June 30, 1887] 



NATURE 



201 



that the colours can never be absolutely pure, and that 

 their order is absolutely invariable ; and the first thing 

 that one must expect in a picture painted by an artist of 

 intelligence is that this order shall not be interrupted or 

 travestied. 



In 624 a very interesting question of perspective is 

 raised. If we imagine a room with an atmosphere very 

 thickly laden with dust, and imagine further this room to 

 be illuminated by a window with an ordinary window- 

 sash : if the sun shines into the room, the sashes will 

 cast their shadows on to the dust-laden air. As the 

 sun is 93,000,000 of miles away, or thereabouts, the 

 shadows of all the horizontal sashes and of all the 

 vertical sashes will be parallel. If on the other 

 hand, instead of letting the sun throw shadows of the 

 sashes in this way, we imagine a strong light not far 

 away from the window to do it, the shadows of the 

 two series of sashes will be no longer parallel, they will 

 diverge, and the nearer the light is to the window the 

 more they will diverge. It is of course quite understood 

 that in the room itself the parallehshadows cast b^ the 

 sun could not appear to be truly parallel, for the reason 

 that one part of the shadow must be nearer the eye than 

 the other, but we cannot help thinking that the differ- 

 ence between this condition and the one which holds 

 when the source of light is close to us, has not been 

 sufficiently taken into account by the artist, so that a 

 sun 93 yards off instead of 93,000,000 of miles would 

 have been very much more likely to produce the effect 

 shown in the picture. 



A REVIEW OF LIGHTHOUSE WORK AND 

 ECONOMY IN THE UNITED KINGDOM 

 DURING THE PAST FIFTY YEARS> 



III. 



■'PHE growth of improvement in lighthouse towers, 

 ■*• lanterns, and apparatus has been glanced at. The 

 source of light, or lamps and their aliment, must now be 

 considered. It is probable that the phari of antiquity 

 were open wood fires of great size on the summit of high 

 towers or headlands. " Igties " and "Jlavimts " are terms 

 used by Pliny and others, and Statius compares the 

 pharos to the moon, not to a star as a modern poet would 

 rather do. Yet Lucan speaks of " lattipada," and Pliny 

 fears that the flames might be mistaken for a constella- 

 tion. But in these times oil could hardly have been 

 used, as no form of lamp known could be applied with 

 success. For 2000 years the iiluminant was mainly wood 

 or coal. The Cordouan, in 1610, was kindled with oak 

 logs. Coal fires were burnt at Harwich in the end of the 

 eighteenth century. The Lizard was a coal fire in 181 2. 

 St. Bees ceased to be one only in 1822. The Isle of May 

 remained a coal light for 181 years. It is now the single 

 specimen of the electric light in Scotland. Sperm oil was 

 not used before 1730, and then but on a small scale until 

 the burners of Argand in 1783 and the reflectors of 

 Teulere in the same year changed the character of light- 

 house illumination. ' The Eddystone in 1759 threw its 

 first beams over the waters from ten pounds of tallow- 

 candles, for which, in 181 1, wax was substituted. 



But in 1837 sperm oil was the general aliment for our 

 catoptric lights. In that year the oxy-oil lamp was pro- 

 posed by Mr. Gurney. The principle of this light, known 

 as the Bude, was a stream of oxygen injected into an oil 

 flame, and it has since been tried with gas flames. It was 

 followed by the Drummond lime-light, and by ignited 

 platinum wire and various pyrotechnic mixtures. The 

 Bude and Drummond lights were tried by the Trinity 

 House without successful result. In 1845 a Parliamentary 

 inquiry on oils led to the choice of rape-seed as a substi- 



' Continued from p. i8i. 



tute for spermaceti, and in i860 vegetable oil was being 

 used everywhere, with perhaps a little gas for small lights. 

 The single lamp of the dioptric system was then in 

 England and Ireland the " fountain," and in Scotland 

 the mechanical or clockwork lamp, as used in France, 

 both having four concentric wicks. It was with this 

 lamp that Fresnel established his first light at the Tour 

 de Cordouan in 1822. So far as can be ascertained, the 

 electric spark was first practically suggested for a light- 

 house in 1852 or 1853 (Holmes), or in 1854 (Watson), as 

 will be later referred to. In i860. Prof G. B. Airy wrote 

 to the Royal Commissioners on Lights : — " At present the 

 great excellence of a lighthouse is, or may be, the 

 optician's part. The great defect and waste is in the 

 source of light." Coal gas had been introduced in 1837 

 at the inner pier light of Troon (Ayrshire), and in 1847 it 

 was used in the Hartlepool Heugh dioptric sea-light. 

 From 1865 to 1867, gas was proposed for lighthouses in 

 Ireland, but not officially adopted. In the same period, 

 mineral oil, which had become familiar to English people 

 in domestic lighting, and had been used in French light- 

 houses in lamps of a single wick and apparatus not larger 

 than the fourth order, was much discussed as a suitable 

 iiluminant for sea-lights. After a long course of official 

 experiment and inquiry, the unreserved use of mineral oil 

 was authorized for lighthouses on land, and the Flam- 

 borough Head was the first Trinity light to receive the 

 new iiluminant (1872). One name is here worthy of dis- 

 tinction. Capt. H. H. Doty may justly be regarded as 

 the chief demonstrator of the " promise and potency " of 

 mineral oil. He also constructed a burner with multiple 

 wicks which produced steady and brilliant flames. This 

 burner is not, however, novel in its elements or combina- 

 tions, and other petroleum burners of equal and superior 

 merit have since been introduced. It is not on the Doty 

 burner itself that his reputation is best founded, but on 

 his strenuous and intelligent advocacy of mineral oil, and 

 on his practical application of it to a multiple burner. 

 It is gratifying to know that his services have been for 

 this reason recognized by grants of money from the 

 Governments of England and France. 



Since 1872 the use of petroleum has been more and 

 more extended, and it is now a trusted and perfectly safe 

 iiluminant. Until recently the variety known as "Young's 

 lighthouse oil " was exclusively adopted by the Trinity 

 House, its flashing-point being not lower than 142°, its 

 specific gravity 08 1. Later varieties of it have a flashing- 

 point of 154°. This fluid does not rise to the level of the 

 top of the burner, but is confined to a certain distance 

 below, whence the cotton wicks are charged with it by 

 capillarity, and it is the vapour or gas that is ignited. 

 The absence of overflow leaves the tips of the burner dry 

 and unrefreshed, and therefore subject more or less to 

 rapid deterioration. But in the heavy mineral oil lately 

 recommended by the Trinity House, the specific gravity 

 is between 0828 and 0*832 (at 60° F.), and the flashing- 

 point is not lower than 250°. This oil, therefore, may 

 probably be allowed to overflow the burner like colza. 

 There is also a very useful variety, under the name of 

 " mineral sperm," which was first introduced by the 

 writer into harbour and ships' signal lights. The flashing- 

 point has reached 270°. 



The saving of expense in using mineral oil in a light- 

 house may be understood thus. A six-wick burner of the 

 best Trinity type consumes, when at full power, 79-4 fluid 

 ounces, or half a gallon hourly. In a year of about 4000 

 hours this would cost perhaps ^70. Vegetable oil in the 

 same quantity would cost perhaps ;^25o. There would 

 be no appreciable difference in intensity of light, but 

 much in favour of mineral oil in the facility of service 

 and in the smaller consumption of wicks. Pari passu 

 with the adoption of this iiluminant has been the im- 

 provement in the pressure and pump lamps and their 

 burners effected by the Trinity House and by Messrs. 



