Nov. 7. 1878] 



NATURE 



19 



LL.D., F.R.S. (Longmans); "Dogma, Doubt, and Duty," by 



arles Hoare (Aston and Mander) ; " Leisure Time Studies, 



.iefly Biological," by Andrew Wilson, Ph.D., F,R.P.S.E. 



vChatto and Windus); "Medicinal Plants," Parts 32 to 35, by 



Robert Bentley, F.L.S., and Henry Trimen, M.B., F.L.S. (J. 



i A. ChurchiUj. 



The additions to the Zoological Society's Gardens during the 



-: week include a Toque Monkey {Mtuacus pileaius) from 



ylon, presented by Mrs. Tranchell ; two Macaque Monke)-s 



'zcacus cynomolgus) from India, presented by Mr. C. Loveless ; 



ireen Monkey (Cerco/itkKtis calHtiichus) from West Africa, 



. ented by Mr. Henry Hands ; a Grison {Galidis vittata) from 



•.th America, presented by Mr. H. Potier; a Common 



Igehog {Erinjceits eiiropsus), European, presented by Mr. 



:.vin Etty Sass ; a Common Boa {Boa constrictor) from South 



erica, presented by Mr. D. W^. Bell ; two South American 



ikes {Zatnenis hippocrepis) from South America, presented by 



:. G. H. Hawtayne; two Small-Scaled Mastigures {Uromastix 



rolepis) from Busreh, presented by Capt. Phillips ; a Meso- 



tamian Fallow Deer {Dama mesopotamica) from Mesopotamia, 



posited ; two Japanese Pheasants {P/iasianus versicolor) from 



an, a Gold Pheasant (Thaumalea picta) from China, a 



.eated Pheasant {Euplocamus lineatus) from Pegu, two Barred- 



iled Pheasants {Phasianus rervesi) from North China, a 



mese Pheasant {Euplocamus pralatus) from Siam, two Swin- 



-s Pheasants {Euplocamus swinAoii) from Formosa, a Bewick's 



~ van {Cygnus bezi'ickii) from North Asia, purchased. 



.V THE VERTICAL DISTRIBUTION OF THE 

 LIGHT FROM LIGHTHOUSES PLACED AT 

 HIGH ELEVATIONS ABOVE THE SEA- 

 LEVEL 



'T'HE strongest beam of rays jM-oceeding from lighthouse appa- 

 ratus in high towers is sent to the sea horizon, as being the 

 ■action in which the light can be seen at the greatest distance 

 clear weather. 



-My late brother, Mr. Alan Stevenson, sugro^ted the dipping 

 lioptric lights below the normal to a plumb line in his Report 

 December 10, 1839, to the Commissioners of Northern Light- 

 uses in the following passage : — " A more serious inconvenience 

 using catadioptric zones is that in very high towers where 

 ae correction of the position of tlie apparatus becomes neces- 

 y so as to direct the rays to the horizon, the means of regu- 

 ;ng the zones in a manner similar to that used for the mirrors 

 inapplicable. The adoption of a high point in the flame for 

 - focus of these zones, however, affords a considerable com- 

 r.sation for this defect, and it might even be entirely obviated 

 constructing each set of zones of the form suited to the known 

 .jht of each tower and the required range of each light if such 

 ^rrection were found to be of sufiScient importance to warrant 

 application."^ 



But though the precaution of dipping the strongest of the light 

 the sea horizon was followed out by Mr. Alan Stevenson in 

 ,h towers it was not alwaj-s attended to, till the year i860, 

 en Mr. J. F. Campbell, the Secretary of the Royal Commis- 

 1 on Lighthouses, brought the subject prominently forward, 

 i si^gested the internal mode of adjustment. Since then the 

 ngest beam has been invariably dipped to the horizon. 

 It must, however, be remembered that when the weather 

 becomes even in the least degree thick or hazj% not to say foggy, 

 the range of the light is greatly curtailed by atmospheric absorp- 

 tion and refraction ; which last produces during fogs irregular 

 difiiision of the light in every plane. So that at high towers 

 where the beam is pointed to a very distant horizon, it is obvious 

 that the strongest %ht is directed to a part of the sea, where it 

 cannot be seen with certainty vmless when the weather is excep- 

 tionally clear. 



In an interesting paper on what he calls the " horizontal visual 

 penarability of the atmosphere" lately published in the Journal 

 of the Scottish Meteorological Society,* Mr. A. Cruikshanks, 



" On the Application of Catadioptric Zones to Lighthouses." by Alan 

 •Stevenson, LL.B. (Edinburgh, 1840.) 



Jovrjtal of the Scottish Meteorological Society, new series, vol v. p. 97. 



M.A., makes the following remarks as to daylight obser\'a- 

 tions : — "This shortening of the range of view arises from 

 various atmospheric causes. Such obstructions to the horizontal 

 ^•iew as low clouds or mists, or falling showers of rain, snow, or 

 hail circumscribe it at once or abruptly from the observer at a 

 distance of from a few 3rards to many miles off. The other great 

 obstruction to the horizontal range of ^'iew is haze, which may or 

 may not co-exist with the other obstructions, and is supposed to 

 consist of minute particles of water, dust, and smoke floating in 

 the air. The haze, unlike the abrupt obstructions to the view, 

 apparently gradually increases with the distance from the 

 obser%-er till objects become invisible at the distance of three or 

 foiu- miles up to seventy, &c., miles." 



Mr. Cruikshanks also gives the following table showing the 

 mean results of twenty-one years' observations made at the middle 

 of the day on terrestrial objects stich as hills and mountains : — 



From this table it appears that during the twenty-one years 

 there were 366 days per annum when an object under five miles 

 distance could be seen, and only ninety days when an object fifty 

 miles distant could be seen. 



It has lately appeared to me that the strongest beam shoxdd 

 be dipped lower than my brother proposed, and as is now 

 everywhere adopted. 



The best of the light should certainly be directed to the place 

 where the safety of shipping most requires it. Now it may in 

 most cases be laid down as axiomatic that the peril of any 

 vessel is inversely proportional to her distance from the danger, 

 whether that danger be a lee shore or an insulated rock. Con- 

 fining otirselves to this one view of the subject, it would follow 

 that the strongest of the light should, in hazy states of the 

 atmosphere, be thrown as near to the shore, or the rock, as 

 would admit of vessels keeping clear of the danger. But such 

 a restriction as this would, if permanent, greatly impair the 

 usefidness of the light by unduly ciu-tailing its range in clear 

 states of the atmosphere ; and of course, cateris paribus, the 

 farther cff a sailor is warned of his approach to the shore the 

 better and safer. Besides, the loss due to atmospheric absorp- 

 tion increases in a geometric ratio, and as the rays diverge in 

 cones from the apparatus, the power of the light is further de- 

 creased in the inverse ratio of the squares of the distances from 

 the shore. 



It is of course wdl known that the sun itself is extinguished 

 by fog, and we cannot expect to compete with that luminary. 

 But seeing there are endless variations in the density of fogs 

 and in the transparency of the air when there is no fog pro- 

 perly so called, it always appeared to me that had we an easy 

 way of doing so, we ought to increase temporarily the dip of 

 the light, and thus during haze and fogs to direct the strongest 

 beam to a point much nearer the shore than the sea horizon. At 

 present we direct our strongest light not only in clear weather, 

 when it can be seen, but also during fogs, when it cannot pos- 

 sibly be seen, to a part of the sea where the danger to shipping 

 is in most siiuafions the smallest, and this is done to the de- 

 triment of that region where, even when the weather is hazy, 

 there is at least some chance of the light being visible, and to a 

 part of the sea where the danger to shipping is imquestionably 

 the greatest. 



The simplest mode of depressing the light temporarily would 

 be to raise the lamp itself in relation to the focal plane of the 

 lens. But this is, for several reasons, very inexpedient. The 

 proper adjustment of the apparatus to the focus, so as to secure 



