SKYWRITING AT NIGHT.

Press, Volume LXVIII, Issue 20472, 16 February 1932, Page 14

SKYWRITING AT NIGHT.

SEARCHLIGHT PROJECTOR,

A BRITISH INVENTION

v ri?OM ot-R OWN cokkespondent.) LONDON, January 5. Skvwriting by night is now a practical operation. Tlic sky has been used ior luminous advertisements in a series of tests at Hendon, Watford, Portsmouth, Southampton, and elsewhere, rnd in various-parts of Germany. Tho >kyivriting lias been clearly read at a distance of sis or seven miles from the projector, and in oue case where the operator followed the last retreating cloud towards the eastern horizon, the legend i-ast unoii it was read 15 jniles away. The system will shortly be in use over London and in other parts of tiie '[ho images cast by this. method may take almost any form. They may be thrown on the sky whole or may appear littlo by little as though a pencil were writing messages m space. lost done so far has been a word ot ! 6 letters or 16 letters set in two lines, lr.it the size of the sign may now be increased to 24 letters, and there is no i eason why a still bigger reflector or two projectors in conjunction should not be used to cast a longer or deeper notice. There is perhaps little danger i hat the night skv will be made iigly l,v this sort of advertisement, for it nl cnpi-cs comparatively little ppace, and ilje rights iA the "use of the process .-ire being rigidly controlled by the inventors. Experiments Made.. The authorities have also been consulted'since the conclusion of the experimental stage, and the Admiralty, War Office, and Air Ministry have stated that they have no objection to •the use of the device. During the tests ;i rrangcmeiits were made to collect evidence from members of the public. At Portsmouth the skywriting was read at places seven miles away from the projector. At Southampton the effective range was six miles. at attord_ miles, and at Hendon on two particular occasions the sign was read at Edmonton, 10 miles away, and at Hornlord. lo miles distant. These distances may not represent a vast improvement <ih "daylight skywriting, but experiI'lice in Gcrmanv especially lias shown that there are many more nights suitable for projection than days suitable ior skywriting with coloured smoke. Actually the light cannot be thrown on the siiv, but clouds at all sorts of heights and even mist in an otherwise clear sky arc sufficient for its purposeThe best renditions are given by high, sol i d clouds with a clean, regular base. Few starlit nights are so free from mist as to be unsuitable for projection. At Hendon, which is rather liable to mists, one clear night in four provides :i layer of mist at about 4000 ft, affording sufficient background for these light images. The projector used for this purpose is in fact the new searchlight, referred to recently in its commercial form in ■•The Times.'' Just as the searchlight is the first of its kind, so the skywriting projector is the first of its kind, and the only one' in the world which gives satisfactory results. Attempts have been made in Germany to produce simitar effects with' an ordinary searchlight beam projected through an optical stencil. The success of that method may be ganged from the fact that the m-w British apparatus has been j adopted in Germany in preference to '" it. For skywriting, as for tho grid searchlight, the secret lies in the breaking up of a mass of light into 300 separate rays by means of mirrors and in deflecting every rav along such a path that the ultimate light image is built up. 3,000,000,000 Candle-Power. An enormous volume of light is used, amounting to about 3,000,000,000 candle-power, and the power of every individual ray is preserved by securing to it parallel walls instead of the coneshaped- walls of the familiar searchlight. The image thrown on the clouds is therefore one of great intensity, and casual observers have reportsd to Major .1. C. Savage, who, with his staff at HJendon, invented the apparatus, that the writing is sharp and clear even when seen from brightly lighted streets. People in- London and Cologne stated that they noticed the signs even when standing under bright are lamps. The searchlight can bo made to give any pattern of writing. It has been made - to reproduce actual signatures. The setting of the mirror plate has been reduced almost to an automatic process, and while skywriting is being developed as the main line, the growing technique is yielding new advantages for the military searchlight. It is possible now to manipulate the grid pattern so that, having caught an aeroplane within its meshes ,of light, the ' big square may contract, turning from grid into chess-board with alternate squares , of darkness -and light, anil openiiint out again into the wider mesh should the aeroplane attempt to spin out of this closer-woven web. At a height of 15,000 ft the grid of 25 sqtltires now being used measures a little more than 3000 ft along any side. "When the grid is contracted to a chess-

board, with brightly illuminated alternate squares, the length of a suic shrinks to about IOOOft. a distance which would bo covered by the fastest bomber in five seconds. The change from grid to chequer pattern occupies only one second. The light may be traversed or otherwise deflected very rapidly,, and .if by. some fortunate chance an aeroplane escaped from the light,- the wider-net would b© spread again over it almost immediately.