March 31, 1870| 
phenomenon of the aurora. It must now be observed that, on 
account of the physical condition of the earth’s surface, the 
electromotor activity of the southern hemisphere must be through- 
out much stronger than in the northern ; whence it happens, that 
not only on both hemispheres between pole and equator, but 
also between the north and south poles themselves, a continual 
disturbance of electric equilibrium occurs; and it is this by which 
the direction of the needle is determined. The narrow belt be- 
tween the north and south-east trades—called by Dove the zone 
of calms—may be termed, for present purposes, the metecrological 
equator. This is known not to coincide with the geographical 
equator, but to oscillate slowly about a limit of 1 to 14 degrees 
north of it. The exferimentim crucis for the theory—or, as we 
will only term it at present the hypothesis—here adduced of the 
trade-winds as the source of terrestrial magnetism, would con- 
sist in establishing that the known alterations which the mag- 
netic pole, as well as declination, gradually undergo, are accom- 
panied by parallel changes of our magnelic equator. But work 
of this description cannot be accomplished by a single private 
individual, and I must content myself with having brought the 
subject forward. 
Amagat on the law of Mariotte 
Prorrssor E. H. Antaca'r has published the results of some 
experiments, still in progress, on the influence of temperature on 
departures from the law of Mariotte. The researches of Regnault 
have shown that this law is not rigorously obeyed by any gas 
excepting hydrogen ; in all other cases compressibility increases 
with pressure, that is, when the gas approaches its temperature 
of ebullition. This phenomenon has received various explanations, 
It has been considered as resulting from reciprocal molecular 
attraction; it has also been elucidated by a theory which was 
first enunciated by Daniel Bernouilli, but has received successive 
additions at the hands of Joule, Kroenig, and Clausius. The 
theory in question takes into account not only the movements of 
translation of molecules, but their rotatory and internal movements, 
as well as the possible movements of imponderable fluids. If 
we admit the first explanation, then, as attraction only depends 
on the mean distance of the molecules, the departure from the 
law in any single case must be the same at any temperature, 
provided the initial and final volumes are the same. In other 
words, let V be a given volume of gas at the temperature ¢ and 
pressure 7. Reduce this volume to V' by a pressure 7’,’ the 
temperature remaining unchanged. On heating the gas to ¢, it 
will expand; let P be the pressure necessary to restore the 
yolume to V, and P’ the corresponding pressure. If the depar- 
ture be only a function of the volume, it is clear that we must 
hare 
EA eee 
va Vv! — Pv’ : 
Vv ; i VER Ve 3 
As 7 is common to both sides of this equation, it is only neces- 
The author has done this in the 
In the 
Deena le 
sary to compare vi with p 
case of sulphur dioxide, ammonia, and carbon dioxide. 
instance of sulphur dioxide— 
at 14°, . = 050838 } 
Pp ( difference, 0°00561. 
at 98°, ee 0°50277 —-) 
(This difference corresponds to an observed height of more than 
one centimetre of mercury.) For ammonia— 
at 13°, z = 95073" 
£ difference, 0003209. 
at 97% jp = 0'50402 
For carbonic dioxide— 
at 13°, £ = 0°50981 
4 difference, 0°00210. 
at 97°, Sp = 050402" 
It appears from the precéding numbers that the departure is not 
only a function of the volume, but also of the temperature at 
which the experiment is performed. This result agrees, however, 
with the second theory. In fact, the as eva of the molecules 
being greater as the temperature rises, it may be readily conceived 
* This number is obviously a misprint, 
NATURE 
567 
that the loss due to their collision is relatively smaller than the 
augmentation of pressure on the walls of the enclosing vessel, 
due to the augmentation of ay zva, this being true even when, 
as the rate is accelerated, the molecular collisions become more 
numerous, 
In a new series of experiments, M. Atmagat kept the initial 
and final pressures as nearly as possible the same in each case, 
thus obtaining the influence of temperature alone. He then 
arrived at the following general results :— 
1, That near roo°, sulphur dioxide and ammonia depatt 
but little from Mariotte’s law, yet more so than air at 
the ordinary temperature. 
2. That near 100°, carbon dioxide is almost a perfect gas. 
3. That near 100°, air exactly follows the law. 
The author is convinced that the higher the temperature of 
liquefaction of a gas is found to be (under the same pressure), the 
less does it depart from the law of Mariotte at the same distance 
from its point of liquefaction. *—[ Archives des Sciences physiques 
et naturelles, 139, p. 169. ] 
SOCIETIES AND ACADEMIES 
Lonpbon 
Linnéan Society, March 17.—Mr. Carruthers exhibited 4 
section of a fossil Osmunda from the eocene beds of Herne Bay, 
in which not only the forms of the cells were preserved, but the 
contents of the cells, and even the starch-granules. Before its 
conservation it had been attacked by a parasitic fungus, the 
mycelium of which is preserved, in precisely the same condition 
as it would be in a recent specimen.—Dr. Hooker read a further 
communication from Sir Henry Barkly on the Flora and Fauna 
of Round Island. The highest point of the island is 1,049 feet 
above the level of the sea; the summit is smooth, with three 
large and remarkable blocks of granite. It is entirely composed 
of tufa, mixed with volcanic sand in perfectly preserved strata. 
The deeper ravines are crowded with Jofty palms. Of the 
twenty-six flowering plants gathered, the greater number belong 
to the orders Gramineae, Pandanacee, Palmacee, Lbenacee, 
Cinchonacee, Composite, and Asclepiadee. The proportion of 
Endogens to Exogens is very large, namely, twelve to fourteen ; 
but this proportion by no means represents the enormous pre- 
ponderance of the former in individuals, probably amounting to 
99 per cent. Some of the Exogens are specifically identical with 
those of the Mauritius, but few of the Endogens ; those of the 
former class which are common to the two islands have probably 
been introduced at some remote period. Of the three crypto- 
gamic plants observed, one was a moss, probably a Sphagnume, 
one a Selaginella, certainly a new species, and one a widely- 
spread fern, Adiantum caudatum. Of the five grasses the most 
abundant is identical with the Indian Lemon-grass. The 
Cyperacee ave represented by one species, Scirpus mearitimus. 
The Pandanace@ are very remarkable ; Pandanus udilis occurs, 
but in one spot only, rare, and no doubt introduced, whilst the 
other, an allied species (P. Vandermeerchii, is quite peculiar to 
the islet). Of Palms there are no less than three species, pro- 
bably all peculiar, the most remarkable being the bottle-stemmed 
species (a Hyophorbe) already described as peculiar to the island, 
The only other Endogen belongs to the order Ziviacew, and is 
an aloe, growing on the summit, and probably a new species. 
Of Eéenacee there are three species, and two Asclepiads with 
trailing stems; one species of AZyrsinee, new ; two Conzposite, 
one of them a Sonchus, both probably introduced; one species 
of Combretacee and one of ALyrtacee ; two Cinchonee, and a small 
tree about twelve feet high, resembling the Blachkwellia of 
Mauritius. It will be seen that while the genera of the Round 
Island Flora are Mauritian, the species are mostly peculiar. It 
is probable that the whole group of islands—Mauritius, Bour- 
bon, Round Island, Ile de Serpents, Rodriguez, with the smaller 
islets, and probably Madagascar—are fragments of a vast con- 
tinent. As regards the Fauna, there are no indigenous mam- 
malia, although goats and rabbits have been introduced and 
have multiplied exceedingly, and no land birds, not even the 
Mauritian pigeons. The island seems, on the other hand—per- 
haps from the absence of mammalia and birds—yery favourable 
to reptile life. Of Chelonians, a female land-tortoise had pre- 
viously been captured on the island. Four distinct Saurians 
were found, the largest exceeding a foot in length, a native of 
* With the above results compare those obtained by Andrews (Proceed- 
ings of the Royal Society, xviii. 42). 
