7a # Mo 
130 
NATURE 
| Dec. 8, 1884 
ing of the glass tube) takes time to travel from the inside to the 
outside of the brass tube; so that for about 1-1o0coth of a 
second that tube was exposed to a pressure of four or five tons 
weight per square inch on its outer surface, and no pressure on | 
the inner. The impulsive pressure on the bottom of the tube 
projected it upwards, so that it stuck in the tallow which fills the 
hollow of the steel-plug. Even a piece of gun-barrel, which I 
substituted for the brass tube, was cracked, and’ an iron disk, 
tightly screwed into the bottom of it to close it, was blown in. 
1 have since used a portion of a thicker gun-barrel, and have 
had the end welded in. But I feel sure that an impulsive pres- 
sure of ten or twelve tons weight would seriously damage even 
this. These remarks seem to be of some interest on several 
grounds, for they not only explain the crushing of the open 
copper cases of those of the Cha//enger thermometers which 
gave way at the bottom of the sea, but they also give a hint 
explanatory of the very remarkable effects of dynamite and other 
explosives when fired in the open air, 
To show how possible is a serious mistake in the measurement 
of pressure, I append a comparison of the indications of the 
very elaborate gauge attached to the old Cha//enger apparatus 
with those of my steel external gauge already described. The 
seale of the Challenger gauge is divided to cwts. on the 
square inch. My gauge gives very nearly 20 mm. per ton ; so 
that, for a rough comparison, we may take I mm. as equivalent 
to 1 cwt. The two instruments were simultaneously attached to 
the pump, and the pressure was therefore the same in both at 
each reading. ‘There can be no doubt whatever, from repeated 
comparisons with glass gauges of all sizes and shapes, that my 
gauge follows Hooke’s law with great accuracy. The only 
possibility of serious error is in the actual value of the unit. 
This important determination has, however, been very carefully 
repeated by the aid of Amagat’s numbers and the indications of 
the silvered gauge already described ; and the result is as above 
stated. 
Steel Gauge. Challenger Gauge. 
Millimetres, Cwts. per sq. in- Ratio, 
° ° tee 
5 ° o'o 
9 1°2 o'13 
15 87 058 
8) 13°9 0°69 
30 23°6 0°78 
so 35°0 0°87 
SY 47°0 0°94 
; 60 58°7 0°98 
72 707 1°02 
The comparison was repeated several times with almost 
exactly the same results. 
It is quite clear that the Challenger gauge does not follow 
Hooke’s law. It lags behind the steel gauge at first (does not 
give any indication, in fact, till the pressure is nearly 50 atmo- 
spheres), then gradually gains on it ; and, at pressures greater 
than 3 tons, aj pears to leave it rapidly behind, The instru- 
ment is, however, graduated up to 4 tons only. My very first 
experiments with this Cha//enger instrument, in which I used a 
simple form of manometer, showed that it was not trustworthy, 
and led me to make various trials for the purpose of getting a 
proper mode of measuring high pressures. 
Finally, it may be interesting to mention that a fairly approxi- 
mate determination of the compressibility of water was made by 
counting the number of strokes of the pump required to produce 
a measured pressure in the interior of the large apparatus. 
[Then foliows a table of the experimental data for each of a 
large series of the Challenger thermometers. These are of no 
general interest. ‘Their importance is confined to the reduction 
of the actual observations made on board the Challenger]. 
THE GRASS BARRIERS OF THE NILE 
peas interesting phenomenon, which so largely con- 
tributes to produce changes in the bed of the Nile 
and to accumulate river formations of great geological 
importance, has been recently investigated by M. Ernest 
Marno, who has just published an elaborate paper on the 
subject, in the last number of Petermann’s Mittheilungen. 
It is accompanied by a map, on the scale of 1 to 500,000, 
of the Bahr-el-Gebel and of the Bahr-el-Abiad, from 
Geseir Abbas to Sohat, and of the Bahr-el-Serat from its 
mouth to 7° 30’ N. lat. After having made its way 
among the hilly region, through several great lakes, 
formerly forming a series of terraces and connected to- 
gether by short rivers, the Nile, or the Bahr-el-Gebel— 
the River of the Mountains—enters an extensive flat 
land, which it crosses over six degrees of latitude to the 
next rocky barrier, which it cuts through at Khartum. 
Over this stretch it runs with numerous windings, first 
north to its confluence with the Bahr-el-Ghazal, and then 
to the east,under the name of Bahr-el-Abiad, and, 
although the direct distance between its issue from the 
hilly tract to Khartum is only 600 miles, the total length 
of the river with its windings is no less than 1100 miles. 
The whole of this region is a wide marsh, and the river 
has no proper banks, its water being mixed with that 
of marshes which cover the whole of this tract. It is 
even a rare occurrence to see dry banks, as the country 
is more like an extensive marsh, through the midst of 
which a somewhat deeper channel has been dug by the 
current of the river. Numerous smaller rivers connected 
together and with thé main channel and its numerous rami- 
fications circulate amidst these marshes, and during the 
rainy season the waz/e, or lateral ponds and lakes, increase 
yet more, covering wide tracts of land, whilst during the 
dry season some stretches of banks re-appear, and the 
lakes which were navigated by steamers some months 
before become simple marshes. Vegetation plays an 
important part in the modifications which are going on 
in this region. The country is covered with rich grasses, 
mostly consisting of such species (Saccharum spontaneum, 
S. irschamum, Vossia) as grow perfectly well even in 
water; this grass can be lifted with its roots by water, 
and grow floating on the surface, so as to render it most 
difficult to draw a line of demarcation between land and 
water. Thick and high papyrus palms grow sometimes 
on the very banks of the main channels of circulation of 
the water, and strengthen these by their complicated roots, 
but they do not cover all the banks, and the outlines of the 
river are mostly indefinite. Some few tree-like Herminiera 
elaphrosylon grow as isolated individuals on the banks of 
the rivers, and of the mazje, whilst the smaller marshy 
and aquatic flora (Pistie, Nymphee, Vallisneri@) nearly 
disappears in comparison with the rich vegetation of the 
above-named species. The fauna of this region closely 
depends upon the season. Mammals and birds leave it 
during the rainy period and wander to the hilly tracts, 
but during the dry season the banks of the sazje and of 
the rivers are peopled with elephants, buffaloes, giraffes, 
antelopes, and by many kinds of birds. Besides this 
region has also its special forms, namely the Ba/eniceps 
Rex, the Protopterus @ethiopicus, and the ganoid fish, 
Polypterus, all being remains from earlier geological 
periods. The people who inhabit this region, the Dinka, 
the Shilluk, and the Nuehr, all belong to a very low 
level of civilisation, living mostly on their herds of cattle ; 
they change their abodes in accordance with the season, 
but they cannot be considered as true nomades, as the 
land occupied by each tribe is strictly limited by other 
tribes, and every encroachment on anothers !and is 
punished by war. ; f 
It is obvious that in this region the fall of the rivers is 
very small and that the regular outflow of water may be 
checked by winds and other occasional circumstances ; 
whilst the great quantities of water poured down into the 
basin during the rainy season cannot find an easy way 
through the flat channels; extensive inundations occur 
theretore every year, and when the rains are especially 
heavy, great masses of floating grass are brought from 
the maze into the main river, and accumulate in its 
windings. New floating islands of grass are brought by 
and by to these barriers, being pressed upon or beneath 
them, and soon the whole of the river throughout its 
width and depth is obstructed by these barriers, which 
the inhabitants call se¢¢s. The grass does not decay in 
