March 9, 1SS3.] 



SCIENCE. 



137 



judged necessary, it was decided to use the 

 electric liglit in it; and the same decision was 

 talien regarding the lighthouse of la Palmj-re, 

 whose luminous intensity was recognized as 

 insufflcieut. 



But the good results given by the electric 

 light at la H6ve and at Cape Gris-Nez called 

 attention to the more general service it could 

 render ; and on the 27th January, 1880, after 

 a long study of the question, M. Allard, di- 

 rector of the French lighthouse department, 

 presented to the minister of public works an 

 important report, recommending the general 

 adoption, upon the whole extent of the French 

 coasts, of electric lighting. This report was 

 approved on the 4th December, 1880, b.y the 

 Conseil g6n(Jral des pouts et chauss^es ; and 

 the principle of electric lighting has just been 

 adopted for the entire extent of the coast. 

 This decisiou was so important that it seems 

 proper to mention here the principal points of 

 M. AUard's report, to make known the argu- 

 ments brought to the support of using the 

 electric light, and the results obtained in vari- 

 ous trials, and, finallj', to give details of the 

 electric installations of this nature actuallj' in 

 use. 



Before mentioning the considerations in 

 favor of changing oil for electricit}-, we must 

 speak a few words on the range of light- 

 houses. The range is the distance to which 

 the light is visible at sea ; the circle of range 

 has this distance as a radius, and the light as 

 a centre. The range of a light depends not 

 onlj- upon the optical conditions in which the 

 light is placed, but also upon its height above 

 the level of the sea. Thus there is a distinc- 

 tion between the geographical range and the 

 luminous range ; the latter being the one under 

 consideration. It increases with the transpar- 

 encj- of the atmosphere, which is verj' variable, 

 and changes with the locality ; thus, on an 

 average, it is much greater on the Mediter- 

 ranean than on the south-western coasts of 

 France, greater on the latter than on the 

 shores of Brittanj", and becomes the least 

 in the British channel. Moreover, the trans- 

 parency varies according to the seasons ; and 

 there are, during the year, a certain number 

 of more or less foggy daj-s, during which the 

 transparenc}' of tlie air and the range of the 

 light are both diminished. It is impossible, 

 therefore, to fix the range as a certain quan- 

 titj' ; and it is necessary to estabhsh a mode 

 of designating the varying range. To do 

 this, observations are made during the year 

 on the variations of the range ; the foggiest 

 nights are then omitted, and the minimum 



range for the remainder of the 3'ear represents 

 the range for that portion of the j-ear. If, 

 for example, thirtj' nights, or one-twelfth of 

 the 3'ear, are' deducted, and, during the remain- 

 der of the 3"ear, the smallest range is twelve 

 nautical miles, it is considered that the light 

 under consideration has a range of twelve 

 miles for eleven-twelftlis of the 3'ear. In 

 short, the range of a light during a portion of 

 a year is the distance at whicli it is alwa3'S 

 visible during that portion. 



In order that the lighting of coasts be effi- 

 cient, it should be continuous, so that a vessel 

 sailing along the coast, as soon as it passes 

 the range of one light, should come within that 

 of the next ; in other words, that the circles 

 of range should cut each other successivel3'. 

 With the system of oil-lights now in use, this 

 is actually the case, but onl3' during half the 

 3-ear : during the other half, the oil-lamps have 

 not sufficient power. It will be ver3' different 

 when the electric light is used. The ranges 

 will be increased, and the circles of ranges will 

 cut each other during eleven-twelfths of the 

 year. 



The acconipan3'ing outline map, Fig. 1 , shows 

 what would be the ranges if the electric lights 

 were used, supposing that each light had a 

 mean intensity of 125,000 carcels. The dotted 

 lines show the present ranges with oil-lamps. 

 When the electric light is adopted, the range 

 of the new lights will be 27.7 nautical miles in 

 the Mediterranean for \^ of the 3'ear, 19 to 21 

 miles in the British channel for J-| of the 3"ear, 

 and 22 to 26.5 miles on the Atlantic coast for 

 the same period. 



If the increase in the range, by using the 

 electric light, is a powerful consideration in 

 favor of this system, objections ma3'. however, 

 be made on the score of econonn'. The report 

 of M. Allard shows that the expense of exe- 

 cuting the entire programme, even including 

 the installations of steam-sirens, will not exceed 

 $1,600,000 ; which is very reasonable compared 

 with the results obtained. Besides, tiie cost of 

 maintenance of electric lights is not, as one 

 might have supposed, much greater than that 

 for oil-lights. Thus the annual expense of a 

 first-order oil-light is about §1,660 per year; 

 while for each electric light-house at la H6ve 

 the cost is $2,270, and for that of Cape Gris- 

 Nez $2,680. If it is desired to compare the 

 cost of a unit of hght for a lighthouse lit by 

 oil with one lit b3' electricit3% it is found that 

 the former costs $81 per unit, while the latter 

 is $22 at Cape Gris-Nez, and $19.40 at la 

 H6ve. 



It should be said here, that there is only 



