March 29, 1883} 
NATORE 
bey 
-From the above table it is seen that in ten cases out of thirteen, 
the formula I have proposed gives results cloer than Mr. 
Stevenson’s, while the means differ by a quite insignificant 
amount. 
If then, as seems probable on all grounds, the higher we 
ascend, the slower the increase of the velocities with the heights, 
Mr. 7=(5)* should hold for a level 
Wea 
Above that, 
again, a formula, 2=(7)* should apply ; and finally, the 
Stevenson’s formula, 
somewhat higher than 775 feet, and not below it. 
formula, z=(5) recommended in my paper. 
I cannot believe, however, that the formula 5 =(})' holds 
up to such a comparatively large height as this inference would 
postulate, since it gives such an excessive value at 1600 feet with 
Vettin’s data (more than twice that observed), and I can only 
conclude, therefore, until experiments in a free atmosphere 
corroborate Mr. Stevenson’s data from Arthur’s Seat, that these 
latter do not correctly represent the actual rate of increase in the 
velocity between such levels in the atmosphere, away from the 
disturbing influences of mountains and valleys. 
In any case, however, I must enter a distinct protest against 
having my name prefixed to the pressure formula i Vt: 
if Mr. Stevenson carefully examines my paper, he will no- 
where find the remotest allusion to such a formula. The 
formula for the velocity which I there recommended for the 
higher levels, was in fact shown to be directly deducible from 
Mr. Stevenson’s first formula for the pressure, viz. if = / A 
to which it is exactly equivalent on the ordinary assumption that 
ji ais 
aa 
Moreover, the paradoxical result which Mr. Stevenson arrived 
atin violation of this assumption, viz. that the same formula was 
practically applicable both to force and velocity, is controverted 
by the conclusion entertained in his letter, that the formula 
Z “ 
aan a/ + agrees best with the recorded results of velocity, 
and the formula = f with those of pressure. 
While these two formulz can hardly be called the same, it is 
somewhat striking to find that on the assumption force varies as 
(velocity)?, which is supposed to be annulled by the diminished 
density as we ascend, they are #dentical, 
Finally, Mr. Stevenson has ey dently quite misunderstood my 
allusion to sea-level. When I spoke of sea-Jevel, I simply 
meant the approximate equivalent to the level of the sea on /and, 
as at Berlin for example, where Vettin’s observations were made, 
When Mr. Stevenson therefore maintains that the velocity of 
the wind at roo feet above sea-level over land, is probably not so 
great as that near the surface over the sea, he entirely misses 
the point of the argument, which lies in the ve/a¢ively excessive 
velocity of the wind at 100 feet above, to that near the surface, 
over land which lies approximately at sea-level. 
The very fact mentioned by Mr. Stevenson regarding the 
greater friction encountered by air in passing over land than over 
water, as well as the results of his experiments, point to a con- 
siderable increase in the velocity from the surface to an elevation 
of 100 feet above it. For the very same reason, I should expect 
to find a mere moderate increase up to the sawe height over 
water, E. DouGLas ARCHIBALD 
On the Formation of Mudballs 
THE letter from Mr. Hart in NaTurE, vol. xxvii. p. 483, on 
the natural formation of snowballs, has recalled to my memory 
the similar formation of balls of mud. 
About eight miles south of Bromley in Kent the soil is clayey, 
ard after rain the country lanes are apt to be very muddy. Some 
five or six years ago there was a very violent storm of rain, 
whether or not accompanied with melting of snow I cannot now 
remember. The steep lanes were in many places regularly 
scoured with water, and it looked afterwards as though the 
whole surface had in places been a sheet of water, for the soil 
Was quite washed off and the flints were left bare. After this 
storm my brother and I noticed in the lanes a considerable 
number of mudballs, usually almost perfectly spherical, but in 
some cases with a tendency to a cylindrical shape. They varied 
in size from small pellets up to four or five inches in diameter. 
On seeing the first one or two, they looked to us like the handi- 
work of some boy with an enthusiasm for mud pies, but the 
number of them, and the fact that they were always found on 
the slopes of hills proved them to be a natural formation. They 
were formed throughout of soft clayey mud, and I do not re- 
member finding any nucleus in the middle when we cut them 
open. We concluded that they were formed by accretion to 
pellets of mud washed down the hillside and rolling as they 
went, I have only once since seen a similar ball, and that was 
in a furrow in a ploughed field in the same country; it is 
possible that this ball may have been made inside an agricultural 
roller, although there were no marks on the field of recent 
rolling aad there had been heavy rain. The comparative rarity 
of the appearance of these balls seems to show that they can only 
be formed with some precise degree of stickiness of the mud, 
Closely similar are the marvellously spherical balls of matted 
vegetable fibre to be found on the seabeach in some places. 
Sir Anthony Musgrave informed me that on the beach in 
Australia, I think near Adelaide, he had seen tens of thousands 
of such balls, all perfectly spherical. It seems rather obscure 
why the fibres should begin to mat together in such a form as to 
be rolled by the surf, but the perfection in shape is clearly due 
to incessant rolling. It is probable that, with a flat bath 
and some cocoanut fibre or oakum, the process of formation 
might be watched, but I have never tried the experiment. It is 
very common to see after rain matted lines of such objects as 
pine-leaves or the flowers of lime-trees, but I have neyer seen 
any apparent tendency to rolling, and such lines are left lying 
flat after the water has drained off. G. H. DARWIN 
Cambridge, March 23 
Snow Rollers 
THE phenomenon described in NAtTuRE, vol. xxvii. p. 483, 
under the title of ‘‘ Natural Snowballs,” is known to British 
meteorolegists under that of snow rollers, and as the latter 
agrees more closely with the phenomenon, I venture to plead for 
its adoption. 
1 believe that the first person wh» carefully examined their 
formation was that excellent and venerable observer, the Rey. 
Dr. Clouston of Sandwick Manse, Orkney, and I am under the 
impression that he published a description of their formation in 
an early number of the PAlosophical Magazine. He has ob- 
served them on the lawn at Saniwick more than once, and has 
always noticed the hollowness at the ends ; in fact, he described 
them to me as resembling ladies’ white muffs, 
I remember only one instance of their being reported in Eng- 
land, namely in the following letter from the late Admiral Sir 
F, W. Grey, which appeared in the AZeteorological Magazine for 
May, 1876. G, J. Symons 
62, Camden Square, N.W. 
S1r,—The snowstorm of Thursday night (April 13, 1876) was 
marked by one circumstance which I have never witnessed 
before, though it may not be uncommon. It was this :— 
On Friday morning I observed th it fora considerable distance, 
and following a regular line, the lawn, to leeward of the house, 
was strewed with masses of snow like boulders, varying from 
the size of a snowball to a cubic foot at least, and as the snow 
melted, a track either straight or curved led up to the large 
ones, following, apparently, the direction of the wind. I had 
observed before dusk that the eddies of the wind and the swirls 
of the snow were very marked, and | have since heard froma 
friend who observed the same thing, that he saw the snow 
rolled along by the wind, and forming masses such as I have 
described. 
As I have said, I know not whether this has been observed 
in other cases, and perhaps it may interest you to have this 
account of it.—Yours faithfully, F. WM. GREY 
Lynwood, Sunningdale, Staines, April 16 
Incubation of the Ostrich 
Ir seems strange that there should have existed an uncertainty 
in the mind of an ornithologist as to the mode of incubation of 
the ostrich in confinement at the Cape of Good Hope. The 
habits of the birds are of course as familiar to the ostrich-farmers 
