5q8 



NATURE 



{April I, 1879 



of rLver, wliicli is about 2 feet 9 inches above summer low-water 

 level. 



Yours truly, 



William Bowen 



The pier master (W. Mants) at Clevedon, near Weston-super- 

 Mare, reports that he timed the rise of the tide there on March 10 

 from two hours flood, and found that it flowed thirty feet per- 

 pendicular in two hours and forty-five minutes. On March 24 

 the tide rose thirty -eight feet at the Clevedon Pier Head, 



J.Y. 



Ice Pearls 



A PHENOMENON of singular beauty presented itself on the 

 morning of March 24. A patch of meadow laud, several acres 

 in extent, had been inundated so far as to leave, pretty regularly 

 di-tributed throughout, stalks of last year's grass projecting several 

 inches above the surface of the pond. During the preceding night 

 the temperature had been below freezing-point, but the wind which 

 rippled its surface prevented the pool from freezing, while it 

 alternately raised and depressed the stalks of grass. The water 

 thus collected by the bending and rising grass-heads formed into 

 large shining beads of ice which lay at the point of junction of 

 the stalk and the pool. The effect was as if each projecting 

 stalk had unfolded a white flower floating on the water, and 

 when a gleam of sunshine smote the surface of the pool, the 

 effect was resplendent. j. Shaw 



Tynron, Dumfriesshire 



- Unscientific Art 



Mr. Coppock's explanation (Nature, vol. xix. p. 484) has 

 occurred also to me ; but may I be allowed to remind him that 

 in consequence of the internal construction of the marine baro- 

 meter (the pipette and the contraction in the tube), when it is 

 sloped the mercury rises and falls very slowly. "As it naturally 

 rises and falls at a decreasing rate, if the barometer be sloped 

 for a few seconds it takes a comparatively long time for the 

 mercury to resume its original position. I have just sloped one 

 of Adie's marine barometers at 30° from the vertical, and I find 

 it takes more than ten minutes to recover itself. I do not know 

 what may be the actual practice on board ship, but I cannot but 

 think that a plan which renders a barometer useless for ten 

 minutes to another or the same observer must be an unusual 

 one. John W. Buck 



New ICingswood, Bath, March 28 



SCIENCE AND WAR—SIGNALLING BY 

 SUNSHINE 

 'T^HE use of the heliostat in the field adds one more to 

 -*• the many applications of science made by our 

 soldiers and sailors. Signals by sunshine may be no 

 novelty, but the present Afghan campaign and the Zulu 

 war will henceforth be cited as the first in which the 

 heliostat was employed as an implement of warfare. 

 There can be little question as to its value to the soldier, 

 for it affords at once a ready and far-reaching mode of sig- 

 nalling ; but sunshine is an obvious sine qua ?ion to its 

 use. In this country, where the Astronomer-Royal tells us 

 the number of hours of sunshine in the week sometimes 

 does not go beyond the units, the heliostat would furnish 

 but an irregular means of telegraphing, and interruptions 

 in the service would be both frequent and prolonged But 

 in India, on the other hand, at special seasons, at any 

 rate, sunshine is the rule rather than the exception, and 

 consequently the heliostat furnishes an excellent means 

 of communication which our scientific soldiers have done 

 well to make use of. 



Heliostat stations are established at this moment 

 throughout the Khyber Pass, and General Sir Sam. 

 Browne, at Jellalabad, has his orders passed up to him 

 by flashes of light from Peshawur and Ali Musjid. Lord 

 Chelmsford has of late also been furnished with heliostats, 

 in order to provide him with better means of commuica- 

 tion along the Tugela. The plan of working is very 

 simple. The mirror of the heliostat is placed so as to 



reflect the sun's image to a distant station, and when the 

 instrument has once been set the clockwork arrangement, 

 it need not be said, suffices to maintain the mirror in its 

 proper position. In this way the distant station in ques- 

 tion always sees the dazzling ray reflected from the 

 miiTor, except when the latter is purposely obscured. 

 The appearance and disappearance of the bright spot or 

 flash constitute the signals. There is no need for any 

 superintendence when once the apparatus has been put 

 in working order, and a trained signalman suffices for the 

 duty. The ordinary Morse alphabet supplies an intelli- 

 gible code, and no one out of the line of signals can read 

 or understand the message. As a substitute for the dot 

 and dash, which go to make up the ordinarj' written 

 Morse code, the light is shown for short and long 

 intervals ; thus the light shown for a short period followed 

 by a long period signifies A, while B is represented by a 

 long period followed by three short ones ; in the case of 

 C, long, short, long, short signals are made in turn, and 

 to form E, the letter most frequently used, the light is 

 permitted to shine for one single short period only. 



The intensity of these sunshine signals can scarcely be 

 imagined by any one who has not seen the heliostat in 

 working order, and the distance to which they might be 

 made to travel, could suitable stations be provided, is 

 practically unlimited. But everybody has noticed at one 

 time or another, just before sunset, the light striking 

 vividly against the windows of a house. In this case the 

 burning spot may be seen for miles away, and forms the 

 most striking object in the whole landscape. The helio- 

 stat signal is obviously brighter still than this, and the 

 appearance and non-appearance of the light is to be ap- 

 preciated at ten or twenty miles distant without the aid 

 of telescope or binocular. 



Signalling by the aid of a mirror is among the earliest 

 experiments of telegraphy, nor, if we are to believe tra- 

 vellers, is the use of a reflecting surface in this way new 

 in warfare ; it is only the hehostat, indeed, which we can 

 claim to have been the first to employ in the field. Several 

 instances are on record of polished metal surfaces being 

 used in this manner by savage nations, and it is but two 

 years ago that the United States forces captured a tribe 

 of Indians to whom the use of the mirror was not un- 

 known. These were the Nez Percys Indians, and, ac- 

 cording to latest accounts, they were still confined by the 

 American Government in a camp near Fort Leavens- 

 worth, where, however, they were left pretty well to their 

 own devices. According to the N'ew York Daily Graphic 

 their chief carried with him a looking-glass, "used to 

 direct military manoeuvres in battle, by means of reflected 

 rays of light. Their various significations, however, have 

 never yet been found out by the white man," we are told. 

 These are not likely to have been very compHcated. The 

 difficulty, in fact, is not so much in reading light-signals 

 of this kind as to reflect the rays in precisely that direc- 

 tion in which the party for whom they are intended hap- 

 pens to be located. How the chief of the Nez Percys 

 managed to do this with his hand-mirror is rather what 

 " the white man " would like to understand. 



One other incident in the history of light-signals de- 

 serves to be mentioned. When Admiral Sheriff was 

 stationed at Gibraltar in 1835, he made a series of ex- 

 periments with a view to employing light as a means of 

 telegraphy. His signals were made by an ordinary 

 toilet looking-glass from his bedroom window, that looked 

 out upon the Mediterranean, and by the aid of this simple 

 apparatus he was enabled to communicate with a friend 

 at Tangiers. His light-signals travelled from " the Rock " 

 right across to the African mainland, a distance of some- 

 thing like twenty miles, and were read and answered with- 

 out difficultv by his colleague on the opposite shore. 



Besides the heliostat, our troops in the field are pro- 

 vided with flags and lamps for signallmg by day and 

 night. The flags are made four feet square, so as to be 



I 



