A Treatise On The Principles And Practice Of Harbour Engineering

2/4 HARBOUR ENGINEERING. Powerful lights in clear weather may easily exceed their geographica! range, i.e. the distance at which, owing to the earth’s rotundity, they cease to reach the eye of the observer. This distance varies with the respective heights of the light and of the observer, and also with the degree of latitude. The latter affects the radius of curvature (R) ; but, assuming it to be known as also the levels (H and h) of the source of light and of the station of ob- servation, the limiting distance D is given by the formula D=Ä:{5/RH+ ^RÄ) where It is a coefficient representing the effect of atmospheric refraction. Identification of Light Signals.—For the purpose of identification, various characteristics are conferred upon lights. Formerly colours were largely relied upon, but the great difference in range of the three chief varieties of light, viz., white, red, and green, militates very much against the efficiency of the method. Red cannot be seen at half the distance penetrable by white light, and green is even less powerful. At a distance of two miles, a white light of 3 candle-power is readily discernible, while from 30 to 40 candle-power would be requisite to bring a red or green light into equal prominence. Moreover, there was not much scope for variation with merely three alternatives. With the introduction of the group Hashing System, devised by Dr Hopkinson in 1875, a new and preferable means of identification came into vogue, and its utility has been still further extended by the introduction of lightning flash-lights under the inspiration of M. Bourdelles. The first consists of a definite series of illuminations and eclipses, variable in extent and sequence. The latter derives its title from the extreme rapidity of its appearance and disappearance, the period of visibility being the minimum required for imprinting a distinct visual impression. Further exposure is now found to be unnecessary, as it is covered by the persistence of the image on the retina. Light rays, accordingly, instead of being uselessly expended in emphasising their effect on one point, may be deflected to another, with much more serviceable results. The principle, in fact, is that of a highly concentrated beam rotating rapidly and reappearing at intervals of a few seconds. The duration of the flash, though short, is ample for récognition, and its frequent appearance, besides affording greater scope for character- isation, enables the mariner to verify his position with greater assurance than was feasible with an arrangement of slowly moving lights It was not impossible, of course, to provide a considerable number of flashes under the system of rotation on rollers, but it could only be done by increasing the number of lens panels at the expense of luminous power; whereas, by the introduction of the mercury seating, a much greater rotary velocity may be imparted to the apparatus, with less effort and friction. In some cases of older lenticular apparatus, the number of panels was as high as twenty-four. The diffusion of light, therefore, was very great, and the beams suffered correspondingly in intensity and penetration.