April 6, 1871 | 

NATURE 
447 
«| (tee. _ 4 eee 
The practice of quoting the titles of foreign papers is aggravat- 
ing in the extreme, especially when accompanied by the remark 
that ape laborious but lengthy paper is not suited for ‘abstrac- 
tion ! 
I seldom go out of our foggy little island without hearing our 
want of familiarity with what is going on elsewhere roundly and 
deservedly abused. But I feel bound to say that, more especially 
in France, I can justly retaliate. 
Russian science is said to be very productive. 
at all events, is beyond most of us. 
New University Club 
That language, 
MARSHALL HALL 
Aurora Australis 
OpsERVING in NATuRE of Oct. 27, an account of a brilliant 
aurora observed in England on the 23rd to the 25th of that 
month, I think it may excite attention to the subject by informing 
your readers that at the same date a splendid display of Aurora 
‘Australis was recorded at most of the meteorological stations in 
this colony. 
The extent of sky covered was not so great as in the fine 
display on 5th April last, which was also coincident with a remark- 
able Aurora Borealis, but the red tint was so much deeper than 
usual, that many persons in this place attributed the phenomenon 
to the glare of the great fire which destroyed the town of 
Lyttelton, and the news of which was just then received by tele- 
graph. James HEcTOR 
Colonial Museum and Observatory, Wellington, N.Z., Jan. 2 

Ocean Currents 
My rejection of the idea that permanent differences of atmo- 
spheric pressure could produce any effect on Ocean Currents, was 
meant to be as sweeping as Mr. Johnston considers it. I believe 
that the idea is repugnant to the most elementary conceptions of 
hydrostatic equilibrium ; and I am particular in so far repeating 
the gist of my former letters, because Mr. Johnston, in his letter 
in NATURE for March 9, reiterates his suggestion that difference 
of atmospheric pressure is a power in the production of Ocean 
Streams, and whether he suggests that it is a supplementary 
power, or a chief one, is nothing to the purpose, if, as I distinctly 
maintain, it is not a power at all. 
My rejection of the idea that the formation of these differences 
of pressure can produce any appreciable effect, is quite as decided ; 
but Mr, Johnston is mistaken when he speaks of my denying 
also the influence of the movement of these differences of pres- 
sure ; for my remarks concerning them were to the very opposite 
purport ; and I pointed out that such movements do sometimes 
give rise to rapid and dangerous sets, known as storm-currents, 
which in their irregular and exceptional nature, differ essentially 
from those regular permanent or periodic currents usually under- 
stood by the general term Ocean Currents, though they may oc- 
casionally modify them both in direction and velocity. 
I would also call Mr. Johnston’s attention to the fourth para- 
graph of his letter, and assure him that I have never, directly or 
indirectly, maintained that the Trade Winds “ would account for 
the whole of the phenomena of Ocean Currents ;’ but I have 
maintained, and do still maintain, that all the phenomena alluded 
to may be very satisfactorily accounted for by a reference to the 
prevailing winds of the different parts of the world ; and that the 
Gibraltar Current is to be attributed, not to the local, partial, 
and peculiar wind of the Straits, but to the great body of the 
west wind of the North Atlantic, which also produces a northerly 
current on the coast of France, known distinctively as Rennell’s 
Current, and a southerly current on the coast of Portugal. 
T have discussed this question so fully in another place, that I 
should be only repeating myself were I to say more about it here ; 
but I may add that though, as Mr. Johnston asserts, under- 
currents cannot be caused primarily by the action of the winds, 
they can be, and frequently are, caused secondarily by that action ; 
and many a ship has owed her safety from the apparently immi- 
nent danger of a lee-shore, to the “under-tow,” or reflux of the 
water swept towards the shore on the surface. If there is a 
deep-flowing outward current in the Straits of Gibraltar, I believe 
it to be exactly of the nature of an ‘© under-tow ;” it seems to 
me probable enough that there is occasionally such an outward 
current ; but I cannot admit that the ove observation of it which 
Dr. Carpenter considers he obtained, after several attempts made 
in vain, has abundantly proved its existence 5 still less can I 

admit that it is necessary to call in difference of temperature and 
density to account for it. 
Mr. Croll considers that there is a similar escape of water, 
underneath, from the northern to the southern hemisphere, and 
his arguments appear to warrant the suggestion, although no 
such under-current, or system of under-currents, has yet been 
observed. I see no improbability in the idea; but so many 
mistakes have, at different times, been made by trusting rather 
to theory than to positive evidence of fact, that we cannot be too 
cautious in admitting the existence of such under-currents, with- 
out any reliable observations. For that recorded by Captain 
Maury, to which Mr, Johnston refers, has, from the vague 
manner in which it is described, no scientific value whatever. 
There is no mention of locality, season, direction of wind, swell 
or surface current, no mention of the relation between the effect- 
ive area of the ‘‘block of wood loaded to sinking” and the 
barrega or breaker which floated it ; the depth is spoken of as 
indifferently one hundred or five hundred fathoms ; above all, no 
mention is made of any means being taken to distinguish be- 
tween an apparent and real set of the breaker. It is quite clear 
that if the loaded block was lowered into still water, the breaker 
to which it was attached was, to a certain extent, moored, and 
the surface drift of the boat away from it would give it all the 
appearance of moving in the opposite direction. No mention is 
made of the method adopted to discriminate ; or, in fact, of any 
method at all being adopted, or any attempt made to eliminate 
or neutralise the many errors which necessarily find: their way 
into such an observation ; all that we are told is that “ it really 
appeared as if some monster of the deep had laid hold of the 
weight below, and was walking off with it.” To such an account 
one is almost attempted to add—very like a whale. 
I have dwelt on the thoroughly unsatisfactory nature of this 
experiment, because, from the description of it having been 
repeated in every edition of the ‘‘ Physical Geography of the 
Sea,” I find it constantly referred to—as Mr. Johnston has now 
referred to it—as a conclusive proof of the existence of strong 
counter under-currents at great depths ; whereas in reality it is 
a proof of nothing, unless, perhaps, of the careless style ot 
observing which was accepted as sufficient twenty years ago. 
The other instance which Mr. Johnston brings forward would 
be really remarkable, if we only had some evidence of it asa 
fact ; he speaks of the warm water of the Atlantic dipping down 
beneath the cold and ‘specifically lighter” water of the east 
Greenland current. It has been well known, long before the 
late German expedition, that at the meeting of the two waters 
there is a distinct line of demarcation, but such a line does not 
necessarily indicate a dip of either water, such as Mr. Johnston 
describes ; as indeed has been very fully shown by the survey of 
the nearly vertical ‘‘ cold wall” of the United States, along 
which the line of demarcation is more distinct than anywhere in 
the world. And besides, can we admit that the water of the 
East Greenland current is ‘‘ specifically lighter” than that from 
the Atlantic? that the cold water is lighter than the hot, the 
salinity of the two being very nearly equal? Captain Maury speaks 
of hot water, like oil, running over cold ; Dr. Carpenter illus- 
trates the same idea in a long glass trough, showing plainly 
enough the way in which he conceives the interchange to take 
place. I do not attach so much value as Dr. Carpenter does to 
this illustration, which represents a system of motion entirely 
different from that of the ocean-currents ; but accepting it as the 
exposition of the views held by the leading supporters of the 
claims of temperature and density, it is utterly antagonistic to 
the idea of this extraordinary dip of warm water said to take 
place near the east coast of Greenland. Whether we consider it 
from a purely theoretical or from a geographical point of view, 
the idea is wholly unsupported, and can only be classed as one of 
those crude speculations which, in every branch of science, do so 
much harm by tending to unsettle the minds of those who indeed 
take an interest in the subject, but have not made it a special 
study. J. K. LaAuGHTon 
Draper’s Experiment simplified 
WiIsHING to repeat Draper's Experiment, and casting about 
for a simple method of performing it, it occurred to me to take 
advantage of the intense heat evolved in the combustion of 
sodium, and, beginning with the entire spectrum, watch its 
degradation as the heat declined ; to which end I formed a 
shallow, conical cup of thin copper wire, half an inch in diameter, 
and, putting therein a piece of sodiwm, applied a spirit-lamp till 
