798 EEPORT— 18H6. 



intensity at disposal that a small percentage of penetrating efficiency is of no prac- 

 tical importance. 



In 1836 Faraday showed by actual experiment that the penetrating power of a 

 light in atmosphere impaired by such obstruction as fog, mist, &c., is but very 

 slightly augmented by a very considerable increase in the intensity, and M. Allard, 

 late Engineer-in-Chief to the French Lighthouse Board, has more recently shown, 

 after long experimental and practical research, that, in an atmosphere of average 

 transparency, a beam of hght equal to 6,250 bees (Carcel) would penetrate 

 53 kilometres, yet when augmented to twenty times that intensity, or 125,000 bees 

 (Carcel), it would only penetrate 7,540 kilometres ; showing that, in the average 

 condition of atmospheric transparency, 2,000 per cent, of increased intensity only 

 gives 42 per cent, longer range. 



The South Foreland experiments have demonstrated that, while with both gas 

 and oil an ordinary intensity of light can be adopted for clear weather sufficient to 

 reach the sea horizon with efficiency for the mariner, a maximum light can be shown 

 with impaired atmosphere fifteen to twenty times this intensity, and that in these 

 respects both illuminauts are practically on an equality. This maximum light of 

 gas and oil is considered by the Committee to be sufficient for all the ordinary pur- 

 poses of navigation, and, for this, mineral oil is the most economical illuminant ; 

 but for some special cases, where the utmost intensity and penetration are demanded, 

 these results can only be attained by electricity, and by this agent an intensity more 

 than ten times that of the maximum of either oil or gas is found to be practically 

 available. 



With regard to the gas and oil lights, the report of the Committee states that : 

 ' It appears from the direct eye observations, made at distances varying from 3 to 

 27 miles in clear weather, that through annular lenses, light for light, there is 

 practically no difference. Both reach the horizon with equal effect. In weather 

 not clear the records indicate practically the same relation. In actual fog, again, 

 the records indicate a general equality of the lights. Both are lost at the same 

 time, both are picked up together: and although here and there a very slight 

 superiority is attributed to the gas, this superiority is of no value whatever for the 

 pvu'poses of the mariner.' A point referred to in favour of gas is the well-known 

 one of greater handiness and ease of manipulation than oil, which is of importance 

 for small beacon lights, where a constant attendant is not provided ; but this does 

 not apply to a coast light, where a light-keeper is always required to be on the 

 watch in the lantern from sunset to sunrise. With oil the great advantage, in 

 addition to economy, lies in the simplicity of its application to a coast light- 

 house in any part of the world, however limited the space the hghthouse 

 is necessarily required to occupy. The final conclusion of the Committee on the 

 relative merits of electricity, gas, and oil as lighthouse illuminants is given in the 

 following words : — ' That, for ordinary necessities of lighthouse illumination, 

 mineral oil is the most suitable and economical illuminant, and that for salient 

 headlands, important landfalls, and places where a very powerful hght is required, 

 electricity offers the greatest advantages.' 



In conclusion it may safely be asserted, now that the relative merits of electri- 

 city, gas, and oil have been accurately determined, that these investigations of the 

 Trinity House Committee will, for many years to come, furnish to the lighthouse 

 authorities of all maritime nations of the world, and their engineers, very valuable 

 data which cannot fail to assist very largely in the development of lighthouse illu- 

 mination, and thus tend very materially to the present aids to navigation, and to a 

 consequent reduction in the loss of life and property at sea. 



The following Papers were read : — 



1. On some poiiits for the Consideration of Unglish Engineers with reference 

 to the Design of Oirder Bridges. By W. Shelfoed, M.Inst.G.E., and 

 A. H. Shield, Assoc.M.Inst.C.E. — See Reports, p. 472. 



