20 NATURE [Wov. 3 1881 
River. Nota trace of coal was found along the whole course of | tion, namely, the meridian. There was also introduced an 
the river, thovgh it was reported by the natives to be seen in 
abundance protruding on the river banks, The whole country is 
thickly covered with wcod. Mr. Thomson contemplates setting 
out on his second expedition during the present month. He 
intends to visit the little-known region between the sea and 
Mount Kilimanjaro, extending from Melinda on the north to 
Pangani on the south. Mr. Thomson hopes to make important 
discoveries in geology and botany. 
A PRETTY full account of the proceedings of the recent Inter- 
national Geographical Congress at Venice will be found in the 
new BSollettino (for August) of the Italian Geographical Society, 
which reports in full the papers on the question of oscilla- 
tions on the coast of Italy. It contains besides a map showing 
the various arcs of meridian and farallels that have been 
measured all over the world, and a map of Europe showing the 
present state of the various trigonometrical surveys in that 
continent. Russia, Turkey, and Greece are almost blank, 
and the Scandinavian peninsula is far belind; the other 
countries are shown covered with triangles. 
LUNAR DISTURBANCE OF GRAVITY? 
ie November, 1878, Sir William Thomson suggested to Mr. 
G. H. Darwin that he should investigate, experimentally, 
the lunar disturbance of gravity and the question of the tidal 
yielding of the solid earth, This. Committee of the British 
Association was subsequently reappointed, and the authors’ 
names were added to thelist of its members. In May, 1879, the 
authors visited Sir William Thomson at Glasgow, and there saw 
an instrument which, although roughly put together, he believed 
to contain the principle by which success might perhaps be 
attained. The instrument was erected in the physical laboratory 
of the University of Glasgow. The following are the rough 
details :-— 
A solid lead cylinder, weighing perhaps a pound or two, was 
suspended by a fine brass wire, about five feet in length, from 
the centre of the lintel or cross-beam of the solid stone gallows 
which is erected there for the purpose of pendulum experime ts. 
A spike projected a little way out of the bottom of the cylin- 
drical weight ; a single silk fibre, several inches in length, was 
cemented to this spike, and the other end of the fibre was 
cemented to the edge of an ordinary galvanometer-mirror. A 
second silk fibre, of equal length, was cemented to the edge of 
the mirror, at a point near to the attachment of the former fibre. 
The other end of this second fibre was then attached to a sup- 
port, which was connected with the base of the stone gallows. 
The support was so placed that it stood very near to the spike 
at the bottom of the pendulum, and the mirror thus hung by the 
bifilar suspension of two silks, which stood exceedingly near to 
one another in their upper parts. 
It is obvious that a small displacement of the pendulum, in a 
direction perper dicular to the two silks, will cause the mirror to 
turn about a vertical axis. 
A lamp and slit were arranged, as in a galvanometer, for ex- 
hibiting the movement of the mirror by means of the beam of 
light reflected from the mirror. It was found to be in incessant 
movement, of so irregulara character that it was hardly possible 
to localise the mean position of the spot of light on the screen, 
within five or six inches. On returning to the instrument after 
several hours, the observer frequently found that the light had 
wandered to quite a different part of the room, and it was some- 
times necessary to search through nearly a semicircle before 
finding it again. The cause of this extreme irregularity of the 
movement of the pendulum was obscure ; and as Sir William 
Thomson was of opinion that the instrument was well worthy of | the plumb-line through 1-rooth of a second of are was distinctly 
careful study, the authors determined to undertake a series of 
experiments at the Cavendish Laboratory at Cambridge. 
Accordingly throughout 1880 they proceeded to make experi- 
ments with an instrument which involved the principle above 
explained. Several modifications of some importance were in- 
troduced. 
destroy the oscillations generated Fy lccal tremors, and, being 
suspended. by two wires, it was only free to cscillate in one direc- 
« Report of the Committee, consisting of Mr. G. H. Darw'n, Prof. Sir 
William Thomsen, Prof. Tait, Prof. Grant, Dr. Siemens, Prof. Purser, 
The pendulum was hung in fluid, in order to quickly | 
Prof. G. Forbes, and Mr. Horace Darwin, appointed for the Measurement | 
of the Lunar Disturtance of Gravity. Account of experiments by G H. | 
Bupwin and H. Darwin, read at the British Associaticn, York, September 
1881. 
apparatus, which we have not syace to explain, by which a 
known very small horizontal thrust might be applied tothe pen- 
dulum. By means of this the actual displacements of the 
pendulum were determinable from the observed displacements of 
the spot of light on the screen. 
The image on the screen was found to be in a state: of con- 
tinual agitation of an irregular character, so that it was not 
possible to take a reading with very great accuracy, But as 
the pendulum was hung in fluid, the agitation was not nearly so 
great as it had been in the instrument at Glasgow. 
The observers also found that the pendulum was subject to a 
diurnal oscillation, and that it stood furthest north towards 6 
p.m., and furthest south towards 6 a.m. Superposed on this 
motion was a gradual change of the mean diurnal position, for 
during two months the pendulum moved northwards, 
The instrument was found to exhibit the flexure of the stone 
piers of the gallows, even when the force employed’ was only a 
slight pressure with one finger. Water poured on the ground 
round the basement of the stone gallows tilted the whole struc: 
ture over, and very small changes of temperature in’ the stone 
piers were found to give distinct effects. It was concluded that 
i foot of displacement in the spot of light on the seale corre- 
sponded with 1” of change in the direction of the plumb-line 
with reference to the base of the gallows. 
From these experiments the authors concluded that the instru- 
ment was susceptible of all the delicacy requisite; but that the 
mode of suspension was unsatisfactory. 
Accordingly in 1881 they proceeded to erect a new instrument 
in which the support for the pendulum was a copper tube, which 
itself formed the envelope for containing the fluid in which the 
pendulum was suspended. The whole apparatus was immersed 
in a large mass of water, and the observations were taken from 
outside of the room by means of a telescope. The unsteadiness 
of the image was diminished, probally on account of the precsu+ 
tions taken against inequalities of temperature in various parts 
of the instrument, and because the pendulum was hung in a very 
confined space. The accuracy with which readings could be 
taken was thus increased. 
Similar diurnal oscillations of the pendulum were again 
observed, and a similar slow change in the mean diurnal posi+ 
tion. The authors therefore concluded that these changes are a 
real phenomenon, and do not depend upon changes of tempera- 
ture in the instrument itself. 
They also noted that there are periods lasting for several days 
in which the pendulum is in a state of continual agitation, so 
| that the readings taken at a few seconds apart do not agree inter 
sé, and that there are other periods of abnormal quiescence. These 
periods do not seem intimately connected with the external 
meteorological conditions, at least as far ary the experiments have 
been hitherto carried. 
The pendulum was found to be practically insensible to the 
| effect of local tremors, such as are produced by hitting the stone 
support or stamping on the ground in the immediate neighbour- 
hocd of the instrument. But it was extraordinarily sensitive to 
steady forces. If a force be applied at a point on the floora 
dimple is produced in consequence of the elastic yielding of the 
soil, and any object on the floor is slightly tilted towards the 
point where the force is applied. Now when a person stocd in 
the room at sixteen feet away from the instrument, and again ut 
seventeen feet, the difference was rendered distinctly evident 
between the amounts of inclination towards the point of pressure 
of the stone basement supporting the pendulum in the two 
cases. 
Although no great pains had been taken to render the instru+ 
ment as sensitive as possible, it was found that an alteration of 
measurable. 
The second part of the paper contains an account of the work 
of some of the previous observers on the same subject. 
M. Zollner’s instrument, the ‘‘horizontal pendulum,” is 
described. It does not appear that any extensive series; of ob- 
servations have heen made with it. 
An account of M, d’Abbadie’s work is next given. He made 
his observations by means of reflections from.a pool of mercury, 
and the site of his experiments was at Abbadia, near Hendaye, 
in the south of France. He found that there were periods of 
agitation and cf quiescence in the mercury, apparently, without 
reference to any perceptible external causes. There were also 
gradual changes of level extending over several months, and the 
