been frequent. 

 The smaller lights, on the other hand, have worked more 

 uniformly, and have given considerable satisfaction. 

 The strong shadows thrown by the high lights have a very 

 weird-like effect in certain positions, and the vibration 

 of the lamp gives to the shadow of the pole that supports 

 it an unsteadiness that has led the unwary to imagine in 

 many instances tVat the pole itself was shaking. Could 

 the shadows be removed from the effects of these high 

 lights the effect would be very fine ; as it is they detract 

 enormously from the beauty of the lamps. The effect of 

 the high lights to those standing below is excessively 

 pleasing, and doubtless in warmer weather will be more 

 highly appreciated than it has been during the past week. 

 It is when crossing streets, and especially when crossing 

 such a busy thoroughfare as that in front of the Mansion 

 House that these lights show their efficiency to advan- 

 tage. It is quite amusing to see how the i^amins of 

 London have taken advantage of the combination of 

 electric lighting and asphalte road to convert the whole 

 City into a gigantic skating-rink. Hundreds of boys are 

 to be seen every night disporting themselves on their 

 roller-skates. 



(TV be continued.) 



WEATHER WARNINGS 



IN a lecture on Solar Physics delivered at South 

 Kensington on Friday last Prof. Balfour Stewart 

 stated that he believed one great cause of weather 

 changes to be solar variability in which we have periods 

 of short length, as well as others extending over many 

 years. 



These weather changes, it is sufficiently well known, 

 are propagated from west to east after they have once 

 appeared. 



Again there are variations in the diurnal dechnation 

 range which may be said to constitute magnetic weather. 



These are also caused by solar variability, and it is 

 suspected that they are likewise propagated from west to 

 east, although more quickly than the well-understood 

 changes of meteorological weather. 



It would thus appear to be at least possible that British 

 magnetical weather of to-day may be followed by corre- 

 sponding meteorological weather five or six days hence. 



Prof. Stewart has made a preliminary trial, which 

 induces him to think that this is the case, and that it 



