Page:Science vol. 5.djvu/129

 tSKOAKT 6, 1835.1

��e considerations Ie<l the Trinity boiise to the Inlon that the Wigham gas sjstem in single fomi 'i In a very few cases be eTuplojeil atltahiglier t highest power at which its single bnrner caul<l B tised under every required condition was also gfeulnable by oil; that its special novelties In dis- jfaii'tivenesi, as JDtroduced at Galley Head, would ^ly be available at widely separated stations; and t where space and considerations of expense per- ise of gas in triform or quadriform, oiec- Icity would alaobeadniBSlble, and, by Its snilabllily _ optical treatment, would be better adapted foi' producing the effects required in coast llluuilnation; and, finally, its own experience with the two gas- lighted towers at Haaborough was not such as to encourage a more extended application.
 * r* without risk ol perplexing the mariner; that

The Conunissi oners o( northern lighthouses, in answer to the letter of the Board of trade, send a report from Messrs. Thomas .'^t^venson anil J. A. Cdcbton, which, in the main, agrees with that of the Trinity house.

From the paper read by Sir Jamea Douglass before tbe British association In Montreal may be gleaned a few facl3 as to the relative powers of the best lights now in use, which are not mentioned tu the corre- spondence just described. Be Btates that the first electric light used In an English lighthouse in 1858 was of 700-candle power, whereas an intensity of 50,000-candIe units is now found to be practically and reliably available for the focus of an optical appa- ratus; so that, with regard W intensity, this luminary ouutripi all competitors. Compact flames arc now hig produced from oils and coal-gas, having an tensity of 1.500 to 2,0<X) candles; while, with the 6-Jet Wigbam burner, an intensity of nearly 3,000 idles has been reached. With regard to economy, ineral oil has the advantage of all its rivals up tu e maximum Intensity at which an oil light is prac- able. and has the further ad vantage over electricity 11 its ready application at any station, how- tr isolated, and in many cases where the use of the Iker itiumlnants would be impracticable. He pro- o show that flxed lights are no longer to be nsidered trustworthy coast-signals, owing to their billty to confusion with other lights, and that the period of a light should not exceed half a minute; further, that time slioutd not form an element In the determination of the distinctive character of a light. On the coast of England the Trinity bouse is con- vening all (ised lights to occulting, where local dan- ger* are required to be covered with red sectors, or Mctors of danger-light. For this Ibe electric light 111 eminently adapted. In cases where this local map- f dangers is not required, flashing llgbti, leonKquence of their higher intensity, are being

eferrfng to Uie optical apparatus of the new

Idyitone lighthouse, be describes it as consisting

o saperpused tiers of lenses with a six-wick

lasa oll-bumer in the focus of each. In this


 * t a part of Mr. Wlgham's systefli baa assuredly

I copied. With n clear atmosphere, the lower

��burner only is worked ,it its minimum Intensity of about 400-caQdle units, giving an intensity of the flaslies of the optical apparatus of about 37|UO0 can- dies; but ill thick weather the fnU power of the two burners is put in action, with an a^regate inten- sity in the flashes of the optical apparatus of about 150,000-candle units. This intensity Is about 23 times greater than that of Ibe fixed light latterly exhibited from the old tower, and aliout 2,380 times the intensity of the light originally exhibited tn the same tower, at about the same cost, from tallow candles.

��THE ESSEX DENEHOLES.

The word ' denebole ' means 'ilenbolc,' and is pronounced 'dandioie.' Those of Kent and South Essex may be described as consisting of narrow ver- tical shafts leading to artificial chambers excavated in the chaik, 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 dcneholes. The tieehive 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 cliS at Cray- ford brickfields is given (fig. 1). Their depth was

���thirty-seven feet, and the greatest width eighteen feet. Tbe 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 (aSamlan dish, etc.) rested, followed by some very tragroenlary and coarse potsherds and contused rubbisli, appar- ently Intended tu fill the hole up to the surface of the ground. Tbe sister-cave did not show an equal strat- iQcalion of iJe'6ri», and appeared to have fallen in at sn early period.

Of the deeper deneholes existing in Hangman's Wood, one (flg. 2) is eighty feet deep. In three ex- amples at Hangman's Wood (not figured) there were six chambers, while in two at llexley only three chambers radiated from the shaft. A final stage in denebole evolution senois lo have been the removal

�� �