May 14, 1885 | 
NATURE 
29 
for the various subjects of the mineral kingdom, the 
work, however, supplies a want which has been long felt, 
and it will prove convenient for purpose of reference. 
The amount of detail will be better appreciated if we 
mention that in the description of the uses of carbonate 
of lime even the hammers used by stonemasons are 
specially figured. 
LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his correspondents. Neither can he undertake to return, 
or to correspond with the writers of, rejected manuscripts. 
No notice is taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space is so great 
that it ts impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts. ] 
Photographing the Aurora Borealis 
I SHOULD be obliged if I might be permitted to state, with 
reference to the negative of the aurora borealis obtained by Mr. 
Tromholt in Christiania on March 15 (NATURE, vol. xxxi. 
Pp: 479)—the first ever obtained—that he now informs me that, 
although the plate was exposed for eight and a half minutes, the 
said impression is so faint and imperfect ‘hat it cannot be repro- 
duce l as a positive. My object in asking to be allowed to mention 
this important fact is to show that the opinion expressed by Mr. 
Tromholt in his work just published, ‘‘ Under the Rays of the 
Aurora Borealis,” that it is almost impossible to photograph 
the aurora borealis on account of the small strength of light and 
its limited chemical action, may be said still to hold good in the 
main. CARL SIEWERS 
Speed and Velocity 
Your reviewer of Williamson and Tarleton’s ‘‘ Dynamics” 
(Nature, February 26, p. 385) speaks of the confusion therein 
of speed and velocity. Does he mean that these words should 
now be used in distinct senses? Ifso, would he kindly specify 
the distinction, which is unknown to me and my friends. 
[Certainly. Velocity is a directed quantity, or Vector. Speed 
is its Tensor.—YouR REVIEWER. ] 
Time.—Thunderbolts.—Vision.—Sunglows 
ON my return from a magnetic tour along the Red Sea, I ask 
leave to refer to some back numbers of NATURE. In vol. xxxi. 
p- 125, Latimer Clark is quite right when he says that mean 
and sidereal time ought to be distinguished by names. I 
should prefer a step farther, and use for the latter the decimal 
angle, thus abolishing our frequent and tiresome conversion of 
time into space, and vice versd. The resulting advantages would 
be obvious. 
Answering Herr Von Danckelman’s remarks in vol. xxxi. p. 127, 
I beg leave to quote my memoir, ‘‘ Sur le Tonnerre en Ethiopie,” 
published in 1858 by the French Institute, among its Mémoires des 
Savants érangers. Facts mentioned there do not support the 
opinion that fatal thunderbolts are all but unheard of in Tropical 
Africa. 
In your published remarks on vision, is it not Lord Rayleigh 
who says that the supposed superiority of eyesight among savages 
may be explained otherwise? Years ago, when reading Bergmann’s 
travels among the Kalmouks, I noticed his remark that when 
examining camels rvelurning to the fold, those natives distin- 
guished sexes with their naked eyes just as well as he could 
through his excellent field-glass. In conclusion, Bergmann says 
that savage eyes are superior to civilised ones, or something to 
that effect. I must confess that I then accepted his opinion as 
being admirably warranted by the quoted facts. However, 
some time afterwards I was travelling on foot in the Pyrenees 
with a Basque illiterate peasant, and a splendid refractor by 
Cauchoix, which I proudly carried myself. My companion 
having tauntingly asked me why I had not left that lumber at 
home, I gave him, foolishly, a lecture on optics, and wound up 
by saying that the glass enabled me to distinguish a cow from an 
ox, even from that distant hill. He said he could do as much 
without my lumber. I then selected a cow grazing, and asked 
him what that was. “ Wait till the brute walks,” said the 
peasant ; and at its first step he exclaimed: “it is acow.” I 
tried him, then, several times, and never found him in fault. 
He affirmed that cows and oxen do not lift their legs in the 
same way. May I request your rural readers to tell us whether 
that remark applies to English cattle? When on the Atlantic a 
sail was announced for the first time. I could perceive nothing, 
because I had not yet learnt what kind of a hazy thing I should 
distinguish. Having then sharp eyesight, I succeeded after a 
short practice, in discerning distant sails before any of my com- 
panions, and could turn tables on them by repeating their own 
saying, ‘‘ Ca créve les yeux.” 
To your lore on far-sightedness in vol. xxxi. p. 506, allow 
me to add two instances. Zach saw from Marseilles, Mount 
Canigou (2700 m.), at a distance of 158 English miles ; he had 
calculated the true azimuth beforehand, and says that the peak 
bursts into view at sunset. Sir W. Jones informs us that the 
Himalayas have been seen at the great distance of 244 miles. 
I quote this from Carr’s ‘‘ Synopsis,” a useful volume, which 
I regret to see behindhand in many cases since the death of its 
clever author. 
May I intrude here a comment on our mysterious sunglows ? 
My companion having a nice eye for discriminating colours, has 
confirmed my notion that on rising from the horizon the suc- 
cessive zxances of fiery red, faint red, rose, mallow, prussian 
blue, and green, are of the same on consecutive days, although 
thermometer, barometer, and wind have not changed. This 
suggests the hypothesis either that the lower strata of our atmo- 
sphere undergoes changes otherwise unperceived, or that there 
are maxima, minima, and perhaps regular epochs in the pheno- 
menon. To those who, unlike myself, remain stationary under 
a rainless sky like that of Egypt, I would recommend a careful 
record of these changes, at least during a few months. 
Cairo, April 22 ANTOINE D’ABBADIE 
Plutarch on Petroleum 
THERE is in “ Plutarch’s Lives,’ in the life of Alexander, 
an interesting notice of the petroleum of Media ; I have not 
found any mention of this passage in ‘‘ Plutarch ” either in 
encyclopzdia or chemical dictionary ; I trust, therefore, that 
you will give me the opportunity of reproducing it in NATURE. 
I transcribe the passage from the translation of John and William 
Langhorne (9th edition, London, 1805) :— 
««, . and in the district of Ecbatana he (Alexander) was par- 
ticularly struck with a gulph of fire, which streamed continually 
as from an inexhaustible source. He admired also a flood of 
naptha, not far from the gulf, which flowed in such abundance 
that it formed a lake. The naptha in many respects resembles 
the bitumen, but it is much more inflammable. Before any fire 
touches it, it catches light from a flame at some distance, and 
often kindles all the intermediate air. The barbarians, to show 
the king its force and the subtlety of its nature, scattered some 
drops of it in the street which led to his lodgings ; and, stand- 
ing at one end, they applied their torches to some of the first 
drops, for it was night. The flame communicated itself swifter 
than thought, and the street was instantaneously all on fire.” 
W. H. DEERING 
Chemical Department, Royal Arsenal, Woolwich, May 6 
Hut Circles 
THE remains of the ancient British habitations on the downs 
on both sides of Dunstable are fairly well known to archzolo- 
gists. I have often wished to expose the floor of one or more 
of these circles, as the task could be accomplished with a spade 
in an hour or two. It is, however, far better that the remains 
should be left alone, as it is not likely that anything would be 
found beyond a few flakes and the other simple forms, such as are 
abundant in the cultivated fields close to the huts. 
On passing some of the circles on the east side of Dunstable, 
in the railway, about ten days ago, I noticed that the remains 
were covered with whitish soil instead of the normal green of 
the short pasture belonging to the downs. Thinking that some 
persons had been digging at these antiquities, I took an early 
opportunity of going to the spot. On reaching the circles I 
found they had been undermined in every direction by a large 
number of moles. A great deal of the material from the actual 
floors had been brought to the surface, and on examining this 
chalk rubble—for such it was—I had no difficulty in securing 
two or three handfuls of flint flakes. Mingled with them were 
