Fuly 6, 1871] 
NATURE 
183 

nor his former promise was ever realised. He avoided public 
discussion, whil as it now appears he fvivately depreciated results 
incompatible with his own. To Archdeacon Pratt I am grate- 
ful for producing evidence of the kind of weapon which I had 
long suspected to have been employed by my distinguished 
adversary. HENRY HENNESSY 
Dublin, July 1 

Oceanic Circulation 
Mr. LAUGHTON treats an experiment which was only intended 
to be illustrative as if it had been advanced as probative, and 
tests it by a doctrine of ‘‘thermometric gradients” which does 
not correspond to the facts of the case. A uniform reduction of 
the temperature of ocean-water from the Equator to the Pole 
would doubtless give a ‘‘thermometric gradient” of infinitesimal 
minuteness. But the water of the circumpolar area, on which 
what Sir John Herschel truly designated the intense action of polar 
cold is exerted, brings with it so much of equatorial heat that a 
very decided increase of its specific gravity must be produced by 
the cooling process to which it is subjected within the polar area, 
This increase will be adequate, as I have attempted to show, to 
produce a continuous downward movement of the whole mass of 
water subjected to the cooling process ; and such a movement, 
however slow, will make itself perceptible in a continuous outflow 
of the chilled dense water along the deepest floors of the great 
oceanic basins, and in a continuous indraught of warmer surface 
water into the polar area. The proof that such is the case seems 
to me to be afforded by the fact that temperatures not much above 
32° seem to be uniformly met with at depths exceeding 2,000 
fathoms, even under the equator ; a fact of which Mr. Laughton 
and those who think with him have not, so far as I am aware, 
offered any account. That there is nothing in depth, Zev se, 
which produces this depression is shown by the absence of it in 
the Mediterranean. 
It would be difficult, if not impossible, to carry out a probative 
experiment that should represent the actual conditions of the 
case. Taking the distance from the pole to the equator at 6,250 
miles, and the average depth to which the chilled water would 
descend at 2$ miles, we should require a trough having a propor- 
tion of 2,500 to 1 between its length and its depth, or (in round 
numbers) a length of half a mile to a depth of a foot. Let it be 
supposed that cold were continuously applied by a powerful 
freezing mixture to the surface of the water occupying one 
extremity of the trough as far as one-tenth of its length, and that 
heat were applied to the surface of the water occupying the 
opposite extremity to a corresponding extent, the intervening 
water being neither heated nor cooled artificially, would, or 
would not, a continuous circulation from the one end of the 
trough to the other come to be established? To me it seems 
that what Sir John Herschel calls the ‘‘common sense of the 
matter” teaches that the continuous descending movement given 
to the water at the polar end of the trough must in time propa- 
gate itself to the equatorial, provided only that the conducting 
power of the sides and floor of the trough were sufficiently bad to 
prevent the chilled stratum which falls to the bottom at one end 
from losing its cold before it reaches the other. 
When such masters of Thermotics as Pouillet and Herschel 
consider that the doctrine of a general oceanic circulation sus- 
tained by differences of temperature is conformable to the facts at 
present known, I would suggest whether it would not be wise if 
those who are interested in the subject, instead of attempting to 
controvert their views on theoretical considerations, were to use 
their endeavours to collect additional data for practically testing 
them. By the kindness of the Hydrographer to the Admiralty I 
hope, in the course of the present season, to obtain some further 
information of a reliable kind ; and I am doing my utmost to 
urge upon our Government a systematic inquiry into what the 
Secretary of the Scottish Meteorological Society has truly desig- 
nated (in a recent letter to me) as ‘‘ the most important problem 
in Terrestrial Physics.” 
July 3 WILLIAM B, CARPENTER 

r 
I sHOULD need Mr. Laughton’s hint if I had ever supposed 
that the cause of the vertical circulation of the ocean could be 
determined by such an experiment as I suggested. The experi- 
ment was specially intended to throw light on the easterly and 
westerly oceanic movements. For this purpose it is only neces- 
sary that the rate of rotation of the shallow cylinder should be 
- duly adjusted to the observed rate of the vertical motions. But 

even in this respect the experiment would afford but an illustra- 
tion, not a demonstration. 
The subject of oceanic circulation is altogether too wide and 
too difficult for discussion in letters. Every point touched on 
by Mr. Laughton requires many columns for its full discussion. 
I just note that the infinitesimal nature of the thermometric 
gradients scarcely seems a sounder objection to the temperature 
theory of oceanic circulation than to the temperature theory of 
atmospheric circulation. In one case, as in the other, we must 
integrate the effects of the solar light on tropical and subtropical 
regions, RICHARD A. PROCTOR 
Day Auroras 
Last evening, about eight o’clock, being in the grounds 
belonging to the Radcliffe Observatory, I was exceedingly sur- 
prised at seeing what I have xo doubt of being true auroral 
streamers, forming a little to the east of the south meridian, 
reaching an altitude of about 25°, and after travelling some dis- 
tance in a westerly direction, vanishing. This lasted at least ten 
minutes, when the sky, which had been overcast nearly all day 
again became so. I pointed the streamers out to several people 
who were near me, some of whom watched them with me, as a 
proof of what I had before doubted, namely, that auroras are 
visible by daylight. : Joun Lucas, 
Assistant at Radcliffe Observatory 
Radcliffe Observatory, Oxford, June 28 
The 
I REGRET that I have misinterpreted the severity of Prof. 
Newcomb’s remarks respecting my chapter on the Solar Parallax. 
The fact is, that so far back as February 1 I was warned by an 
eminent astronomer that Prof. Newcomb had vowed here last 
November that he would annihilate all who upheld the finality 
or correctness of Mr. Stone’s researches. 
Prof. Newcomb must be sensible that his offer to supply infor- 
mation as to the history of inquiries into the solar parallax during 
the last few years is a very generous one; and that it will be 
immensely to my advantage to profit by his exceptional fami- 
liarity with the subject. I thank him very earnestly. I have 
an especial distaste for inquiries into the historical parts of scien- 
tific subjects, and shall rejoice to be saved the labour of looking 
up authorities, &c., in this particular matter. If I find my 
account requires alteration, I shall admit the fact without a par- 
ticle of hesitation. It is indeed most desirable (though not, 
perhaps, for students of science, for whom I specially write, and 
who need trouble themselves little on the matter) that to each 
worker in the subject of the solar parallax his due proportion of 
credit should be assigned ; and asin this case not only J, but Sir 
John Herschel, as well as the Council of the Astronomical 
Society, would seem to have done Prof. Newcomb less than 
justice, the sooner recantation is made the better. 
Prof. Newcomb refers to ‘‘the kind spirit in which I have 
taken his remarks ;” meaning rather, perhaps, the appreciative 
way in which I have spoken of his labours. His critique, 
regarded as a whole, was not, I take it, kindly meant; and 
though I by no means feel annihilated by it, I should be speaking 
untruly if I seemed to admit its justice. If I failed to note how 
I viewed his comments, it was only because I found a pleasanter 
subject to speak about in those important researches whereby he 
has advanced astronomy. RICHARD A. PRocTOR 
P.S.—I take this opportunity of noting that the remark in my 
former letter respecting the work of Mr. De La Rue and F. Secchi 
in 1860 must not be understood as implying that the account in 
F. Secchi’s book Ze So/ei/ is incorrect. On the contrary, I have 
no doubt itis strictly accurate. I was fortunate in securing a copy 
of Ze Soleil before Paris was beleaguered, and derived consi- 
derable assistance from its perusal. 
Solar Parallax 

Lee Shelter 
PERHAPS it is worth noting that a lee shelter is almost as 
effectual asa screen to the windward. The fact may be quite 
well known and understood ; but I did not become aware of it 
till I was on Bognor Pier, when a strong gale was blowing 
directly on the broadside. There are seats backed and covered 
overhead and on the sides, alternately, on the one or other side 
of the pier, and on this occasion all the seats to the windward 
were occupied, so that, wanting a rest, I had to put up with one 
