Feb, 6, 1890] 



NATURE 



331 



stratus clouds, and that the space included within the halo is 

 frequently of a more intense grey, or of a deeper blue than the 

 rest of the sky. 



The son of Sir W. Herschel observed the phenomena at 

 Oxford, and noted: — "The sun was near the horizon. On 

 either side of it, at a distance of five or six diameters of the sun, 

 was a mock sun, not very bright, of the colours of the rainbow, 

 the one on the right being the brighter. There was a scarcely 

 perceptible rainbow, of which red was the only colour visible, 

 joining the two mock suns. This rainbow was brightest directly 

 over the sun. As far oft again as the first was a second rainbow, 

 hazy, but fairly bright, which was equally visible from earth to 

 earth. Vertically above the sun, a third, a very bright rainbow, 

 touched the second, being inverted, and having its centre straight 

 overhead. It did not look quite as large as the second. The 

 weather was clear, but the clouds on and above the horizon were 

 of a uniform grey colour, fading off gradually to a nearly clear 

 sky overhead. There seemed always to be a much lighter shade 

 of grey in the clouds where the sun and the two mock suns 

 were." 



The coloured parhelia observed indicates the refraction and 

 dispersion of solar light by vertical prisms, whilst the phenomena 

 of inverted arches are produced by the light which passes 

 through horizontal crystals, at different azimuths. 



Mr. Frank E. Lott, at Burton-on-Trent, observed a third 

 parhelia on the part of the first halo vertically above the sun, 

 whilst Mr. H. G. Williams, of Caterham, observing the pheno- 

 mena about 4 p.m., noted that the sun appeared about 10° above 

 the horizon. So far, the observations of two or three parhelia 

 with two halos and two inverted arches agree with many former 

 descriptions. In the diagram appended, however, and in the 

 majority of sketches received, the inverted arch is not given as 

 the arc of a circle, but hyperbolic. 



Mr. A. J. Butler, observing at \Valton.-on-Thames, remarks : 

 "The hyperbolic band above the sun was carefully noted ; " and 

 Mr. C. A. Carus-Wilson, in the following observation made at 

 Staines, supports this view : — 



" The sun was just setting behind a bank of hazy mist, appear- 

 ing as a crimson disk enveloped in blue grey cloud ; I first 

 noticed a distinct bow, of light grey tint, and coloured for a 

 short distance at its left extremity with the ordinary rainbow 

 tints — red inside. There then appeared a part of a second 

 bow outside the other, coloured throughout the whole length 

 visible — red inside. From the sun vertically upwards to the first 

 bow, there was a band of white light, quite distinct from the 

 light grey tint of the lower bow, and above the lower bow this 

 band continued as a hyperbolic brush of white light : this brush 

 was much brighter and better defined than the vertical band, A 

 hasty measurement, with a pocket sextant, of the angular radii of 

 the two bows, gave 46° and 23° for the outside and inside bows 

 respectively." 



Mr. H. W. Pyddoke also remarks : — " The most noticeable 

 thing of all was the shape of the upper bow, which was like a 

 hyberbole except at its ends where it bent round again very 

 slightly ; " and other correspondents concur in this description 

 of the shape of the first inverted arch. 



From the descriptions and figures given it is evident that the 

 two parhelia on the parhelic circle are the respective centres of 

 halos similar to the first halo concentric with the real sun ; the 

 intersection of these two circles with that surrounding the sun 

 gives the appearance of a hyperbolic curve at the top of it. An 

 exactly similar appearance was drawn by Pastorff as occurring on 

 December 29, 1789, and is found in his " Beobachtungen dtr 

 Sonnenflecke " ; and V AstTonoviie for August 1889 contains a 

 drawing and description of a very similar appearance. 



Lunar halos followed the solar halos on the 29th ult., and 

 on the following day Mr. G. B. Buck ton, F.R.S., observed 

 three fine parhelia and a halo at Haslemere, and describes 

 them as follows : — 



" The sun shone brightly, but through a moderate haze. On 

 the right and on the left, at equal altitudes with the sun, an 

 oblong bright patch of light appeared. That on the left was the 

 brightest, and formed a blurred image of the sun with all the 

 prismatic colours of the rainbow, but the colours were reversed 

 in order. The upper and lower parts of these mock suns were 

 drawn out, and formed portions of a large circle of about (by eye 

 estimate) 20° radius. These images were connected with the 

 haze, but a lower stratum of finely striated cloud came between 

 the eye and these patches. Immediately above the true sun a 

 third patch of light occurred, through which a portion of an in- 



verted circle was seen, the greater part of which was lost in the 

 blue of the sky above. The right-hand mock sun was fainter 

 than the other, on account of the grey haze being more dense." 



Mr. Buckton's observation is a demonstration of the principle 

 laid down — namely, that parhelia always appear at the same 

 elevation as the true sun, and are united to each other by a 

 white horizontal circle, whose pole is the zenith. This circle 

 changes in elevation with the true sun ; and the apparent semi- 

 diameter is always equal to the distance of the luminary from 

 the zenith. 



Mr, Nagel, of Trinity College, Oxford, notes that :—" The 

 solar halos on the afternoon of January 29 were very clearly 

 seen in Oxford ; the tangential arc to the outer halo was ex- 

 tremely brilliant, and the two mock suns at the extremities of 

 the horizontal diameter of the inner halo were well marked. 

 During part of the time the halos lasted, a whitish incomplete 

 circle was seen about So'' from the sun, and consequently beyond- 

 the zenith. This circle seemed to correspond to that first 

 described by Helvelius in 1661." 



It is evident from the descriptions given that the parhelia 

 are not, as is sometimes supposed, images of the real sun at all, 

 but only the junctions of two of the circles formed. The upper 

 and the lower parts of these mock suns were drawn out and 

 connected with the first halo, whilst their sides were observed to- 

 be drawn out and to merge into the parhelic circle. 



THE INSTITUTION OF MECHANICAL 

 ENGINEERS. 



T^HE forty-third annual general meeting of the Institution 

 -^ of Mechanical Engineers took place on January 29, 30, 

 and 31, in the theatre of the Institution of Civil Engineers. 



The papers down for reading and discussion were as follows : 

 on the compounding of locomotives burning petroleum refuse in 

 Russia, by Mr. Thomas Urquhart, Locomotive Superintendent, 

 Grazi andTsaritsin Railway, South-East Russia ; on the burning 

 of colonial coal in the locomotives on the Cape Government 

 railways, by Mr. Michael Stephens, Locomotive Superin- 

 tendent ; and on the mechanical appliances employed in the 

 manufacture and storage of oxygen, by Mr. Kenneth S. Murray, 

 of London. The latter paper was communicated through Mr, 

 Henry Chapman, 



Mr, Urquhart's paper is one of a series of excellent and 

 thoroughly useful descriptions of work done by that gentleman 

 on his railway, and had been for some time promised to the 

 Institution. In order to satisfy himself as to the utility and 

 saving of fuel in compound locom.otives, he obtained the 

 sanction of the Government for altering one locomotive by way 

 of experiment. The altered engine was put to work, and the 

 driver was allowed over a month's running to get fully ac- 

 quainted with the handling in regular service. Comparative 

 trials were then made of the compound against a non-com- 

 pound locomotive with the same weight of train, on the 

 same days, so as to expose them both to the same circum- 

 stances in regard to weather. It was clearly proved that the 

 compound burnt 22 per cent, less of the petroleum refuse 

 ! used as fuel than the non-compound engine, and the author's 

 I experience has left no doubt in his own mind that compound 

 j locomotives are the engines of the future in all countries. Mr. 

 i Urquhart's results are thoroughly borne out by those obtained 

 \ in this country by Messrs. Worsdell and Webb. Some engi- 

 neers suppose that this great economy in fuel is due to the 

 higher working steam pressure, and therefore greater expansion 

 in the compound engines as compared with the non-compound 

 engines. 



The paper by Mr. Michael Stephens is a description of the 

 South African coal-fields, their discovery, and general working 

 within the last sixteen years. It appears from the paper that 

 the local coal cannot be burned to advantage without a special 

 arrangement of fire-bars — as may be well imagined, since it 

 contains nearly 30 per cent, of incombustible matter. 



Mr. Kenneth S. Murray gives an interesting account of the 

 commercial preparation of oxygen from the atmosphere by 

 means of the alternate heating and cooling of the monoxide of 

 barium. About thirty years ago the eminent French chemist 

 Boussingault made the discovery that, at a temperature of about 

 1000° F., the monoxide of barium would absorb oxygen readily 

 from the atmosphere, with the resulting formation of the dioxide ; 



