metal being found in certain islands north of Spain was sufficient 
to secure for them the distinctive title of Jvsule Cassiterides, or 
Tin Islands. G. F. RoDWELL 
The Greenwich Date 
Unper this heading, in your numberfor Nov. 28, a letter 
signed ‘* James Pearson” ends thus :—‘ The query then is—in 
what part of the globe and in what meridian does October 20 
end and October 21 begin?” As well ask where a circle ends 
and where it begins. See an article at the end of Bayle’s Dic- 
tionary, entitled, in the second Rotterdam edition, 1702, ‘‘ Disser- 
tation sur le jour,” vol, iii. p. 3118 ; in the London edition, 1741, 
** Dissertation concerning the Space of Time-called Day,” vol. x. 
p. 365. The difficulty, as Bayle shows, is in the nature of 
things. Let an equatorial railroad go round the world in twenty- 
four hours, with a station at every 45th meridian. At noon of 
October 20, Mr. West takes ‘‘a return ticket”? westward ; Mr. 
East takes one eastward. Both reckon by solar time. At every 
station Mr. West finds it noon, and on his return home reckons 
noon October 20 ; but the station-master reckons noon October 
21. Mr. East at 45° sees the sun set at 60’clock. At 90° he 
finds midnight ; at 1 3 the sun rises at 6 o’clock on October 21 ; 
at 180° it is noon. ere the two passengers pass each other, 
Mr. West reckoning it to be noon of October 20, Mr. East noon 
of October 21. At 135° W., Mr. East sees the sun set 5 at 90° 
he finds midnight ; at 45° the sun rises at 6 o’clock on October 
22,  Onhis return home Mr. East reckons it to be noon of 
October 22. Here then are three different reckonings, and prac- 
tically the keeping of Sunday, Christmas Day, &c., on different 
days in different countries exists at this instant and must exist for 
ever. Practically also those who sail eastward round the world 
get one more dinner than those who stay at home. Those who 
sail westward round the world get one dinner less than those 
who stay at home, and two dinners less than those who sail east- 
ward, when both voyages are completed. ; 
GEORGE GREENWOOD 
Brookwood Park, Alresford, Nov. 30 
Mr. Pearson’s query, in NATURE of November 28, does not 
admit of any exact or scientific answer, for there is no natural line 
vof demarcation or change, and the settlement is entirely a matter 
of usage or convenience. It is not very many years since the 
dates at Manilla and Macao were different; and till the cession 
of the Alaska Territory to the Americans, the date there was 
different from that in the British Territory adjoining. The rule 
now generally held is, that places in E. long. date as if they were 
arrived at by the Cape of Good Hope, and places in W. long. as if 
they were reached via Cape Horn—a rule that the width of the 
Pacific renders practically convenient. Afloat, the rule is for a 
ship making a passage to change her date on crossing the meridian 
of 180°, or as soon after as the captain may find convenient ; 
repeating or omitting a day, according to the direction in which 
she is going ; but a ship merely cruising across the meridian, 
with the intention of returning, does not generally change her 
date, so that ships having different dates may and do. occasion- 
ally meet—a very marked instance of which occurred during the 
Russian war, when our squadron from the Pacific joined the 
China squadron on the coast of Kamschatka. 
And thus, according to established usage, October 21 at Ade- 
laide, and October 21 at the hypothetical place in gh. 35m. W. 
long., are different days; in the two places October 21 has a 
different meaning. J. K. Laucuton 
Royal Naval College, Dec. 1 
THE Rey. J. Pearson is correct in the method of finding the 
corresponding Greenwich date, although its numerical perfor- 
mance is incorrectly performed in his letter, 
It is absolutely necessary for practical purposes to draw the 
line somewhere, and it is drawn in England an her colonies as 
well as in America and Russia, at the meridian 180° E. of 
Greenwich. The limit, therefore, of the longitude to be added 
to or subtracted from the Greenwich date will not exceed twelve 
hours. 
_ It is usual for sailors, when crossing this meridian, to skip a 
day, or to reckon the same day over again, according as the 
meridian has been reached from the eastward or westward. 
105 
An instance of this apparent anomaly is furnished in the 
Appendix to the “ Nautical Almanac” for 1874. The time of 
the phenomenon of the transit of Venus over the sun’s disc takes 
place generally about December 8, 16h. Greenwich astronomical 
time. Its recorded local astronomical time for the middle of the 
transit at Auckland, New Zealand (long. 174° 42’ E.), is Decem- 
ber 9, 3h, 40m. ; but for Woahoo (long. 158° W.) the time of 
the first, contact of Venus with the sun’s limb takes place at 
December 8, 3h.°4°7m: EpWARD ROBERTS 
Blackheath Road, Greenwich, Dec. 2 
Comets’ Tails 
CaN any of your readers refer me to a work by a recognised 
authority in astronomy in which I can find the method by which 
the direction of a comet's tail, as regards that of the heliocentric 
radius-vector of the head, has been calculated from observation ? 
Or, more briefly, have we axy proof whatever that there is other 
than an occasional chance coincidence of these two directions ? 
Gas 
REMARKS ON THE ZOOLOGY OF THE 
FAROE ISLANDS 
AS I have already announced in this paper, I started 
with the Danish expedition in September from 
Copenhagen, and arrived after a very fortunate voyage of 
four days in Torshavn, the little capital of Faroe in the 
isle of Str6m6. There I intended to remain while our 
steamer, with the geologists and engineers, went to the 
southern island (Suderé), where the miocene coal deposits 
are to be seen some hundred feet above the level of the 
sea in the basaltic rocks near the village of Qualb6. As 
to their researches about the extension of the coal-fields 
in Suderé, directed by Prof. Johnstrup, and as to the pos- 
sibility of taking the coals over to Copenhagen at a reason- 
able price, I cannot say anything now, as the report must - 
first be made to the Minister of the Interior, who will per- 
haps afterwards publish the results. Some words, however, 
about my own zoological researches in Torshavn will, I 
think, have some interest for the readers of NATURE. 
I remarked in my preceding paper that no wild 
mammals were known to occur in the islands, except 
some species of the genus,Mus. This is, as I now 
know, not quite correct ; for some thirty or forty years 
ago the northern hare (Lepus a/pinus) was introduced into 
the islands, and it seems to have met with very favourable 
conditions of life, as it is now spread in considerable 
numbers over Strém6, and has also been brought to 
Oester6. The hare finds ample food in the grasses cover- 
ing the ground ; the large rocks spread everywhere pro- 
tect him, and no mammals or birds of prey endanger his 
life, with the exception of Corvus corax, or the little alco 
aesalon, which sometimes might take the younger ones. 
The occurrence of the Masco tslandicus is too rare to do 
any serious damage to the hares. Besides these, they 
have also endeavoured to introduce the “ripers” (Zetrao 
lagopus) so common in Iceland and Norway, but those 
set free have perished without breeding. These birds 
require food and protection from trees, which, as it is 
known, do not occur in these islands. 
The rats found in the northern islands of Faroe (al- 
though they have not yet come to all the islands) be- 
long to the species Mzs decumanus, which here, as 
nearly everywhere in Europe, has nearly destroyed the 
smaller black rat (1/us rattus), still, however, to be found 
in some houses of Suderé. In the “ fields” still another 
species of rat is said to occur, not heretofore seen by 
naturalists. Mr. Randrop of Torshavn, who has taken great 
trouble in order to secure a specimen of the animal, the 
footsteps of which he has seen, thinks it is the Lemmus 
norvegicus, but he could never get it. Among the large 
aquatic mammals the “ Grindehval” (Delphinus globiceps) 
is known to be of great importance here, as nearly every 
year large flocks of it are taken, which they drive to the 
eT Pe ees 
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