- darting streamers in the zenith. 
[Fume 155 18; 

stantly darting with a flickering motion from the surrounding haze 
of similar light, and meeting in the zenith; the length of their 
course was as much as 15 to 18°; they appeared to 
indiscriminate!y from all points in azimuth. 
I immediately became aware that the whole sky, down to the 
very horizon, was illumined by a white, colourless aurora ; but I 
was so fascinated by the incessant play of the streamers over- 
head, that for some time I could notice nothing else. At last 
I turned away in order to observe accurately the full-extent of 
the aurora in all directions. I found that it reached quite down 
to the horizon all round, except in one place, viz., in the S.S.E., 
and at that point there was a symmetrical are (of a great circle, 
as far as I could judge) the summit of which was about 7° or S 
above the horizon. This are was perfectly well defined ; within 
it was blue sky, and above and around, over the whole heavens, 
nothing but the auroral light, except in the gaps between the 
I carefully took the bearings of 
this remarkable arc, and found by means of a compass the next 
day, that it was bisected by the magnetic meridian. The phe 
nomena underwent no diminution during the time (an hour and a 
half) I was watching it. The sky appeared to be quite free from 
I have often witnessed fine displays of aurora ; one in the 
winter of (I think) 1848, in this county, the colours and streamers 
of which were magnificent, far finer than those I saw on the 24th 
and 25th of last October at Edinburgh ; but I never before ob- 
served the stars to be so much dimmed as they were at Nairn in 
September, notwithstanding the light on that occasion was colour- 
less On other occasions, I have always thought the stars quite 
unaffected by the auroral light, both to the naked eye and in the 
tel , but on this they were obviously dimmed as by a haze. 
My impression is, that no aurora that I ever saw could be 
visible in daylight, with the exception perhaps of this last, and 
the only portion of this that could possibly be seen in daylight 
was the well-defined arc low down in the S.S.E. Ithinkitis 
just possible that in a clear and cloudless sky such an are as this 
might be visible. HENRY Cooper Key 
Stretton Rectory, Hereford, June 6 
Red and Blue 
I was much interested by the letter of Mr. T. Ward (NatuR5, 

aged estates tre change teal ohihat the time of the flow 
the tide that great inundations occur in the Delta. 
On the coast of the United States, however, there is apparently 
an intimate relation between low and high tides. In 
general, so longas the barometer remains low, easterly winds are 
blowing on the coast and heaping up the waters in every 
During the second stage of the storms the winds 
violently from the west or north-west, often at a right angle to 
= - 
a 
bay. 
Diow — 

the whole coast. These high westerly gales cause very low tides “— 
along the United States when the barometer is rapidly rising. 
— tides are not experienced with high pressures if the air 
is calm. 
High tides only occur on the coast of Europe after 
westerly — 
winds have been blowing for some days in the Atlantic Itis2 
well-recognised fact among the fishermen on the east coast of Scot- 
land that high tides are due to this cause. I think Hugh Miller 
was right in maintaining that the friction of the south-west winds 
on the wide surface of the Atlantic must be quite as powerful in 
maintaining the flow of the stream through the Florida Channel 
as the action of the Trade winds in forcing the tropical waters 
into the Gulf of Mexico. 
Owing to the great rapidity with which barometric disturbances 
are propagated in our temperate Istitudes, it is dificult to con- 
ceive how barometric pressure of itself can have an a i 
influence on the currents of the ocean. The rate of their propa- ~ 
gation in winter is from thirty to sixty miles an hour. In the 
fourth number of the “Board of Trade Weather Report” an 
instance is given, in which the rate is affirmed to be upwards of 
seventy miles anhour. The velocity in this case, however, as I 
may try to show on another occasion, is estimated about ten. 
miles an hour too high But let us suppose that no winds 
accompanied these rapidly propagated depressions and elevations _ 
of the barometer. A difference of an inch of pressure existing 
between places on the ocean two or three hundred miles apart _ 
would create only a very slow moving current, even though the 
diminished area of pressure were i . But these low 
pressures pass so rapidly onwards that the ws imertie of the 
waters of the ocean would hardly be overcome before they were 
againsubjected tothe opposite influence of an crease of pressure. 
The effect of barometric pressure on the level of inland seas, 
like the Baltic and Mediterranean, must be still less than m the 
open ocean. Winds are often localised, bat great depressions of 
the barometer extend over immense areas; Im most cases far 
vol. iv. p. 68) describing the appearance of a blue colour when | Jarger than the area of the Baltic. Any higher level from this 
looking at white chalk marks on a black board while the 
was shining in the eyes, as I have frequently noticed a precisely 
complementary phenomenon. 
While walking along the chalky rads of East Kent in bright 
sunshine, and reading under an umbrella, I have frequently 
noticed that the letters appear of a deep blood-red colour ; the 
black colour of the type reappearing immediately om passing 
over the shadow of a tree on the ground, or on allowing the 
sun to shine directly om the book. This was so striking when 
first seen that I had to convince myself that the page was not 
printed in redink. This 3s obviously the exact converse of the 
observation of Mr. Ward, who saw a blue colour from white 
marks on a black surface, while I saw a red colour from black 
marks on 2 white surface. 
cisely similar colour when looking down on the platform of a rail- 
way Station with the setting sun shining on the eyes, the cracks 
between the beards alsoappearing red. «60s HEBERT M‘LEoD 
~ Influence of Barometric Pressure on Ocean Currents 
A tow barometric pressure and an increased height of the ocean 
was, I believe, first assumed to stand in the relation of cause and 
effect by Mr. Piddington. The abnormally hich tidal waves 
that sometimes rus up the Hooghly dunmg Calcutta hurri- 
canes were ascribed to the low pressures which accompanied 
them. There is no doubt that unequal pressure is a tree cause of 
currents in the ocean. But I think it, as well as difference in 
ihe eravity, may be rezarded as infinitesimal in amount, com- 
pared to the influence of the winds 
The high tidal waves at the mouth 
perienced during the first stage of the hurricame, orso leng as 
the wind blows from a northerly quarter. The waters in the Bay 
of Bengal are then propelled towards the south It is only after 
the wind chances to the south and the barometer i: rising that 
the waters are driven against its northern shores. It is when the 
A short time since I observed 2 pre- | 
Pea. | Small scale that this acute observer was enabled to give 2 con- 
| 
; 
| 
sun cause would be brought about by the flow of the waters from 
either end, as the pressure might be assumed to be the same e= 
both sides of that narrow sea. The mere effect of of 
barometric pressure, it will be admitted, would be quite mappre- 
ciable in any inland lake in Brita But any one who is im the 
practice of fishing in the smallest of our lakes may always observe 
that there is an under current or “dre” created when the wind 
blowsstrongly towardsthe shore, in consequence of the accumula- 
tion there of its waters. The Nisgara is sometimes suddenly raised 
two feet by strong west winds blowing over hke Erie. It is long 
since M. Volmey, as regards the Mediterranean, stated that east 
winds caused a rise or flood of from two to three feet m the 
harbour of Marseilles, and that westerly winds produced oppasite 
effects. It was by a careful deduction of effects produced on a 
sistent outline of the causes which produced the ocean currents 
in general The currents of the ocean may be regarded as 
coinciding very closely with the average force and direction af 
the winds over its surface. Simce, however, Humboldt assares 
} us that the surface water of the Gulf Stream im the Flonds Straits 
is sometimes reversed by the force of the winter “ northers,* 
of Hooghly are not ex- | 
t 
it does seem vain to attempt to trace surface currents 
in any partof the North Atlantic, vexed as its surface is by winds 
so inconstant in their force and direction. 
Pilmuir, Leven, Fifeshire 
St. Mary’s Loch, Selkirkshire 
To the student of Nature it may seem easy to decide whether 
the water of any civen Iske is good for domestic uses. Bat as 
regards St. Mary’s Loch, where the question bas to be settled 
by dint of a squabble im the Auld Reekie municipality, with all 
its complementary dust, smut, and heat, the true aspects of 
Nature are lable to misrepresentation. 
Although not resident in Edimbargh, nor 
jedices and ratings, your correspondent -has 
R. RUSSEEEL 
sabject to its pre- 
taken considerable 
