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must become more and more vertical as the rays fall less horizontally, and 
this will continue until they reach the critical angle, when total internal 
reflection ceases, and it suddenly disappears. Its apparent tilting over on 
the spectator is probably an illusion, produced by the rapid approach and the 
rising of the dark veil without any gradual disappearance which can be 
watched and estimated. It will be evident that the illumination of the in - 
numerable particles floating in the atmosphere causes the aerial shadow to 
be visible by contrast. Another interesting phenomenon visible in the 
mountain districts admits of an equally simple explanation. At times broad 
beams, apparently of bluish light, may be seen extending from the zenith 
downwards, converging as they approach the horizon. The spaces between 
them have the ordinary illumination of the rest of the sky. If we suppose, 
as is frequently the case, that the lower strata of air are colder than the 
upper, the reflection spoken of in the case of Adam’s Peak will be down- 
wards instead of upwards. If several isolated masses of clouds partially 
obscure the sun, we may have several corresponding inverted veils of dark- 
ness, like blue rays in the sky, all apparently converging towards the same 
point below the horizon. This latter phenomena is called by the natives 
u Buddha’s rays.” 
A Simple Form of Heliostat. — The heliostat is such an useful instrument, 
not merely for the physicist, but for the photographer, that any mode of 
improving it is of interest. At the meeting of the Physical Society on 
April 29, the Secretary read a communication from Sir John Conroy, Bart., 
u On a Simple Form of Heliostat.” The defect of Fahrenheit’s heliostat, in 
which the beam of sunlight is reflected by a mirror moved by clockwork in 
a direction parallel to the axis of the earth, and then in the required direc- 
tion by a fixed mirror, consists in the great loss of light. The author sub- 
stitutes two silvered mirrors for the looking-glasses usually employed, and 
he has shown that the loss of light with this arrangement is less than when 
the light is once reflected from a looking-glass. 
Flectric Communications without Wires. — It would seem from recent, ex- 
periments that it is perfectly possible to convey a message for a certain dis- 
tance along the earth without any conducting wire whatever. But M. Tb. 
Du Moncel has explained to the French Academy (May 8) that the idea of 
communication without wires is far from novel, having been experimentally 
tested thirty years ago, both in England and America. Thus, messages 
were sent from Gosport to Portsmouth (and, we believe, across to the Isle 
of Wight), a distance of about 3 kilometres. 
Ice-making Machines — Professor Hoffmann has the following account in 
the u Chemical News,” May 12, of a novel form of ice-machine. He says 
that since the beginning of 1873 Nehrlich & Co., of Frankfort-on-the-Main, 
make the Windhausen machine with two cylinders of one size only, with 
especial regard to the demand in breweries. It requires a 40-horse power 
engine, and is guaranteed to yield hourly 2,500 cubic metres of air at tem- 
peratures of from —30° to —50°. If we assume that these temperatures 
refer to initial temperatures of from 10° to 30°, the total reduction of tem- 
perature amounts to 60°, whence the amount of the negative heat units may 
be calculated as 50,000, corresponding at most to 400 kilos, of ice. If the 
production of ice were the object in view, the same quantity of air might 
