Dec. ii, 1884] 



NATURE 



I2 5 



Gegenwart." It is clearly printed, has numerous illustra- 

 tions, and the information, which is excellently arranged, 

 is brought down to the latest date and is very full. The 

 volume and the series are of a kind more numerous and 

 popular in Germany than in England. 



LETTERS TO THE EDITOR 

 [ Tie Editor doesnot hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 

 [ The Editor urgently requests correspondents to keep their letters 

 as short as possible. The pressure on his space is so great 

 that it is impossible otherwise to insure the appearance even 

 of communications containing interesting and novel facts.'] 



The Prime Meridian Conference 



1 x La Nature of November 22 (p. 399) appears what is repre- 

 sented as information obtained at the meeting of the Academy 

 of Sciences at Paris on November 17. It is stated that the 

 proposal made by Prof. Janssen at the Meridian Congress at 

 Washington, relative to the application of the decimal system to 

 the measurement of angles and time, obtained a majority of 24 

 votes against 21, notwithstanding the "opposition tris-vive" of 

 the English and Americans. The vote to which reference is 

 made was not on the merits of Prof. Janssen's proposal, but 

 merely whether the opinion of the President that the Congress 

 was not competent to entertain it, should be upheld or not. The 

 decision being in favour of considering it, the proposal was 

 accepted unanimously. On turning to the Comptes Rendus of 

 the Academy I find it simply stated that M. Janssen observed 

 that his proposition had been accepted almost unanimously, and 

 without a vote in opposition. 



La Nature further refers to the British delegates as having 

 made the discussion on the prime meridian a question of 

 "amour-propre" and as having converted to the British cause 

 most of the representatives present. This statement is no less 

 inaccurate and misleading than the former. As M. Janssen 

 himself remarked at Washington, England did not make the 

 proposal to adopt the meridian of Greenwich, and though the 

 Uiitish delegates differed from their French colleagues as to the 

 considerations which should govern the choice of a prime 

 meridian for longitude, there was not a word said by them to 

 justify what is stated by La Nature, and it is manifestly absurd 

 to speak of the conversion of the representatives to the British 

 cause, inasmuch as it is a perfectly well-known fact that almost 

 every one of them came to Washington with instructions from 

 their own Governments to vote for the Greenwich meridian. 

 In justice to M. Janssen I wish to add that the Comptes Rendus 

 makes no reference whatever to anything having been said by 

 him on this subject. 



It is greatly to be regretted that a journal professing to be 

 scientific should have given a colour to the discussions which 

 took place at Washington that forcibly suggests a deliberate 

 intention of exciting national jealousies and animosities. 



Richard Strachev, 



December 5 Late Delegate at Washington 



It is to be regretted that the French delegates have declined 

 to accept some of the resolutions of the Prime Meridian Con- 

 ference, but it is to be hoped that their non-adherence is only 

 temporary ; at the same time it must be admitted that their con- 

 tention that Greenwich is not a scientific starting-point for a 

 universal meridian has much to be said for it ; the zero of 

 longitude ought certainly to be defined somewhere on the 

 equator, and if it were to be hereafter so defined at a point on 

 the equator having the same meridian as the Greenwich instru- 

 ment it is probable that all difficulty would be removed. "J he- 

 French are known to attach importance to ideas, and doubtless 

 do not like the apparent supremacy which would be conferred 



on Greenwich if it were made the actual centre of departure. 

 The point in question lies somewhere in the Atlantic Ocean, 

 and is therefore on perfectly neutral territory. 



One of the great obstacles to the introduction of the French 

 metrical system into this country lies in the forbidding and 

 inconvenient nomenclature attached to it. If the long com- 

 pound names were translated into short English monosyllables, 

 such as met, iim, mini, &c. , not only would their use be greatly 

 advanced and facilitated, but the French nation would in time 

 borrow back from us our nomenclature. Such words offend at 

 first sight by their new and startling aspect, but this all wears 

 off in an hour or two ; they require however to be started by 

 some one in authority. There is a strange and unreasonable 

 prejudice in the present day against the introduction of new 

 monosyllabic words without derivation, which happily for us did 

 not prevail in the days of our early forefathers. 



It is desirable that at future meetings of the Conference the 

 question of astronomical nomenclature should be considered ; 

 the practice of using the same names for sidereal and mean time 

 is extremely inconvenient. I have suggested that the sidereal 

 hour should be called a sid, or sider, and the second a cron, so 

 that sidereal time would be indicated by the letters s, m, and c. 

 Some such change is greatly needed, and new names should also 

 be assigned to minutes and seconds of arc. 



Londm, December 1 Latimer Clark 



The Electric Light for Lighthouses and Ships 



The application of the electric light to lighthouses and ships 

 appears to me to be capable of considerable extension by a 

 modification of the apparatus used. In lighthouses the practice 

 is to have a fixed light in the lantern, with an apparatus either 

 catoptric or dioptric, or a combination of both, for the purposes 

 of bringing the rays of light from the arc into a parallel beam 

 and sending them to the horizon. Sometimes, if not generally, 

 this beam is cylindrical, and sweeps round at intervals of time as 

 the combination of lenses and reflectors is rotated. 



In the case of ships the head-light is an ordinary arc light, and 

 searchers in use on men-of-war are arc lights set in the focus of 

 a parabolic reflector, and pointed straight at the object it is 

 wished to light up. 



The arrangement that I would suggest as partly applicable to 

 lighthouses and fully applicable to ships would be to use a fixed 

 arc-light and large parabolic reflector in combination with aSIarge, 

 light, plane or suitably curved minor to direct the beam of light, 

 rendered parallel and cylindrical by the parabolic reflector, in 

 any direction by means of this mirror only. 



To apply this principle to a lighthouse, this light movable 

 mirror would be placed in the lantern at an angle of something 

 less than 45° with the vertical ; the arc light and the fixed para- 

 bolic reflector would be placed below, centrally, in the tower ; 

 the light would then come from the parabolic reflector on the 

 plane mirror, and so be sent in the required direction. 



In using this mirror, where the light has to sweep over an 

 angular area of less than 360 , I would use ato-and-fro motion, so 

 that if the time of each sweep from side to side was 30 seconds 

 of time, then any vessel in the middle line would have the light 

 at this interval, but at any angular distance from the centre line 

 the duration of the flashes would differ until, at the extreme 

 range, two would be seen almost together, with almost 60 

 seconds interval between them and the next two, the sum of the 

 time of two intervals always being the double of the fixed time 

 for that light, and the difference between two intervals for all 

 positions off the central line would enable the distance from 

 the centre line to be determined by a vessel within the 

 range of the light. An arrangement similar to this would 

 answer for masthead lights for ships, the arc light and para- 

 bolic reflector being below deck, a light metal tube, termin- 

 ating with a lantern to carry the plane mirror, going from the 

 deck up to the required height in font of the foremast ; the 

 movement in azimuth of this mirror might be of the same kind 

 as that mentioned for the lighthouse, but a much quicker motion 

 from side to side, through iSo" in about five seconds, would then 

 give this time for all points in a straight line ahead, but vary at 

 the sides in the manner already mentioned. As the light plane 

 mirror has only to be moved, a clockwork arrangement would 

 answer perfectly well for this purpose. In rough weather, when 

 the vessel rolled, the light would have a tendency to vary too 

 much in the vertical direction, but it would not be difficult to 

 make the correction by a gravity counterpoise. 



For war-ships such an arrangement, but on a more powerful 



