March Tal 88 3] 
NA TURE 
413 
opinions ; that we must be prepared to receive fresh ideas from 
our new views of the action of intense heat on gases and 
meteorites. f 
I have only one point to add to my own observation (at two, 
not ten, minutes past six, as misprinted), that the object, when 
nearest, presented through its length (but rather below than 
above) a remarkable ‘‘ doz/ing’” appearance (as seeds in a cap- 
sule), while the edges appeared smooth and quiet. 
The Rookery, Ramsbury, February 20 ALFRED BATSON 
Aurora 
A NEWSPAPER paragraph that has come under my notice 
describes ‘‘a strange phenomenon” seen at Brixham on Thurs- 
day morning at 1.30—the 15th instant is to be inferred from the 
date of the paper. It would seemto have been an aurora—yet 
another example of exceptional auroral activity attendant on the 
passage of large sun-spots, as there was a spot of importance 
approaching the sun’s central meridian at the time. Any definite 
information concerning this particular manifestation, or indeed 
aurora generally near the date in question, appears worthy of a 
place in your journal. The sun-spot maximum is passing— 
perhaps past—and such opportunities should not be lost. 
February 24 F. BoE. 
DIURNAL VARIATION OF THE VELOCITY OF 
THE WIND ON THE OPEN SEA, AND NEAR 
AND ON LAND? 
ees the three-and-a-half years’ cruise of the 
Challenger, ending with May, 1876, observations of 
the force and direction of the wind were made on 1202 
days, at least twelve times each day, of which 650 days 
were on the open sea, and 552 daysnear land. The obser- 
vations of force were made on Beaufort’s Scale, (o—12) 
being the scale of wind-force observed at sea. ‘he five 
oceans have been examined separately, viz., the North 
and South Atlantic, the North and South Pacific, and the 
Southern Ocean, and thereafter the results grouped toge- 
ther. The*mean diurnal periodicity in the force of the 
wind on the open sea and near land respectively is 
shown on Fig. 1, where the figures on the left are Beau- 
fort’s Scale, and those on the right their equivalents in 
miles per hour. The solid line represents the mean force 
on the opea sea, and the dotted line the mean force near 
land. 
As regards the open sea, it is seen that the diurnal 
variation is exceedingly small, showing only two faintly- 
marked maxima about midday and 2 a.m. respectively. 
On examining, however, the separate means for the five 
oceans, no uniform agreement whatever is observable 
among their curves. The slight variations which are met 
with are different in each case, not one of the maxima or 
minima being repeated at the same hours in more than 
two of the five oceans. It follows, therefore, that the 
force of the winds on the open sea is subject to no dis- 
tinct and uniform diurnal variation. The difference 
between the hour of least and that of greatest mean 
force is less than a mile per hour. 
Quite different is it with the winds encountered by the 
Challenger near land, where the observations of the force 
of the wind give a curve as pronouncedly marked as the 
ordinary diurnal curve of temperature. The minimum 
occurs at 2 to 4.a.m., and the maximum from noon to 4 
p-m., the absolute highest being at 2 p.m. The curve 
constructed for each of the five oceans, from the obser- ! 
vations near land, gives one and the same result, viz., 
a curve closely agreeing with the curve of diurnal 
temperature. 
The 650 daily observations on the open sea give a 
mean velocity of 17} miles per hour, but the 552 near 
land give a velocity of only 12} miles per hour. The 
difference is greatest at 4 a.m., when it amounts to up- 
« Part of this article is abridged from a forthcoming volume cf the 
“Reports” of H.M.S, Challenger, by permission of the Lords Commis- 
sioners of H.M, Treasury. 
wards of 6 miles an hour, but is diminished as the tem- 
perature rises, till at 2 p.m. it is less than 3 miles an 
hour, 
At Mauritius, which is situated within the south-east 
trades, the minimum velocity of the wind is 9'7 miles per 
hour, occurring from 2 to 3 a.m., from which hour it 
rises to the maximum 18'5 miles from I to 2 p.m., the 
influence of the sun being thus to double the wind’s velo- 
city. At Batavia, situated in a region where the mean 
barometric gradient is much smaller, the differences are 
still more decided. From 1 to 6 a.m., 85 per cent. of the 
whole of the observations are calms, whereas from noon 
to 2 p.m. only 1 per cent. are calms. In all months the 
minimum. velocity occurs in the early morning, when the 
temperature is lowest, and the maximum from 1 to 3 
p-m., when the temperature is highest. At Coimbra, the 
mean maximum hourly velocity in summer is five times 
greater than the minimum velocity, whereas in winter it 
is only about a half more. At Valencia, in the south- 
west of Ireland, one of the stormiest situations in western 
Europe, the three summer months of 1878 gave a mean 
hourly velocity of 13°3 miles per hour, the minimum 
oscillating from 1o to 11 miles an hour from 9 p.m. to 
6 a.m.,and the maximum exceeding 16 miles an hour 
from II a.m.to 5 p.m. The absolutely lowest hourly 
mean was 10 miles at I1 p.m., and the highest 18 
miles at I p.m., the velocity about midday being thus 
nearly double that of the night. The results of observa- 
FIG I. 
I2MILES PER HOUR 
bo 
bo 
tions at many other places might be added to these, in- 
cluding those published by Wild, Hann, Koppen, Ham- 
berg, and others, which go to establish the fact that the 
curves of the diurnal variation of the velocity of the wind 
generally conform to the diurnal curves of temperature. 
The curves of the diurnal variation are most strongly 
marked during the hottest months. The maximum velo- 
city occurs at I p.m., or shortly thereafter, being thus 
before the maximum temperature of the day (occurring 
therefore at the time when insolation is strongest) ; and the 
minimum in the early morning, when the temperature 
falls to the lowest, or when the effects of terrestrial radi- 
ation are at the maximum. The rule appears to hold 
good with all winds, whatever be their direction, as shown 
by Hamberg. The exceptions to this rule are so few, and 
of such a nature, that they are in all probability attri- 
butable to causes more or less strictly local. 
With respect to cloud, Hann has pointed out that for a 
number of places the mean maximum hourly velocity is 
1o2 per cent. above that of the minimum with clear 
skies ; 77 per cent. with skies half covered with clouds ; 
and 50 per cent. with skies wholly covered. At Vienna, 
however, these rates of increase are, for clear skies, IOI, 
and half-covered skies, 66 per cent., whereas when the 
sky is overcast the variation becomes irregular and but 
faintly marked. Hann has also examined the Vienna 
observations of the wind on those days when the velocity 
