23 
NATURE 
[Mov. 10, 1870 

there was reallya convergence or whether the beams were parallel, 
and the convergence an effect of perspective, can only be decided 
if some approximative measures of the distance of the streamers 
be ascertained. It appears to have been at a greater distance 
from the earth than is usually attributed to aurora borealis, 
having been seen in different parts of Europe and I believe in 
America. Doubtless the comparison of these observations will 
give some parallax or approximation to measurement of the dis- 
tance. I remember about seven or eight years ago seeing an 
aurora at Chester, where the flashes appeared close to the 
observer, so that gleams of light continuous with the streamers 
could be seen between the houses of the town and myself, like 
the portions of a rainbow intervening between terrestrial objects | 
and the observer. I tried then to ascertain if there was any 
reflection or other cause of optical illusion, but could not see it 
as other than a real effect ; I seemed, so to speak, to be in the 
aurora. The effect on the 25th was very different, and gave me 
the idea of great distance. 
The light was sufficient to enable me to tell the time by my 
watch easily, but not to read newspaper print. 
Between half-past six and seven o’clock it faded away, and at 
from half-past seven to ten had become an ordinary white 
aurora, confined to the northern portion of the heavens. 
115, Harley Street, Nov. 2 W. R. GROVE 

On the evening of the 24th ult. the aurora was most beautifully 
seen here, and if you have space for it, I will add a further spec- 
troscopic observation to those you have already recorded. I 
found no continuous spectrum, but two of the lines described by 
your other correspondent. 
1. A line in the light green, much reminding one of the line 
from the larger nebulee, but more brilliant and with a peculiar 
flickering in it. This line was well seen in all parts of the sky, 
but was specially bright in the auroral patches of silver light. 
2. A line in the red, very much like the lithium line, but 
rather more dusky. This line was only wellseen in the rosy patches 
of the aurora, but could be faintly traced wherever the rose tint 
at all extended. 
When the display of rose-coloured light was at its height, the 
spectrum from the most vividly coloured portion gave the red 
line very distinct, while the green line still remained bright by its 
side. Lam‘quite inclined to agree with your correspondent, T.F., 
in the conjecture that both these lines are due to hydrogen, though 
(probably through difference in temperature or pressure) they do 
not quite agree with the lines of that gas as taken from the dis- 
charge in a vacuum tube. 
The spectroscope was one of Mr. Browning’s small direct- 
vision 5-prism instruments adapted for star purposes. 
It may be worthy of note that the belts of Jupiter are highly 
coloured at the present time. The equatorial zone is of a distinct 
dark ochre colour, deepening to red brown as it approaches the 
lower edge (in an inverting telescope) ; two thin belts above are 
slate purple, and a darker belt below is of a deep purple, with a 
faint trace of rose colour. 
The planet was thus seen on Noy. 2, at 9 p.m., not far above 
the horizon, and in bright moonlight, in a 84 Browning’s silvered 
spectrum with achromatic eye-pieces—144, 305, and 450; best 
I think with 144. 
Guildford, Nov. 5 J. R. Capron 

Clouds 
I vo not think Prof. Poey’s ‘‘ New Classification of Clouds,” 
published in NATURE of Sept. 8th, does much to advance science. 
Tsee no use in any classification of clouds, unless it is based on 
their mode of formation, and, so far as I see, there are but three 
ways in which it is possible for clouds to be formed, These 
are :— 
1. The cooling of a mass of air iz situ by radiation, 
forms stratus. 
2. The cooling of a mass of air by diminished pressure when 
it flows in an ascending column. This forms cumulus. A 
modification of this process is when (according to Espy) sudden 
expansion takes place above, so as to diminish the pressure 
through the entire height of a column of air, and, in consequence 
of the cold due to the diminution of pressure, to produce con- 
densation of vapour throughout the column. This is Espy’s 
explanation of waterspouts. 
3. The cooling of a mass of air by coming into contact with 
a cooler mass of air than itself. This forms cirrus, 
This 

Of course these three modes of formation may be modified 
and combined in endless ways. To mention one of the simplest : 
A cloud which has begun to form as a cirrus or cumulus, may 
become a centre from which heat is radiated, and thus go on 
forming as a stratus. 
It isin the highest degree unphilosophical to reject stratus asa 
species of cloud on the ground that it is ‘‘not a cloud properly 
so called, but a mist or hoar frost.” A cloud and a mist do not 
differ fundamentally. 
Prof. Poey is, however, right in saying that cumulus is not a 
distinct species of cloud. It is only a cloud which (in conse- 
quence, I believe, of the loss of electrical tension) has begun te 
run together into raindrops. JosEPH JOHN MURPHY 
Old Forge, Dunmurry, Co, Antrim 
Extreme Seasons 
A GREAT deal ot speculation has been indulged in to account 
for the extreme seasons that have prevailed over so large a part 
of the northern hemisphere during the last few months. In this 
country, as we are subject to extreme seasons, more particularly 
as regards the rainfall, the subject is one of peculiar interest. 
In a paper read before the California Academy of Sciences in 
February on the subject of our extreme seasons, I brought 
forward a number of observations to show that these were 
due to broad polar and equatorial currents occupying large 
portions of the earth’s surface continuously, and without much 
perpendicular or horizontal disturbance, except at the borders 
where the currents meet. 
showed that from October to the middle of February a northerly 
current prevails over this portion of the American continent, ex- 
tending from one to two hundred miles to the westward of San 
Francisco to the eastern edge of the Mississippi valley, whilst a 
southerly current prevails over the eastern side of the continent 
as far as the Atlantic. The southerly current to the westward 
extends uninterruptedly across the whole breadth of the Pacific 
to the coast of Japan. This same distribution of air currents 
without much perpendicular or horizontal mixing has apparently 
continued during the summer, and acounts, I think, satisfactorily 
for the extreme heat that has marked the continental climates 
overso large a partof the northern hemisphere. Nor is it surprising 
that the summer temperature on the continents should be so univer- 
sally hot, as a horizontal wind, either from the north or from the 
south, blowing over the land in summer must necessarily be 
a hot wind. That there is no cosmical cause for this elevated 
temperature is proved by the extremely low summer temperature 
prevailing over the Pacific between this place and Japan. The 
mean temperature, as ascertained by observations made on board 
the mail steamships between here and Japan was, for Noy. 1869, 
70°°2, for January, 62°°9, for May, 1870, 61:9, for July, 65°7, 
giving a mean of 2°°7 less for May and July than for January 
and February. The difference in favour of the winter temperature 
would be still more marked were the coast temperatures elimi- 
nated, as they perhapsshould be; as these were much abovethemean 
in summer and below the mean in winter, As to the causes that 
lead to the peculiar distribution of theair currents in certainseasons, 
I have not the slightest idea, but I think that, admitting the fact, 
it affords a satisfactory explanation of anomalous temperatures 
both in winter and summer. 
San Franciso, California, Sept. 4 JAMES BLAKE 
Cyclones 
CYCLONES are commonly regarded as exceptional phenomena 
of the atmospheric circulation ; and we see in text-books state- 
ments as to the seasons of the year at which they are most apt to 
occur, descriptions of the premonitory signs which herald their 
approach, and directions to aid ships in avoiding the most 
dan serous portions of the storm-field. In short, each cyclone is 
regarded as an exceptional fact, an isolated burst of fury from the 
old storm-god Hurakan. F 
The writer has lived all his life on the great highway ot 
cyclones, at Charleston, South Carolina ; and from the observa- 
tions of many years, has been led to conclude that this commonly 
received view embraces only those cyclones which, on account of 
their rotatory violence, really do threaten destruction on land 
and sea; and that consequently it overlooks a most important 
series of phenomena, which, though they do not so forcibly 
arrest attention, are even perhaps more significant in a scientific 
point of view. Though destructive cyclones or hurricanes are 
_™ re 
The facts I then brought forward 
