HELIOMETER 



HELIUM 



military signalling, especially in 

 mountainous districts, and its 

 messages are conveyed by long and 

 short flashes in the Morse code. 



Heliograph signalling has been 

 carried out at a distance of 70 m., 

 and it has the great advantage of 

 being practically secret and ex- 

 tremely rapid in operation. A 

 heliograph cannot be read by any- 

 one more than a few yards off the 

 exact alignment. See Signalling. 



Heliometer (Gr. helios, the sun, 

 and meter). Telescopic instrument 

 indispensable for making accurate, 



Heliometer at the Radclitfe Observa- 

 tory, Oxford, for making accurate 

 astronomical measurements 



By courtesy of the Clarendon Press 



measurements of small distances, 

 e.g. the diameter of a planet, or the 

 distance between the components 

 of a double star. The essential prin- 

 ciple of the first heliometers, de- 

 signed to measure the variation of 

 the sun's diameter at different sea- 

 sons of the year hence the name 

 was that of so employing two 

 symmetric sections of a lens as to 

 produce a double image at the focus. 



The first instrument for measure- 

 ment by double-image was probably 

 due to Savary ; Dollond improved 

 on it ; and Fraunhofer gave it the 

 modern form. Fraunhofer' s helio- 

 meters, especially the great Konigs- 

 berg heliometer (1824-29), are 

 still employed. One of the finest 

 instruments at present in use is at 

 the Cape Observatory. See Astron- 

 omy ; Telescope. 



Heliopolis (City of the Sun). 

 Town of ancient Egypt, the chief 

 seat of religious learning, formerly 

 containing a famous university for 

 the education of the priests. 

 Called On in the Bible (Gen. 

 xli, 45), it was known to the 

 Egyptians as Annu. Portions of 

 the great temple of the sun still 

 remain, as well as one of the 

 obelisks of red granite, 66 ft. in 

 height. Cleopatra's needle for- 



merly stood here. Near is New 

 Cairo, a residential suburb con- 

 nected with Cairo by electric rly. 



Helios. In Greek mythology, 

 god of the sun, identified in later 

 times with Apollo, and sometimes 

 called Hyperion. He crossed the 

 sky day by day from east to west 

 in a chariot drawn by four horses. 

 See Apollo. 



Helioscope. Apparatus com- 

 monly affixed to telescopes to en- 

 able direct observation of the sun 

 to be made without injury to the 

 eyes. In its simpler forms it con- 

 sists of smoked lenses or glasses. 

 See Telescope. 



Heliostat (Gr. helios, sun; 

 statos, fixed). Mirror mounted on 

 an axis parallel to the axis of the 

 earth and moved by clockwork so 

 that it rotates with the same angu- 

 lar velocity as the sun, the image of 

 which it reflects. In short, it fol- 

 lows the sun, and in consequence 

 the rays of the sun when reflected 

 from it pass always in a fixed direc- 

 tion. Foucault's heliostat reflects 

 the sun's rays horizontally. Helio- 

 stats are used in spectroscopic work. 

 See Spectroscopy. 



Heliotrope (Heliotropium peru- 

 vianum). Perennial plant with 

 shrubby stem, of the natural order 

 Boraginaceae. It is a native of 

 Peru. It has broad lance-shaped, 

 wrinkled and 

 hairy leaves, 

 and clusters of M 



lilac or dark- 

 blue, salver- 

 shaped flowers, 

 which are 

 strongly but 

 pleasantly 

 scented, 

 whence its 

 name of cherry- \ 

 pie. 



Heliotropin. 

 White crystal- 

 line substance, 

 with the odour 

 of heliotrope, 

 used as a per- 

 fume for soaps 

 and toilet pre- 

 parations. 



Chemically it is the methylene 

 ether of pyrocatechinic aldehyde, 

 and it is also known as piperonal. 

 The crystals dissolve in spirit, and 

 it is this solution which is em- 

 ployed as a perfume. Heliotropin 

 is also used in medicine. 



Heliotropism (Gr. helios, sun ; 

 trepein, to turn). Turning of 

 plants towards the source of light. 

 All plants with green leaves require 

 sunlight for their physiological 

 processes, some more than others, 

 and the whole form of a herb or 

 tree may be altered by its efforts 

 to satisfy its wants. This may be 

 easily seen in the plants on the 

 edge of a thick wood, under a 

 hedgerow, or the pot-plants grown 

 in a window. There is always a 

 bending away from the shade and 

 towards the light. 



Helium. Colourless gas with a 

 characteristic spectrum. It was first 

 detected in 1868 by Lockyer in the 

 spectrum of the sun's chromo- 

 sphere during an eclipse. The spec- 

 trum of helium is characterised by 

 five lines, one each in the red, 

 yellow, blue-green, blue, and violet. 

 The particularly brilliant yellow 

 line was the one by which its pre- 



iieliopoiis, fcgypt. iviosque in inc 



New Town. Top right, granite 



obelisk erected c. 2433 B.C. 



Heliotrope. Leaves and flower clusters 

 of the sweet-scented herb 



sence in the sun's chromosphere 

 and also in many of the fixed stars 

 was detected. 



Helium. was not known to occur 

 in terrestrial matter until in 1895 

 Sir William Ramsay, while search- 

 ing for argon in certain minerals, 

 e.g. cleveite, obtained the helium 

 spectrum. It was also found in 

 many other minerals, chiefly those 

 which contain uranium such as 

 broggerite, fergusonite, and mona- 

 zite. Helium was afterwards de- 

 tected in the atmosphere in which 

 it exists to the extent of four parts 

 in a million. It is contained in 

 several natural gas supplies, also 

 in many mineral springs. 



The density of helium is 1'98, 

 and next to hydrogen it is the 

 lightest gas known. Attempts to 

 produce chemical combinations of 

 helium have failed, and it is there- 



