150 



SCIENCE 



[Vol. V., No. 107. 



arm of one of the men. The cost thus far is 

 $1,188,000. The completed structure weighs 

 81,000 tons. 



In this connection, some of the heights of 

 notable structures may be of interest : Tower 

 of Pisa, 179 feet; Bunker Hill monument, 221 

 feet ; Great mosque, Cairo, 282 feet ; Trinity 

 spire, New York, 284 feet ; Campanile, Flor- 

 ence, 290 feet ; top of capitol, Washington, 

 307 feet ; Milan cathedral, 355 feet ; St. Paul's, 

 London, 365 feet ; Antwerp cathedral, 402 

 feet ; Lutheran Mariankirche, Lubeck, 430 

 feet ; St. Stephen's, Vienna, 441 feet ; St. 

 Rollox chimney, Glasgow, 450 feet ; Great 

 pyramid, 450 feet (originally 485 feet) ; St. 

 Peter's, Rome, 455 feet ; Strasbourg cathe- 

 dral, 468 feet; Cologne cathedral, 511 feet; 

 Philadelphia city hall, to be 535 feet ; Wash- 

 ington monument, 555 feet. 



Many memorial stones were contributed by 

 the states, and by different organizations in 

 this country, and by foreign countries. Some 

 forty of these stones were set in the interior 

 faces. One hundred still remain in the store- 

 house, and will probably be affixed as slabs 

 to the interior walls in convenient places. 



Charles E. Greene. 



THE ELECTRIC LIGHT FOR LIGHT- 

 HOUSES AND SEARCH-LIGHTS. 



The recent experiments in England {Nature, 

 vol. xxx. p. 362), upon the relative merits of 

 electric, gas, and oil lights for lighthouse 

 illumination, have called attention to the very 

 marked failure of the arc-light to penetrate 

 through a misty or foggy atmosphere ; this fail- 

 ure being due to the vigorous absorption of the 

 blue rays of the spectrum hy such an atmos- 

 phere, — rays in which the arc-light is espe- 

 cially rich. A very striking case of similar 

 failure was presented to the writer's notice a 

 few evenings ago. One of the streets of Wash- 

 ington has recently been lighted by arc-lights on 

 each side, upon posts several feet higher than 

 the gas-lamps ; so that, in looking along the 

 street, the rows of electric lights above the gas 

 offer a good opportunity for comparison. For 

 several nights both were lighted ; and one of 

 these nights chanced to be extremely foggy for 

 a few hours in the evening, the ground being 

 covered with slush from melting snow. For 

 this reason I went out of my way to see the 

 effect upon these lights, and was rewarded by 

 the sight of the arc-lights — overpoweringly 

 bright close at hand — becoming almost as 



faint and yellow as the gas-lamps at a distance 

 of less than half a mile. The extent of the 

 arc-lights was only five blocks, and the treasury 

 building at one end, and patent office at the 

 other, prevented a view from a greater dis- 

 tance ; but there can be no doubt, that, if the 

 relative rates of absorption had continued in 

 the same ratio for a greater distance, the arc- 

 lights would have appeared fainter than the 

 gas-lamps at a distance of not much over half 

 a mile, and would have entirety disappeared 

 long before the latter. The arc-lights are said 

 by the compan} 7 to be of about two thousand 

 candle power, and the gas-lights probably equal 

 between fifteen and twenty candles ; so that 

 the enormous difference of absorption under 

 these circumstances is evident at a glance. 

 To be sure, this was a very thick fog ; but 

 this is the very condition of things where pene- 

 trating power is most necessary for lighthouse 

 lamps, and where the arc-light seems to fail 

 utterly. 



For search-lights, in naval warfare, as pro- 

 tection against torpedo attack in thick weather, 

 and for other similar purposes, the case is just 

 as bad, or even worse ; for the light must trav- 

 erse the necessary distance twice, — to the 

 dangerous object, and then reflected back to 

 the ship. For determining the best quality of 

 light for submarine search, experiments upon 

 the selective absorption of sea-water for vari- 

 ous kinds of luminous radiant energy would 

 seem to be desirable. 



Professor Langlej 7 has shown, within the last 

 year or two, that our atmosphere absorbs much 

 more of solar radiant energy than has been 

 heretofore supposed, and that this is very 

 largely in the blue end of the spectrum ; so 

 that sunlight, if we were rid of our atmos- 

 phere, would be much bluer than we see it. 

 He has shown, too, that this takes place by 

 diffusion of the light b} T reflection in all direc- 

 tions from particles in the atmosphere, so that 

 we get about half our daylight from the sky, 

 even in a perfectly clear clay ; and that this is 

 the cause of the blue sky. 



The same explanation is sufficient to account 

 for all the phenomena of the wonderful red 

 afterglows following the sunsets of a } r ear ago, 

 if we can explain the presence of reflecting 

 particles in a more or less stratified arrange- 

 ment (Krakatoa dust, very likely) at an un- 

 usual height in the atmosphere. These would 

 reflect sunlight to us in much greater amount 

 and for much longer (semi-intermittent) inter- 

 vals than the ordinary dust and clouds at a 

 lower level of the atmosphere ; and this selec- 

 tive absorption would account for the wonder- 



