February 6, 1885.] 



SCIENCE, 



113 



These considerations led the Trinity house to the 

 opinion that the Wigham gas system in single form 

 could in a very few cases be employed at its higher 

 powers without risk of perplexing the mariner; that 

 the highest power at which its single burner could 

 be used under every required condition was also 

 obtainable by oil; that its special novelties in dis- 

 tinctiveness, as introduced at Galley Head, would 

 only be available at widely separated stations; and 

 that where space and considerations of expense per- 

 mitted the use of gas in triform or quadriform, elec- 

 tricity wonld also be admissible, and, by its suitability 

 for optical treatment, would be better adapted for 

 producing the effects required in coast illumination; 

 and, finally, its own experience with the two gas- 

 lighted towers at Hasborough was not such as to 

 encourage a more extended application. 



The Commissioners of northern lighthouses, in 

 answer to the letter of the Board of trade, send a 

 report from Messrs. Thomas Stevenson and J. A. 

 Crichton, which, in the main, agrees with that of the 

 Trinity house. 



From the paper read by Sir James Douglass before 

 the British association in Montreal may be gleaned 

 a few facts as to the relative powers of the best lights 

 now in use, which are not mentioned in the corre- 

 spondence just described. He states that the first 

 electric light used in an English lighthouse in 1858 

 was of 700-candle power, whereas an intensity of 

 50,000-candle units is now found to be practically and 

 reliably available for the focus of an optical appa- 

 ratus; so that, with regard to intensity, this luminary 

 outstrips all competitors. Compact flames are now 

 being produced from oils and coal-gas, having an 

 intensity of 1,500 to 2,000 candles; while, with the 

 108-jet Wigham burner, an intensity of nearly 3,000 

 candles has been reached. With regard to economy, 

 mineral oil has the advantage of all its rivals up to 

 the maximum intensity at which an oil light is prac- 

 ticable, and has the further advantage over electricity 

 or gas in its ready application at any station, how- 

 ever isolated, and in many cases where the use of the 

 other illuminants would be impracticable. He pro- 

 ceeds to show that fixed lights are no longer to be 

 considered trustworthy coast-signals, owing to their 

 liability to confusion with other lights, and that the 

 period of a light should not exceed half a minute; 

 further, that time should not form an element in the 

 determination of the distinctive character of a light. 

 On the coast of England the Trinity house is con- 

 verting all fixed lights to occulting, where local dan- 

 gers are required to be covered with red sectors, or 

 sectors of danger-light. For this the electric light 

 is eminently adapted. In cases where this local map- 

 ping-out of dangers is not required, flashing lights, 

 in consequence of their higher intensity, are being 

 adopted. 



Referring to the optical apparatus of the new 

 Eddystone lighthouse, he describes it as consisting 

 of two superposed tiers of lenses with a six-wick 

 Douglass oil-burner in the focus of each. In this 

 respect a part of Mr. Wigham' s system has assuredly 

 lx-en copied. With a clear atmosphere, the lower 



burner only is worked at its minimum intensity of 

 about 400-candle units, giving an intensity of the 

 flashes of the optical apparatus of about 37,000 can- 

 dles ; but in thick weather the full power of the two 

 burners is put in action, with an aggregate inten- 

 sity in the flashes of the optical apparatus of about 

 159,000-candle units. This intensity is about 23 

 times greater than that of the fixed light latterly 

 exhibited from the old tower, and about 2,380 times 

 the intensity of the light originally exhibited in the 

 same tower, at about the same cost, from tallow 

 candles. 



THE ESSEX DENEHOLES. 



The word ' denehole ' means 'derchole,' and is 

 pronounced ' danehole.' Those of Kent and South 

 Essex may be described as consisting of narrow ver- 

 tical shafts leading to artificial chambers excavated 

 in the chalk, their depth varying with the distance 

 of the chalk beneath the surface. They are found 

 singly, in groups of twos and threes, or in larger col- 

 lections of perhaps fifty or sixty pits. 



Our illustrations show two types of the varieties of 

 form exhibited by deneholes. The beehive shape is 

 especially common in the shallower pits, which are 

 wholly, or almost wholly, in chalk. A drawing of a 

 pair of such pits discovered in a chalk cliff at Cray- 

 ford brickfields is given (fig. 1). Their depth was 



thirty-seven feet, and the greatest width eighteen feet. 

 The walls showed no signs of metal picks, and the 

 chalk blocks must have been prized out, but they were 

 well and symmetrically worked. In one was a layer 

 of very hard clay, washed into a cone at the bottom, 

 and containing flint flakes, scrapers, and a ' core : ' 

 above that a layer of Roman pots and pans (a Saurian 

 dish, etc.) rested, followed by some very fragmentary 

 and coarse potsherds and confused rubbish, appar- 

 ently intended to fill the hole up to the surface of the 

 ground. The sister-cave did not show an equal strat- 

 ification of debris, and appeared to have fallen in at 

 an early period. 



Of the deeper deneholes existing in Hangman's 

 Wood, one (fig. 2) is eighty feet deep. In three ex- 

 amples at Hangman's Wood (not figured) there were 

 six chambers, while in two at Bexley only three 

 chambers radiated from the shaft. A final stage in 

 denehole evolution seems to have been the removal 



