340 
the most perfect barrow of its kind in the west of England, 
it was used by the farmer as a shelter for sheep or pigs, 
but it is not known when it was opened. 
(To be continued.) 
OUR BOOK SHELF 
The Useful Plants of India, with Notices of their chief 
Value in Commerce, Medicine, and the Arts. By Col. 
Heber Drury. Third edition, with Additions and Cor- 
rections. (W. H. Allen and Co., 1873.) 
THE first edition of this useful work was published in 
1858, since which period our knowledge of the economical 
products of our vast Indian possessions has greatly in- 
creased ; and we have here an enumeration of 600 herbs 
or trees from which more or less valuable substances are 
obtained. The species are aranged in alphabetical order, 
the natural order and native and English names of each 
are given, followed by a description, and an account of its 
economic uses, taken from various standard works, or 
from the author’s own observation. The list is not con- 
fined to natives of the country, but includes also such in- 
troduced plants as are largely cultivated and of great eco- 
nomic importance, as cinchona, tea, cacao, tobacco, and the 
Australian eucalyptus, now so extensively planted toreplace 
the forests which have been destroyed in many parts of 
the peninsula to the great deterioration of the climate. In 
an appendix are statistics of the cultivation of cinchona, 
indigo, tea, and some of the fragrant woods, a table of 
exports and their value, and lists of synonyms in the Hin- 
dostanee, Bengalee, Tamil, Teloogoo, and Malayalam lan- 
guages. The technical descriptions, and other details, 
have been worked out with great care, and with abundant 
reference to original authorities, as far as was possible toany 
one undertaking a work of this description at Trevandrum, 
and without access to the libraries and herbaria which 
are at the command of students in this country. Col. Drury 
has, however, obtained the assistance of Dr. Hugh Cleg- 
horn, and other practical botanists, and his work is one 
that can be fully relied on as giving an accurate and 
nearly exhaustive account of the economical productions 
of our Indian empire, A. W. B. 
LETTERS TO THE EDITOR 
[ The Editor does not hold himself responsible for opinions expressed 
by his correspondents. No notice is taken of anonymous 
communications. ] 
External Perception in Horses 
May I be allowed to express my entire agreement with the 
theory about smell in dogs, brought forward by Mr, Wallace 
and Mr. Croom Robertson. The latter gentleman’s arguments, 
in your lastnumber, seem tome as sound in fact as they are 
metaphysically acute. 
May I assure him, from long observation, that horses and 
donkeys ‘‘think with their noses” as certainly, though not, I 
believe, as acutely or as continuously as dogs do. But the eye- 
memory of a horse seems to me so much more exercised than 
his nose-memory, that he is, perhaps more able, when lost, to 
find his way home than is the dog, who has smelt everything, 
but looked at very little, C. KINGSLEY 
Feb. 28 
External Perception in Dogs 
Mr. G. Croom Rosertson’s and Mr. Alfred W. Bennett’s 
observations may be easily tested by the cases of blind dogs. A 
blind dog in my house finds her way about as truly as a sighted 
dog, so that a stranger on seeing her would not be aware of her 
blindness. As she lost her sight by illness, she has of course 
the precedent knowledge derived from seeing. 
To a considerable extent this case answers Mr. Bennett's 
requirements. HYDE CLARKE 
St. George’s Square, March 1 
NATURE 
eS eee 
[Mar. 6, 1873, 
Mr. Wallace on Instinct 
_ IN reference to the letters of Mr. Darwin and Mr. Wallace, 
the following passage from Boswell’s Life of Johnson may be 
worth recalling :— 
‘©The custom of eating dogs at Qtaheite being mentioned, 
Goldsmith observed that this was also a custom in China; that 
a dog-butcher there is as common as any other butcher; and 
that when he walks abroad, all the dogs fallonhim. Johnson.— 
‘That is not owing to his killing dogs, sir. I remember a 
butcher at Lichfield, whom a dog, that was in the house where 
I lived, always attacked. It is the smell of carnage which pro- 
vokes this, let the animals he has killed be what they may.’ 
Goldsmith.—‘ Yes ; there is a general abhorrence in animals at 
the signs of massacre. If you put a tub full of blood intoa 
stable, the horses are like to go mad,.’” (Croker's Ed., vol. iii. 
p- 275.) W. R. NICOLL 
Aberdeen —— 
Effect of Light on the Electric Conductivity of 
Selenium 
Tr is of course impossible not to feel intense interest in the 
statement (NATURE, vol. vii. p. 303) which Mr. Willoughby 
Smith makes and which Mr. Latimer Clark endorses. That I 
have been unable to obtain the same result has doubtless been 
due to my having worked under conditions different from those 
existing in Mr. Smith’s experiments. My failure has not been 
one of degree, but has been absolute. I have not only been 
unable to find that light increases the electric conductivity of 
selenium, but I have failed to get a current through selenium at 
all, even through a thickness of o'r millimetre. As I do not 
know how to put myself at once in direct communication with 
Mr. Smith, perhaps you will permit me to ask him through your 
columns to guide me on the following points :— 
(z.) What was the form of battery employed, and what its 
power of overcoming British Association units of resistance ? 
(6.) What was the molecular condition of the “metal” (sic) 
employed,—vitreous or crystalline? 
(c.) Where can ‘‘ bars” of selenium be obtained which will 
afford the results stated ? 
(@.) Are there any unstated conditions essential to the success- 
ful production of the phenomenon ? 
Harry NAPIER DRAPER 
In the description given in NATURE. of February 20 last, 
of the very remarkable variations in the electrical resistance of 
bars of selenium due to the action of light, no detail is given 
to show how such an excessively high resistance as 1400 
megohms is measured. 
I am anxious to repeat the investigation of this very inte- 
resting, and as far as I know, wholly unexpected property of 
selenium, my idea being to measure the resistance of the bars — 
when exposed to the light of the solar spectrum, noting the 
position in which the effect is at a maximum, and the extent to 
which the resistance is affected in the different parts of the 
Spectrum. 
But before I can do this I must be able to measure these enor- 
mously high resistances satisfactorily, and I would therefore ask 
if you or any of your readers would tell me how I am to do this, 
using resistance coils up to 60,000 B.A.U., and a reflecting gal- 
vanometer with a resistance of 1,200 B.A.U. M. L. SALE 
Brompton Barracks 
The Zodiacal Light 
Since I last wrote upon this subject my views have been 
strongly confirmed. Both branches of the zodiacal light have been 
visible for some time past, and it is either getting brighter, or 
four months’ continual practice enables me to detect its presence 
under unfavourable circumstances much more readily. 
The night of January 30 was wonderfully fine ; the ground 
of the heavens was intensely black, and the Milky Way was 
simply one blaze of light from the zenith to the very horizon : 
only on such nights as these are observations of the zodiacal 
light worth recording ; all others must be very imperfect. 
That night the western branch was very distinct from the 
horizon up to the Hyades in Taurus; at this point its breadth 
was much greater than on November 27 ult. ; here it probably 
crossed first the branch of the Milky Way which tends towards 
Orion, then the Milky Way itself, and so was not visible for 
about 40° on the Ecliptic ; but it became visible again in Gemini, 
though very faint, and it did not quite reach Prasepe in Cancer. 
