58 NATURE 
Moreover, displacements of 0:53 to o-95 metre along 
the west coast of Kagoshima Bay converge with those 
in the west and north of Sakura-jima towards an 
elliptical area which agrees roughly with the area of 
greatest depression. 
Propagation of Sound-Waves.—Prof. Omori divides 
the sounds which accompanied the eruption into three 
groups : (1) the early sounds heard from about 10 a.m. 
to the afternoon of January 12; (2) the strong detona- 
tions from 6.30 p.m. on January 12 to 6 a.m. on 
January 13; and (3) the much weaker sounds of the 
after-explosions for about ten days following the 
great eruption. All these sounds were heard within 
two entirely detached areas, and it is remarkable how 
similar these areas are in form and to some extent in 
magnitude. The area which includes the volcano 
extends in each case in an easterly direction, the mean 
radius of the boundary being 111, 114, and 102 km. 
for the above three classes of sounds. The second 
area lies to the north of the other, and is elongated 
from west to east, the mean radial distance. of its 
central line from the volcano being 195, 177, and 
196 km. The width of the silent zone was 40-50 km. 
for the strong detonations and about 108 km. for the 
after-explosions, the axis of the silent zone, in both 
cases, being at a distance of about 120 km. from 
Sakura-jima. The greatest distance to which the 
detonations were heard is about 500 km. (or 310 miles) 
towards the north-east, but the air-vibrations were 
strong enough to shake houses and doors’ for about 
85 km. farther in the same direction. 
C.. Davison. 
. 
THE BRITISH ASSOCIATION AT 
NEWCASTLE. 
SECTION E. 
GEOGRAPHY. 
OPENING ADDRESS (ABRIDGED) By EDwarD A. REEVEs, 
F.R.A.S., F.R.G.S., PRESIDENT OF THE SECTION. 
THE surveying equipment of the pioneer explorer of 
early days, say, of from twenty to sixty years ago, 
usually consisted of a sextant and artificial horizon, 
a chronometer or watch, prismatic compass, boiling- 
point thermometers, and aneroid. With the sextant 
and artificial horizon the astronomical observation for 
latitude and longitude were taken, as well as those 
for finding the error of the compass. The route was 
plotted from the compass bearings and adjusted to the 
astronomically determined positions. The latitudes 
were usually from meridian altitudes of the sun or 
stars, and longitudes from the local mean time derived 
from altitudes east or west of the meridian, compared 
with the times shown by the chronometer, which was 
supposed to give Greenwich Mean Time. 
The sextant, in the hands of a practical observer, is 
capable of giving results in latitude to within ro” or 
20", provided it is in adjustment, but the difficulty is 
that the observer has no proper means of testing for 
centring and graduation errors. 
The great drawback to the sextant for survey work 
is that it is impossible to take accurate rounds of 
horizontal angles with it, since, unless the points are 
all on the same level, the angles must be too large. It 
is essentially a navigator’s instrument, and nowadays 
has been almost entirely superseded by the theodolite 
for land-surveying. ; 
As regards the longitude, the difficulty was always 
to obtain a steady rate for the chronometer, owing 
principally to the unavoidable oscillations and con- 
cussions met with in transit. Formerly it was cus- 
tomary to observe lunar distances for getting the Green- 
wich Mean -Time instead of trusting to the chrono- 
NO. 2447, VOL. 98] 
~ : 
[SEPTEMBER 21, 1916 
meters, but these, even with the utmost care, are very 
unsatisfactory. : 
In more recent years the occultation of a star method 
of finding the Greenwich Mean ‘Time superseded 
almost entirely the lunar distance, but all these so- 
called ‘‘ absolute ’’ methods of finding longitude are fast 
becoming out of date since the miére general introduce 
tion of triangulation and wireless telegraphy. 
Heights of land were usually obtained by the boiling- 
point thermometer or aneroid, 
This, then, was the usual equipment of the pioneer. 
With such an outfit the greater part of the first map- 
ping of Africa and other regions of the world was 
carried out, with results that were more or less trust- 
worthy according to the skill of the explorer and the 
time and opportunities at his disposal. : 
In recent years considerable improvement has been 
made in the instruments and methods of the geo- 
graphical surveyor; the introduction of the invar tape 
for the measuring of the base lines, the more general 
application of triangulation, the substitution of the 
theodolite for the sextant, the use of the plane-table for 
filling in the topographical details of the survey, the 
application of wireless telegraphy to the determination 
of longitudes, these and other improvements have all 
tended to greater accuracy and efficiency in geograph- 
ical and topographical mapping, so that in many re- 
spects the rough approximate methods of the earlier 
explorers are fast being superseded by instruments and 
methods more in keeping with modern requirements 
in map-making. 
Still, the principle underlying all surveying is the 
same, and the whole subject really amounts to the 
best and most accurate methods of measurement with ~ 
a view of representing on a plane, on a greatly reduced 
scale, the leading features of a certain area of the 
earth’s surface in their relatively correct positions; and 
so it resolves itself into geometrical problems of similar 
angles and proportional distances. This being the 
case, it is clear that it becomes in the main a question 
of correct angular and linear measurements, and all 
the improvements in survey methods have had for 
their object the increased accuracy of accomplishing 
this, together with greater facility for computing the 
results, 
What we do now is exactly what was attempted by 
the early Greek geometricians and others in ancient 
times, only we have far more accurate instruments. 
If, for instance, we compare our modern micrometer 
theodolite with the old scaph of the Greeks the con- 
trast is striking, although both had the same object 
in view as regards taking altitudes of heavenly bodies. 
Many of the old instruments, in spite of their great 
size, were extremely rough, and the angles could only be 
read with approximation or toa great extent by estima- 
tion, while the theodolite, which is now generally used 
on geographical surveys, although it has circles of only 
five inches in diameter, can, by means of the micro- 
meters, be read tq 2” of arc, or even to 1” by careful 
estimation. This, when one comes to think of it, is 
a triumph of refinement, since it really means that 
we can measure to within about 1/80,000 part of an 
inch, which is something like the space occupied by 1” 
on the arc of a circle of 5-in. diameter. At least this 
is the theoretical accuracy, but in practice there are, 
of course, errors-in sighting, setting the micrometer 
wires, and those arising from other sources which 
have to be taken into consideration. 
The continued striving after greater accuracy of 
measurement applies not only to angular measuring 
instruments, but to linear distance measurement as 
well; and the improvements in apparatus for this 
purpose, could we follow them in detail, ~-ould be 
most interesting. From the rough methods that would 
suggest themselves naturally to early intelligent men, 
a | 
