Dec. 4, 1879] 



NATURE 



109 



by Mr. Andrew Garrett, and described in the Froc. 

 Acad. Nat. Sci. Philadelphia. One of the most interesting 

 species is Partula hyalhia (Broderip), found abundantly 

 in three groups of islands. In Tubuai, 100 miles east of 

 Rurutu, it is abundant, and the Austral group appears to 

 be its metropolis. It is found, though sparingly, in 

 nearly every valley in Tahiti. It was also obtained by 

 Mr. Garrett at Mangaia, one of the Cook's, or Hervey 

 Islands, 400 miles west of Rurutu. The variation in 

 examples from the three groups is remarkably slight. It 

 is a strictly arboreal species, and has a uniform white 

 colour. 



Another species, Stenogyra juncea, Gould (sp.), is 

 found very widely through Polynesia, in all groups north 

 of the equator, and south of all islands from the Marquesas 

 and Paumotus, to the Viti group, and probably ranging 

 further west ; they are found under loose stones, beneath 

 decayed wood, among dead leaves, &c, and range from 

 near the sea-shore to 2,000 feet above the sea. Another 

 well-known genus, Suctinea, is now recorded from Rurutu, 

 slightly differing from a Tahitian species, S. pudorina 

 <Gould). 



C/iondrella{Vts.st)\% remarkable for having no tentacles ; 

 during locomotion the animal is nearly or quite concealed 

 by the shell, which is carried diagonally. In creeping, 

 only the extreme tip of the muzzle is seen from above, 

 while the eyes are plainly visible through the transparent 

 shell. The extreme interest of the fauna of oceanic 

 islands becomes continually more evident. 



DISTINGUISHING LIGHTS FOR LIGHT- 

 HOUSES 



SIR WILLIAM THOMSON writes a long letter on 

 this subject to the Times of Tuesday, the letter being 

 the result of a most interesting experimental cruise of ten 

 days on board Her Majesty's ship Northampton, in the 

 English Channel, from which he has recently returned, 

 having had many good opportunities of observing the 

 lights on the south coast of England. This has revivified 

 his conviction of need for a threefold reform in our light- 

 house system, which he has been urging and re-urging 

 since 1872 with hitherto but partial success : — A great 

 quickening of nearly all revolving lights ; the application 

 of a group of dot-dash eclipses to every fixed light; and 

 the abolition of colour as a distinction of lighthouse lights, 

 except for showing dangers and channels and ports by 

 red and white and green sectors. Of about 120 revolving 

 lights on the English, Scottish, and Irish coasts, there 

 are in all eighteen in which the periods are ten seconds 

 or less and the times of extinction seven seconds or less. 

 In these quick revolving lights the place of the light is 

 not practically lost in the short intervals of darkness ; the 

 eye sweeping deliberately along the horizon, with or with- 

 out the aid of a binocular, to "pick up the light," passes 

 over less than the breadth of its own field of view in the 

 period of the light, and thus picks it up almost as surely 

 and quickly as if it were a fixed light. And so in respect 

 to compass bearings, whether taken roughly and quickly 

 by inspection or more accurately by azimuth compass, the 

 bearing of the ten-second or quicker revolving lights is 

 taken almost as easily and accurately as if the light were 

 continuous. Sir William contrasts this with the case of 

 the ordinary minute-period revolving light, or even the 

 half-minute period to which some formerly slow-er lights 

 have been quickened. He shows how difficult it is to 

 pick up these slow lights, and his own experience proves 

 that a fixed light like the Eddystone is much more valu- 

 able than the slowly revolving Start. 



The Wolf light he found most irregular in its periods, 

 the successive periods of light varying from nineteen to 

 forty seconds, and of darkness from nineteen to thirty- 

 four. These irregularities are apt to lead to most serious 

 mistakes, as Sir William shows. 



" Except in one unimportant case — the Dungeness Low 

 Light, which flashes every five seconds — all the revolving 

 lights of the English Channel are too slow, and it would 

 be an unspeakable improvement if, with that exception, 

 every one of them had its speed sextupled. There is no 

 mechanical difficulty in the way of doing this. Generally 

 the same mechanism would suffice with a mere change of 

 adjustment of the governor ; but the lightkeeper would 

 have to wind up the weight oftener or longer. 



" Revolving lights are, however, but a small minority 

 of all the lighthouses of the world. Of the 623 lights of 

 the British and Irish coasts, just 1 10 are revolving lights, 

 and the remaining 513 are fixed, and there is a crying 

 want of distinction for fixed lights. The distinction by 

 colour alone ought to be prohibited for all lighthouse 

 lights, on account of its liability to confusion with ships' 

 and steamers' side-lights. Southsea Castle, with its red 

 and green port and starboard side lights, seems as if 

 actually planned to lure on to destruction an unsuspecting 

 enemy carefully approaching the coast with Thomas 

 Gray's happy rule well impressed on his mind :— 

 " ' Green to green, and red to red, 

 Perfect safety, go ahead.' 

 He does so, and is wrecked on Southsea beach. 



" My proposal for supplying the want is to distinguish 

 every fixed light by a rapid group of two or three dot-dash 

 eclipses, the shorter, or dot, of about half a second dura- 

 tion, and the dash three times as long as the dot, with 

 intervals of light of about half a second between the 

 eclipses of the group, and of five or six seconds between 

 the groups, so that in no case should the period be more 

 than ten or twelve seconds. This proposal has been 

 carried into effect with perfect success in Holywood Bank 

 Light, Belfast Lough, now the leading light for ships 

 entering the Lough, but which until 1874 was inclosed in 

 a red glass lantern and was only visible five miles, and 

 was constantly liable to be mistaken for a sailing vessel's 

 port side light entering or leaving the harbour of Belfast, 

 or the crowded anchorage of Whitehouse Roads. In 

 1874 the red glass was removed, and the light was 



marked by dot, dot, dash ( — — , or letter U), 



repeated every ten or twelve seconds, and has been so 

 ever since. It is now recognised with absolute certainty 

 practically as soon as seen in ordinary weather from the 

 mouth of the Lough, ten miles oft", and has proved most 

 serviceable as leading light for ships bound for Belfast or 

 entering the Lough. 



" It is much to be desired that the dot-dash system 

 should be seriously considered by the lighthouse authori- 

 ties of our islands. Hitherto, when attention has been 

 called to it, it has been dismissed with a pleasantry, 

 'Winking lights won't do,' or else something utterly 

 different has been gravely considered and justly con- 

 demned. It is satisfactory now to know that the Deputy- 

 Master of the Trinity Board, Sir Richard Collinson, 

 K.C.B., has, after its character was correctly put before 

 him by the recent Select Committee of the House of 

 Commons on Electric Lighting, given it his approval in 

 the concluding answers of his evidence." 



The Times, in commenting on Sir William Thomson's 

 letter, speaks of the subject as one of great national im- 

 portance, Sir William speaking with the twofold authority 

 of a distinguished man of science and of a practical 

 yachtsman. The Times endorses emphatically all Sir 

 William's recommendations, and insists especially on 

 doing away with colour as a distinctive feature of lights. 



" If," the Times concludes, "the recommendations of 

 Sir William Thomson should eventually lead to a reform 

 of this importance and magnitude, he will be a benefac- 

 tor to humanity ; but even without this his advice cannot 

 fail to commend itself to navigators. It bears one of the 

 most distinctive marks of genius— simplicity ; and now 

 that it has been brought fairly under the notice of the 

 public, we may confidently hope that in the future, what- 



