a 
_— = 
Cl 
NATURE 
201 
! 
of highly trained and experienced geologists than to that 
of geatlemen, who, whatever their attainments muy be, 
certainly do not in this Report evince much acquaintance 
with geology. J..G, 
Reports of the Mining Surveyors and Registrars for 
Quarter ending March 31, 1872. Victoria. 
THERE is nothing in these Reports calling for special 
notice. The total quantity of gold got respectively from 
alluvia (or, as the Reports have it, a//uviums) and quartz 
reefs during the quarter were as follows :—Alluvial, 
171,851 oz. 10 dwt.; quartz, 164,670 oz. 8 dwt.; total, 
336,521 oz. 18 dwt. The quantity of gold, the produce 
of the colony, exported during the same period was 
398,131 oz. 10 dwt. 
LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his correspondents, No notice is taken of anonymous 
communications. | 
Aurora Spectrum 
In connection with my letter in last number of Nature, I 
have in a diagram approximately placed the aurora lines side 
by side with the spectrum of hydrogen and of some of the 
principal air lines (as given in Dr. Watt’s index) and with the 
following results :— 24 
Line No. 1. Close upon, if not identical with, an air-line 
marked by Huggins N O, and Pliicher O. ’ é 
No. 2.—Not apparently coincident with any prominent air- 
line. The coincidence with a line of oxygen noted by so 
careful an observer as Mr. Proctor is puzzling ; and if the in- 
strumental power used was sufficient to ensure absolute identity, 
seems to indicate a second or unusual spectrum of that gas. 
No. 3 is not near any principal air-line. 
No. 4 is nearly coincident with a faint line of oxygen (confir- 
matory of Mr. Proctor’s observations). ; 
No. 5 corresponds to a rather strong N line. 
No. 6 does not coincide with any principal air-line, very faint 
‘lines of O and N being the nearest. 
No. 7. Upon close examination the positions of this line as 
respectively fixed by Mr. Proctor and Lord Lindsay are not 
inconsistent, and the line closely corresponds with a strong line 
of oxygen situate on the less refrangible side of solar G. 
Nos. 1, 4, 6, and 7 fairly correspond in intensity with their 
representative air-lines. None of the lines are identified with 
Ha, H8, or Hy, and it would appear that the aurora, if a 
spectra of atmospheric gases, mainly selects oxygen and 
ignores Ha and the stronger N lines. The modification of 
compound spectra by conditions of temperature and pressure, 
is however only a partially explored subject, and we have more- 
over no certain data of conditions in the case of the aurora, 
which will assist us in bringing it to bear. 
I accidentally omitted from the names of some observers of 
the zodiacal light that of Prof. Piazzi Smyth, whose observations 
in the south may be said to have conclusively demolished the 
supposed identity of the light, and the aurora (at least so far 
as bright lines are concerned) made it extremely improbable that 
anything beyond a continuous spectrum will ever be seen in the 
pure zodiacal light, though a further search should be by no 
means neglected. J. Ranp CAPRON 
Guildford, Jan. 10, 1873 
Polarisation of the Zodiacal Light and of the Aurora 
In the interesting article by Mr. Rand Capron in the last 
number of NATURE—after collating the various results of the 
Specir: scopic exam nitions of the aurora and zodiacal light 
which have ap eared at different times in your pages, together 
with those wiich have been collected by Dr. Schellen—he 
terminates his anuysis of the general results by remarking that 
he is ‘‘ not aware whether the zodiacal light and the aurora have 
been examined with the polariscope,” and suggests that the 
“light, though faint, mighc be tested with a Nicol’s prism and 
Savart’s bands.” 
I would refer him to a paper in the March number of the 
“Montaly Notices of the Astronomical Society” for 1871, in | 
which an observation by Mr. Burton (late assistant to the Earl 
of Rosse) on the polarisation of the zod acal light is described. 
_ Mr. Burton was one of the eclipse party stationed at Agosta, 
in Sicily. He made use of a Savart’s polarisco xe, set so as to 
give a black centre where the binds were parallel to the plane 
of polarisation. On looking to the brightest parts of the zodiacal 
light Mr. Burton believed that he could detect faint traces of 
polarisation, sufficiently strong to enable him just to recognise 
that the bands were black centred when their direction coin- 
cided with the axis of the cone of light, that is, when the direc- 
tion passed through the position of the sua. 
To make sure that he was not examininz the remains of air 
polarisation given by the slight remaining twilight, he examined 
the light of other parts of the heavens, but was unable anywhere 
else to detect any trace of bands. In contradistinction, however, 
to this must be set an observation of my own, yielding a nega- 
tive result, though made on the same evening and with a similar 
instrument, as well as with the same Savart used by Mr. Burton. 
I was, however, unable to detect any trace of bands either upon 
the cones of zodiacal brightness or upon the adjacent parts of 
the sky ; but it is very possible that Mr. Burton’s eye may be 
more sensitive to faint lights than my owa, 
In February last I also met with a negative result in examin- 
ing a faint trace of the zodiacal light visinle in England. I then 
used a double-image prism as well as a Savart, thinking that its 
two oppositely polarised fields in juxtaposition might afford a 
more delicate test for so faint an object. 
Capt. Tupman while cruising in the Mediterranean has aiso, 
I believe, repeatedly obtained negative results when making use 
of a Savart on the zodiacal light. 
And I understand that Mr. Lockyer, together with the other 
observers of the Indian Eclipse of December 1871, totally failed 
to detect any traces of polarisation in the briiliant displays of 
ae zodiacal light which they observed while crossing the Indian 
cean, 
I am therefore disposed to conclude that any traces of polar- 
isation must be very slight, if indeed any polarisation at all is to 
be attributed to the zodiacal light itself and not to the veil of 
atmospheric impurities lying between us and objects near to the 
horizon. Certainly we may conclude that there is no such 
polarisation as is found in the light of the solar corona or--as we 
might expect—if the zodiacal light were caused by a great cloud 
of cosmical dust made up of particles smaller in diameter than 
the wave-length. 
Indeed there cannot be as great a percentage of polarisation, 
or, to speak more exactly, as great a difference between the 
component radial to the sun’s place and the component at right 
angles, as in the case of a sunbeam dispersed by the dust in our 
own atmosphere. For if any one will examine the track of a 
sunbeam passing through a room with a Savart, he will not fail 
to be struck with the distinctness of the bands. We seem there- 
fore justified in concluding, that if the zodiacal light is composed 
of cosmicai dust, such dust particles must be considerably 
coarser than those which float in our own atmosphere. 
As to the polarisation of the light of the aurora, I examined, 
both with a double-image prism and Savart, a faint auroral dis- 
play on November 10, 1871; as also the light of the great 
aurora of Sunday, Feb. 4, 1872, but in neither instance was 
able to detect any traces of polarisation. 
A, CowPER RANYARD 
The Diathermacy of Flame 
THERE are some statements in Capt. Ericsson’s reply to my 
letter (NATURE, vol. vii. p. 149) which demand discussion. 
In the first place he calculates the supply of gas in 47s pipe and 
appliesit to vy burners. As his pipe did not supply my flames, 
but his own, which were at least fifteen times larger than mine, 
the app.icability of his figures is rather obscure. 
Capt. Ericsson says, ‘‘The apparatus contrived by Mr, 
Williams for determining the diathermancy of flame, as de- 
scribed by himself, is exceedingly faulty, the temperature it records 
being that produced by heat derived fram severalesources. The 
radiant heat transmitted to the bulb of the thermometer by the 
flames, acting conjointly with the unknown degree of heat imparted 
by the surrounding medium, it will be evident that Mr. Williams’ 
device is worthless as an indicator of radiant intensity.” Does 
Capt. Ericsson really mean that the maximum temperature indi- 
cated by a thermometer exposed to several varying sources of 
heat is not determined by the maximum radiators or convections 
