July 22, 1897] 



NATURE 



285 



and they have placed, this year, in the Exhibition of the 

 Imperial Institute a " univalve," or one-lens apparatus, of the 

 ihird order. Both these lights are on the/«?« c'clair system. 



Successful as this system appears to be, it should for the pre- 

 sent be regarded as on its trial, and as awaiting the collective 

 judgment of mariners on certain points. And meanwhile it 

 should be estimated as supplementing, not displacing, the 

 well-tried forms of apparatus at the disposal of the lighthouse 

 engineer. 



A notable adjunct to the new rapidly rotating lights is the 

 mercury float carriage, by which the effect of weight and fric- 

 tion is largely diminished. Another excellent quality of this 

 arrangement is its suitableness for stations where earthquakes 

 prevail, as it provides an elastic connection between the optical 

 apparatus and the pedestal, instead of the rigid bearings in the 

 older forms of carriage. If the Curaumilla lighthouse in Chili 

 had been fitted with the mercury-float, it is probable that it 

 would not have been wholly ruined by the shock. 



(6) The principle of occultation — that is, of interrupting the 

 fixed beam of a cylindrical lens of an oil light by a dark shade 

 moving round or over the flame, or in a gas light by cutting oflf the 

 gas — is acknowledged to be of extreme usefulness, and has been 

 applied to many of the fixed lights of our coasts. It is not that 

 any power is really added to the light, but that the quick alter- 

 nation of light ancl dark in various groups or periods imparts a 

 guasi-intensity to the beam, and a valuable set of characteristics 

 for the mariner. The law of contrast has a physiological effect 

 here, as it has in another manner with the/^?<.r Eclairs. It is an 

 extreme example, but it rests on trustworthy evidence, that the 

 occulting light of Ventotene Island, near Naples, which is o.»the 

 sixth order only, has been seen from a distance of nineteen 

 miles, its normal range being nine miles. Using the words of 

 Cicero with another application, it may be truly said, " Eo 

 magis elucet quo magis occultatur." 



Investigations into the amount of light reflected and trans- 

 mitted by certain kinds of glass — lighthouse glass among the 

 rest — have been most ably conducted by Sir John Conroy, Bart., 

 at Oxford, in 1888. lie traversed by newer methods and with 

 surer results the ground of many observers, from Augustin Fresnel 

 to Lord Rayleigh ; and he demonstrated that " the values of the 

 transmissioncoetiicients for light of mean refrangibility show that 

 for I centimetre the loss by obstruction amounts to 2 '62 per 

 cent, with crown glass, and I'I5 with flint glass." The re- 

 fractive index of lighthouse glass lying between i"52 and i '54, 

 the loss may thus be practically taken as 2\ per cent, for 10 

 millimetres of thickness. 



The intensities of lighthouse apparatus have of late been 

 diligently considered by lighthouse authorities with the view of 

 determining, once for all, the relative values of the six orders of 

 lights, and of the lamps appropriate to them, and also of pub- 

 lishing the results in the Admiralty Light-list. A large amount 

 of uncertainty and misconception seems to have prevailed on 

 this question. M. Allard, in 1876, in a celebrated memoir, gave 

 an elaborate exposition of the whole subject, but his conclusions, 

 always tending to excess, have not been accepted by more recent 

 investigators. 



Other estimates, official as well as private, and more or less 

 discordant, have from time to time appeared, both in this 

 country and France. The factors of evaluation, such as radius, 

 lens-surface, vertical section, flame, reduction for losses, &c , 

 have been combined in different ways by different persons, and 

 some of their conclusions must be regarded as merely empirical. 

 But in 1891 and 1892 a serious attempt was made by a committee 

 of the chief engineers of the three Lighthouse Boards to compile 

 an accurate schedule of intensities with photometry as a basis. 

 Taken as a whole, the values arrived at are fairly acceptable, 

 not certainly erring on the side of excess. 



But as yet only the lights with oil or gas for illuminants have 

 been determined. 



Electricity appears even more difficult to deal with, and no 

 intensities have been assigned officially to any of the electric 

 lights in the British Islands. 



Thus it is uncertain whether, for instance, we are to consider 

 the Isle of May as of six millions of candles, or of twenty- 

 six millions, both estimates resting on competent professional 

 authority. The lights on the fen tWrt«> system, oil and electric, 

 are obviously less amenable still to formulas which may give 

 their coefficients of intensity, although French writers do not 

 hesitate to define these, and deduce thirty or forty millions of 

 candles as " ayant la consecration de la pratique." 



NO. 1447. '^'OL. 56] 



It is much to be desired that as a sort of sequel to the publi- 

 cation of the intensities of lights— at all events, of the British 

 lights — there should be provided a form for every vessel under 

 the control of the Board of Trade, in which a return should be 

 made of the appearance of every light approached or passed, 

 with statement of the weather, distance, name and character of 

 the light, &c. These returns, duly kept and handed to the 

 Board of Trade on the first opportunity, would grachially 

 constitute an invaluable record of the merits or demerits of our 

 lights, instructive to the engineer, and, through him, beneficial 

 to the mariner. I have repeatedly, Ijut in vain, urged this 

 expedient on the authorities. 



The administration of lighthouses in this country has under- 

 gone no change in the past decade, nor, indeed, since 1861, 

 when the Royal Commission on Lights recommended that a 

 Central Board should l^e constituted instead of the quadriform 

 government then, as now, in force, and gave excellent reasons 

 for the recommendation. The further experience of thirty-six 

 years has amply confirmed the earlier conclusions on this 

 subject, and has brought into stronger relief the example of the 

 French Administration. Some slight approach to the desired 

 reconstruction may be indicated in the Report of the Committee 

 of Inquiry on the Mercantile Marine Fund of 1896, paragraph 

 71, in the following words: "From the evidence brought 

 before us we unanimously recommend the formation of a 

 small committee containing representatives, possessing as far as 

 possible nautical knowledge, of the Trinity House, the Scotch 

 Board and the Irish Board, which should be summoned at least 

 once a year to advise the Board of Trade upon the desirability 

 of all new works, whether in respect of lighthouses, steamers, 

 buoys or signals, together with all renewals, alterations and 

 important repairs." 



But however we may regard the system of government of our 

 lighthouses in contrast with the French system, and desire its 

 amelioration, it is impossible to deny that the United Kingdom 

 has during the Victorian era produced men who individually 

 have done fully as much in every part, theoretical and practical, 

 of lighthouse science as have the distinguished men of the sister 

 country. In one group we can point to the names of Faraday„ 

 Airy, Thomson and Chance. In another to those of Stevenson^ 

 Douglass, Hopkinson and Matthews. In yet another to those 

 of Farrar, Nisbet, Sydney Webb, Trevor, Wharton and Nares. 



These men have enriched and illustrated lighthouse mathe^ 

 matics, engineering, optics, mechanics and nautical and general 

 administration, in a manner and with a success to be gratefully- 

 remembered in our day, and never to be forgotten in the new 

 developments of the years to come. J. Kenward. 



\^Note. — I would invite the attention of visitors to the Imperial . 

 Institute to the very ingenious and effective illustration of the 

 progress of British coast lights during the past sixty years, by 

 means of two large illuminated maps and a relief plan. This , 

 is exhibited by the Trinity House, whose collection of models, 

 lenses, reflectors and burners is also very commendable. — ^J. K.] 



THE LIMITS OF AUDITION} 

 T N order to be audible, sounds must be restricted to a certain 

 -^ range of pitch. Thus a sound from a hydrogen flame 

 vibrating in a large resonator was inaudible, as being too low 

 in pitch. On the other side, a bird-call, giving about 20,(XXD 

 vibrations per second, was inaudible, although a sensitive flame 

 readily gave evidence of the vibrations and permitted the wave- 

 length to be measured. Near the limit of hearing the ear is 

 very rapidly fatigued ; a sound in the first instance loud enough 

 to be disagreeable, disappearing after a few seconds. A 

 momentary intermission, due, for example, to a rapid passage 

 of the hand past the ear, again allows the sound to be heard. 



The magnitude of vibration necessary for audition at a 

 favourable pitch is an important subject for investigation. The 

 earliest estimate is that of Boltzmann. An easy road to a 

 superior limit is to find the amount of energy required to blow 

 a whistle and the distance to which the sound can be heard 

 {e.g. one-half a mile). Experiments upon this plan gave for the 

 amplitudes x io"*cm. , a distance which would need to be 

 multiplied 100 times in order to make it visible in any possible 

 microscope. Better results may be obtained by using a 



1 Abstract of a lecture delivered at the Royal Institution on April 9, by 

 the Right Hon. Lord Rayleigh, F.R.S 



