JuNE 8, 1899] 
NATURE 139 
terminations in 1888 and 1893, and the Council of the Paris 
Observatory have recommended that this work should be under- 
taken in concert with Greenwich Observatory. As a pre- 
liminary to the actual longitude operations, it seems essential 
that the instruments to be used by both parties of observers 
should be thoroughly tested at contiguous stations. 
SPURIOUS EARTHQUAKES. 
[XN compiling the seismic record of any country, we are liable 
to errors from two sources. We cannot help omitting a 
large number of slight earthquakes, which it is difficult to 
separate from the countless tremors that are artificially pro- 
duced. On the other hand, we include a smaller, but still im- 
portant, number of shocks which are not seismic in their origin, 
though they simulate earthquakes in many ways. Errors of 
the former class are, of course, difficult to prevent, though they 
tend to become less frequent when attention is given to the 
subject. Those of the latter class may sometimes be eliminated 
by a study of the different kinds of disturbance which have been, 
or might be, mistaken for true earthquakes. 
The majority of spurious earthquakes in this country are 
produced by the firing of heavy guns, the bursting of meteorites, 
and the fall of rocks in underground channels. Explosions and 
landslips produce disturbances which are at first frequently mis- 
taken for earthquakes, but their real origin cannot fail to be soon 
discovered and remembered. Perceptible tremors are also pro- 
duced in buildings by thunder, but it is improbable that per- 
manent errors can thus arise, for the long duration of the sound, 
in conjunction with the small area affected, provides a simple 
test. I shall, therefore, confine myself to the first three causes 
mentioned in this paper. 
FIRING OF HEAvy GuUNs. 
The sound and tremor produced by the firing of heavy guns 
are sometimes perceived at great distances; but, as I propose 
to deal with this subject in another paper, I will merely mention 
here that observations of the sound at distances exceeding one 
hundred miles are by no means uncommon. 
On two or three occasions within the last few years I have 
been able to trace supposed earthquakes to this source. On 
the first (January 7, 1890), two shocks were felt in the south- 
west of Essex at 12.30 and 1.25 p.m. All the places from 
which I received accounts lie close to a line running north-east 
from Woolwich, with one exception, in which the direction is 
north by east. Ihave no report of the direction of the wind 
in the immediate neighbourhood, but southerly and south- 
westerly breezes were generally prevalent over the whole country 
on that day. Near the boundary of the district affected, the 
disturbance was supposed to be seismic by observers who felt 
the Essex earthquake of 1884: somewhat nearer the origin, the 
sound resembled the report of a heavy gun; while, six miles 
from Woolwich, the noise and shock were referred without 
hesitation to their true cause, the discharge of a 110-ton gun 
at Woolwich at the times mentioned. ! 
On May 5-6, 1893, a number of shocks were felt at nearly 
regular intervals in the Isle of Man. At Douglas, where they 
were very slight, they were regarded as earthquakes ; further 
south, at and near Castletown, they were described as resembling 
the reports of heavy guns, but the likeness was not striking 
enough to raise doubts as to their seismic character when it was 
once asserted ; at the Chickens Lighthouse, off the extreme 
southern point, the keeper informed me that no earthquakes 
were felt, but that there must have been some man-of- war prac- 
tising to the south-south-west; and this, on inquiry at the 
Admiralty, was found to have been the case, II1.M.S. Neptune, 
a first-class battle-ship, having been engaged in heavy-gun 
practice to the south of the island during the very times when 
the reported earthquakes were heard and felt. 
Tests.—The principal tests by which the true character of 
these disturbances may be distinguished are the following: (1) 
When several are noticed on one day, they are of not very 
unequal intensity, and may occur at nearly regular intervals. 
(2) The disturbance is apparently communicated through the 
air. (3) The gradually increasing confidence in one direction 
with which the shocks are attributed to gun-firing is no doubt 
the most important test. (4) In many cases, the position of 
1 The supposed earthquake at Chelmsford on January 7 (NaTURE, vol. 
xli., 1890, p. 369). 
NO. 1545, VOL. 60] 
the disturbed area or the time of occurrence may lead to sus- 
picions regarding the seismic nature of the shocks. (5) If the 
disturbed area were extensive, a few good time-observations 
would give a velocity approximating to that of sound-waves 
in air. 
THE BURSTING OF METEORITES, 
The explosive bursting of meteorites is one of the commonest 
causes of spurious earthquakes. The mistake in such cases, it 
is probable, arises not so much from any close similarity between 
the two phenomena, as from the exclusion in the minds of the 
reporters of all artificial causes, and the consequent inference of 
a seismic origin. The explosion of a large meteor, weighing 
one or several tons, on entering the atmosphere is heard, 
according to the late Prof. H. A. Newton, for ‘‘a hundred 
miles around, shaking the air and the houses over the whole 
region like an earthquake.”” (NATURE, vol. xxxiv., 1886, p. 
533). Many such cases might be quoted, ‘and a very large 
number must be known to those who have made a special study 
of the subject, but the following are probably sufficient for the 
present purpose. 
West of England Meteorite, January 25, 1894.—At an 
early stage of its flight, this meteor, according to Mr. Denning, 
passed over Chester at a height of fifty-eight miles. It 
travelled in a direction from N.N.W., to S.S.E., passing almost 
over Droitwich and Worcester, at a height of about twenty- 
three miles, and disappearing with an explosion when about 
sixteen miles above a point four miles north of Ashchurch (near 
Tewkesbury). Two minutes after its disappearance three 
detonations were heard at Worcester, ‘‘the last being of ex- 
ceptional violence, shaking buildings and causing the earth to 
vibrate.”? At Brinscombe (near Stroud), about a minute or a 
minute and a half after its disappearance, ‘‘there came 
(apparently from the north-east) a series of explosions, which 
sounded . . . likeanumber of field-pieces fired in rapid succes- 
sion, followed by a volley of musketry.” One of the most interest- 
ing observations from the present point of view is that made by an 
observer at Parkfields (near Ross). ‘‘ A slight earthquake,”’ he 
says, ‘‘ was felt here. . . . I heard a loud rumbling noise like an 
explosion, lasting two or three seconds, which I took for 
thunder. A young lady who had just gone up to her room tells 
me that there were two slight shocks, the motion being similar 
to that of a steamer at sea. ... I am informed that it was 
accompanied by a lurid light, which lasted some time, and that 
objects at a considerable distance were plainly visible. The 
night was dark and cloudy, with some rain.” The recorded 
observations are too few in number to determine the boundaries 
of the sound-area and disturbed area, but a perceptible tremor 
was felt at Alvechurch (near Redditch), which is thirty-nine 
miles from Ross, and the sound was heard at Alvechurch ana 
Brinscombe, places which are forty-four miles apart.? 
Central England Meteorite, November 20, 1887.—At about 
8.20 a.m. a loud sound, accompanied in many cases by a 
distinct trembling or shaking of houses, was heard over a large 
area, chiefly in the counties of Cambridge, Bedford, Hertford, 
Buckingham, Oxford and Berkshire. Many observers at once 
attributed the phenomena to an earthquake, and Mr. H. G. 
Fordham, who has made a careful study of them,” commenced 
his inquiries under that impression. It soon appeared, how- 
ever, that the disturbance proceeded from the air rather than 
from the ground, and this fact, together with the actual observ- 
ation of a meteor at the time mentioned, placed its true origin 
beyond doubt. 
The accompanying map is, in part, a reproduction of that pre- 
pared by Mr. Fordham; but my object being somewhat different 
from his, I have added some details and omitted others. Every 
place where the sound was heard without any mention being 
made of an accompanying tremor is denoted by a small cross. 
If the concussion was strong enough to make doors, windows, 
and other loose objects rattle, the place of observation is shown 
by a large dot ; if the tremor only is mentioned, without any 
other indication of its intensity, it is marked by a small dot. 
The sound-area as thus drawn is about 105 miles in length and 
nearly forty miles in breadth, ‘ 
From the grouping of the places, where the sound was especially 
loud and definite, Mr. Fordham believes that the track of the 
meteorite would probably be best defined by a line drawn from 
Barrington (near Cambridge) to Wantage. Assuming this line 
1 NaTuR clix . 324-325; Lhe Times, January 29, 31, 1894. 
2 Tee eee eae ant 353 eee € ‘Nat. Dae. see 
Trans., vol. v., 1888, pp. 33-62. 
