Fune 16, 1870| 
NATURE 
128 
covering, with poles, &c., has been carried from the north 
of England to the Pyrenees without ever being asked to 
do service, one is move inclined to trust in future to the 
resources of any locality, rather than again to travel in 
company with such a burdensome luxury. 
Provided the morning be not hopelessly cloudy, it will 
generally be found preferable to start work by observing 
the declination. This choice may at times save the 
travellers a day or more of precious time in changeable 
weather; for the sun then not unfrequently shows himself 
for a short time in the early morning hours, and when the 
clouds have been permitted to close around him, he 
remains persistently hidden for the remainder of the day. 
For finding the magnetic declination, the sun is our most 
useful helpmate ; but during the rest of the observations his 
presence can with benefit be dispensed with. The uniiilar 
is therefore at once put together, and whilst the torsion of 
the silk thread by which the declination magnet is about 
to be suspended is being removed, the theodolite is fixed 
ona tripod stand, and several transits of the sun are taken 
over the vertical wires. The azimuth circle having been 
read at each passage of the sun, the telescope is placed 
horizontal, and the position of a fixed mark determined. 
A few altitudes of the sun might then be taken, for finding 
the error of the chronometer, before removing the theodo- 
lite from the tripod stand to make way for the unifilar. 
Great care must of course be taken in levelling each 
instrument before commencing an observation ; and, after 
railway travelling, all the screws should be well examined. 
The tripod stand remaining firmly fixed in its first position, 
the unifilar is placed upon it, and the telescope is directed 
to the fixed mark and its azimuth read. The magnet is 
then suspended, brought to rest, and the positions of the 
azimuth circle and of the magnet scale noted. A second 
reading of the scale is afterwards taken in the inverted 
position, and this invertion should be twice performed, for 
greater security. A glance at his note-book will at once 
tell the observer whether he has entered all the data 
necessary for the future calculation of the declination. 
The chronometer gives the azimuth of the sun for a known 
latitude and longitude, the sun fixes the chosen mark, and 
the angle between the magnet and the mark then tells us 
what are the bearings of the compass at the station of 
observation. Having thus secured the declination, an 
interesting set of scale readings may be taken whilst the 
magnet still remains suspended. -These readings, if taken 
every five minutes during any considerable time, will 
furnish materials for a comparison with the magnetograms 
of the fixed observatories; and thus afford satisfactory 
data for determining the relation that exists between the 
magnetic perturbations at different points of the earth’s 
surface. 
The dip circle is the next instrument to which attention 
must be directed. The observation of the dip of the 
magnetic needle is in theory the simplest, and in practice 
the most trying work of the survey. To attempt it when 
the atmosphere is saturated with moisture may be an 
excellent exercise of patience, but can scarcely lead to 
any trustworthy results; and a dusty locality will easily 
give the observer hours of labour before he has finished 
with a single needle. At each station, two or three needles 
should be used ; each observation being the mean of 32 
readings taken in different positions of the instrument and 
needle. The method of observation can offer no difficulties ; 
after levelling the instrument, the needle is magnetised and 
gently lowered until it rests on its agate planes. The dip 
circle is then turned round till the needle takes up a ver- 
tical position; the reading of the azimuth circle at this 
point, when increased by go degrees, gives the direction 
of the magnetic meridian. The instrument may now be 
clamped with the needle in the meridian, and the readings 
of the angle of inclination commenced. Whilst the dip 
is being found, the torsion may be removed from the silk 
thread that serves to suspend the vibration magnet, 
A complete knowledge of the direction of the earth’s 
magnetic force at the station of observation having been 
acquired, it remains to determine the intensity of this force. 
Knowing the direction, it suffices to measure the intensity 
of the force in any given direction, in order to ascertain 
the total effect of the force. The horizontal component 
of the intensity is then the only element that still remains 
to be observed, and this is found by the method of vibra- 
tions and deflections. A magnet in the form of a small 
telescope, having a delicately graduated scale at the end 
farthest from the observer, is hung by a thread, and made 
to vibrate through an arc of a few seconds. The time 
occupied by 100 such vibrations is noted by the chrono- 
meter to within the twentieth of a second, and this is 
repeated some twelve times to obtain an accurate measure 
of the time of one vibration. The square of this interval 
of time serves to determine the product of the horizontal 
component of the earth’s magnetism, by the magnetic 
moment of the vibrating magnet. 
The quotient of these same quantities is next ascertained 
from the deflecting power of the vibration magnet. For 
this purpose another magnet is suspended by a very 
delicate silk thread, from which the torsion has been 
removed, and the vibration magnet is placed on a graduated 
brass bar at known distances from the suspended magnet. 
The opposite poles of the deflecting magnet are then pre- 
sented alternately to the centre of the free magnet, and at 
the same distance due east and west. The various readings 
of the azimuth circle, and of the scale reflected by the 
mirror of the deflected magnet, are noted for each position, 
and the changes of temperature, which enter into the 
results, are frequently recorded. 
The series of observations is then completed, and, when 
the instruments have again been packed with all possible 
care, the observers are ready for the journey of the morrow. 
Very much of the success of the survey depends on the 
steady rate of the chronometer; and hence no pains 
should be spared to preserve it from all jolting movements 
whilst travelling, and altitudes of the sun should never be 
omitted. But it is, above all, advisable to lose no oppor- 
tunity of comparing the chronometer at any good obser- 
vatory that can conveniently be visited on the way. 
A full set of the above observations may be taken by 
one person within the space of half a dozen hours, but 
then all must have gone on smoothly from starting to the 
end. A loud clock, a clear-toned bell, or even the rustling 
of the leaves in a high wind, will not unfrequently compel 
an observer to recommence his series of vibrations. Or 
again, the breaking clouds will entice him out with his 
theodolite, and then the sun will always seem purposely to 
hide himself at the moment he approaches the fatal wire. 
But the near approach of friends is what tries most the 
powers of endurance. The second has arrived for taking 
the observation, the eye and ear are all attention, and a 
person shows his interest in the work by spoiling all with 
an ill-timed question. Or again, the suspended magnet is 
just coming to rest after a tedious oscillation, the observer's 
patience is about to be rewarded by a perfect reading, when 
a violent movement of the magnet makes him aware that 
the bunch of keys or pocket-knife of a friend, who ap- 
proaches with the most scupulous avoidance of all noise, 
have attracted the attention of the giddy little needle. 
But a good observer will never be overcome by such 
trifling difficulties. 
A two months’ vacation passed in this alternation of 
travel, observation, and repose, will witness the accumula- 
tion of data sufficient for the calculation of the isogonics, 
isoclinals, and isodynamics of a large extent of country, 
and the observer will return to his routine labours in- 
vigorated in mind and body, and with the consoling thought 
that he has added some little at least to that mass of well- 
established facts, which must ever be the only founda- 
tion of any true advancement in the Natural Sciences. 
S. J. PERRY, 
