526 
NATURE 
[ Oct. 27, 1870 
reference to Wiillner’s investigation on the variability of spectra 
at different pressures and temperatures, that the results obtained 
must only be applied with the greatest care to the conditions of 
pressure of the sun’s atmosphere, as the changes in the spectra 
are due far more to temperature than to pressure. But even 
under the assumption that this conjecture should become verified 
by special experiments, this circumstance would influence the 
results brought forward in this communication but in a slight 
degree. For the nature of the function (Formula 5) which 
served us in determining the temperature of the atmosphere is 
such that the pressure #, under which the hydrogen spectrum 
becomes continuous may be varied within very wide limits with- 
out thereby causing any considerable alterations of the requisite 
temperature. Thus it was shown above that, by introducing the 
extremes of the pressure assumed which were in the proportion 
of 1:10, the temperature values resulting were only in the pro- 
portion of 1: 1°5. 
Nevertheless, the separation of the influences which pressure 
and temperature exercise on the nature of the spectrum of 
luminous gases must be regarled as a problem the solution of 
which is of the highest importance for astrophysics. 
THE BRITISH ASSOCIATION 
SECTIONAL PROCEEDINGS 
Section A.—MATHEMATICAL AND PHYSICAL SCIENCE 
Rainfall: its Variation with Elevations of the Gauge.— 
Mr. Charles Chambers, F.R.S. The fact is well known to 
meteorologists that the quantities of rain received in gauges 
placed at different heights above the ground diminish as 
the elevation of the gauge increases. Several attempts 
have been made to explain this phenomenon, but none of 
them are so satisfactory as to discourage the search for other 
causes that may contribute substantially or mainly to its 
production. Hence the submission for the consideration of 
the British Association of this further attempt. One of the 
principal causes of rain is undoubtedly the transfer, effected by 
winds, of air charged with moisture in a warm damp district to a 
colder region, where the vapour it contains is partially condensed. 
The temperature of the lower as well as of the higher horizontal 
strata of the atmosphere being reduced by this transfer, it may fairly 
be inferred that condensation of vapour may also occur in the 
lower as well as in the higher horizontal strata. The rain caught 
by a gauge at any given elevation will therefore be the sum of the 
condensations in all the strata above it, and thus the lower a 
gauge be placed the greater will be the quantity of rain received 
by it. Again, it is known by observation, that there is at all 
times a greater or less difference of electrical tension between the 
atmosphere and the surface of the ground. If then—in accord- 
ance with the views of Prof. Andrews as to the continuity of the 
liquid and gaseous states of matter, from which it follows that 
the change of other physical properties must also be continuous— 
we regard the particles of vapour suspended in the electric bodies 
in relation to the dielectric principal constituents of the atmo- 
sphere, they will be polarised by induction from the ground. 
This polarisation will give rise to an attraction between every 
particle and the neighbouring particles above and below it ; and 
being stronger in the particles near the ground than in those more 
remote, the tendency of the particles to coalesce—which will in- 
crease, by their mutual induction, as two neighbours approach 
each other—will be greatest near the ground. Thus it may be, 
each particle gathering to itself its neighbours in succession till 
their united density exceeds that of the atmosphere generally, 
some rain drops are formed, and that in greatest abundance near 
the ground. If this be the true cause of any substantial part of 
the phenomena in question, then, as the variation in intensity of 
electrical polarisation of the particles will vary with the height 
most rapidly near the ground, so the variation in the rainfall near 
the ground should be more rapid than at a greater elevation, and 
such is indeed the fact. Also, if the idea be correct, it will pro- 
bably serve to explain other phenomena which it was not specially 
conceived to meet; and so it does. For, first, it requires that 
the rainfall over even ground, where the electrical tension is rela- 
tively weak, should be less than over similarly situated forest 
* A perfect transparency of the gas mass to all rays emitted by itself is 
here assumed, a supposition which is the nearer the truth the smaller the 
weights compared. 
land, where at the tops of the trees, ends of branches, and edges 
of leaves, the tension is high, and this is in accordance with obser- 
vation. And, secondly, the tension being relatively high at the 
tops of the elevations of a mountainous district, the rainfall should 
be greater there than in the neighbouring plains; this, again, is 
borne out by observation. Further, at the commencement of a 
passing thunderstorm, a sudden heavy shower of rain will often 
fall for a few mements and then suddenly cease. May not this 
arise from the approach, by the agency of opposite wind currents 
of detached masses of differently charged clouds, the process 
just described of formation of rain drops going on rapidly in each 
mass as the two come near each other, and stopping when, by a 
flash of lightning between them, the two masses are brought 
into the same electrical condition? An experimental test of this 
idea would be to repeat Dalton’s measures of the pressure of 
vapour in the vacuum space of a mercurial barometer tube— 
filling that space with air and a little water, and compare the 
values found when the mercury was charged with electricity and 
when not so charged. Ifin the former case a less pressure was 
found, we might conclude that the particles of vapour are 
really susceptible of electric induction, and the amount of differ- 
ence existing would enable us to estimate whether the attractions 
of the particles upon each other were strong enough to cause 
ihe formation of rain-drops hypothetically attributed to them 
above. 
SrEcTIoN C.—GEOLOGY 
On the Mountain Limestone of Flintshire and part of Den- 
bighshire,—Mr, G. H. Morton. Minute details of the physical 
structure of the region, and lists of the fossils, showed that these 
beds have been erroneously referred to the Millstone Grit, and 
that they were really Mountain Limestone, the shales and sand- 
stones being intercalated among the typical rocks. The white 
limestone was an ancient coral reef, with the organisms ex- 
quisitely preserved. Mr. Hughes protested against co-relating 
with the Yorkshire beds, while Mr. Bailey supported the opinions 
of the author. ; 
On the formation of Swallow-holes, or Pits with vertical Sides, in 
Mountain Limestone.—Mtr. L. C. Miall. The author distinguished 
between cavities formed by direct excavation and those produced 
by subsidence of part of the roof of a cavern. The curious 
pits near the Buttertubs Pass at the head of Swaledale, were re- 
garded as typical of the first kind, and their appearance and 
mode of formation were described, especially the vertical fluted 
sides and the isolated fluted pillars, which were ascribed to the 
action of dropping water, aided by pebbles. A basin is first 
formed upon a ledge of rock, and as the excavation proceeds it 
produces a semi-cylindrical scar, with sharp ridges upon the face 
of the limestone wall, as if cut by a gauge. The presence of 
a thick surface-covering of alluvium or drift was necessary to 
absorb and retain the rainfall, and to distribute it slowly and 
regularly. The limestone of a bare plateau furnishes fissures in 
great variety, but they are not true swallow-holes. Regular 
and well-marked joints were also necessary to the production 
of fissures, as they permitted the ready escape of the waters of 
erosion. The texture of mountain limestone, and its power of 
receiving and retaining sharp impressions, gave the peculiar 
features to the swallow-holes excavated in it. Some swallow- 
holes were due to the subsidence of an undermined crust. These 
frequently lie in a line, sometimes in a ring round a hill-side. 
A particular description of some near Ripon was given, and the 
estoy of eye-witnesses as to their sudden appearance was 
quoted. 
lations. Many conical hollows in drift are probably due to con- 
cealed cavities of subsidence. . 
On the Stratigraphical Distribution of the British Fossil 
Gasteropoda—Mx. J. 1, Lobley. This was the third of a series of 
reports by the author on British fossil mollusca. By the help of 
diagrams were shown the distribution of the species, and the 
range, increment, decrement, and maximum development of the 
genera, families, and orders of the Gasterofoda. he cainozoic 
deposits contain the greatest number of genera and sub-genera, 
though they are numerous also in both the mesozoic and palo- 
zoic rocks. A large number of genera and sub-genera are 
characteristic of single formations, and these are especially 
numerous in the carboniferous limestone, the lower lias, the 
middle Eocene, and the older and newer Pliocene. Details of 
the range and of the distribution of species of each of the 
Swaltow-holes are often disguised by surface accumu- ~ 
