124 
NATURE 
[Fune 16, 1870 
The Report of the Meeting of the British Association 
for the Advancement of Science 
Ir would be a great boon to the English public if the papers 
which are read before the members of the Association, or rather 
those who have the means as well as the inclination to attend 
such important annual gatherings, could be published within the 
first few weeks immediately after the meeting, and at such a 
price as many in this land would by no means grudge. Accord- 
ing to the present arrangement, some monthis have to pass before 
even such as can afford the heavy price placed upon these 
reports can obtain them, the result of which is, that the poor and 
meagre reports of the various newspapers are all that the 
majority of persons have to inform them about the advancement 
of science. Could not some such report as that authorised by 
the committee of the Church Congress be produced? The 
first Congress was reported fully, the papers entire, and the dis- 
cussions almost so, in a well-printed volume of nearly 500 pages, 
for the moderate sum of half-a-crown, prepaid ; or four shillings 
if purchased afterwards. There are many, I am convinced, who 
would gladly prepay three or even four half-crowns for the report 
of the papers only which are read before the British Associa- 
tion. 
Birkenhead G, A. A. 
TERRESTRIAL MAGNETISM 
ate great progress made during the last few years in 
our knowledge of the phenomena of terrestrial 
magnetism has naturally attracted not a little attention 
to this interesting subject, and the persevering efforts of 
many leading scientific men are the surest guarantee of 
the ultimate success of the labours undertaken in this 
cause. Hitherto theory has gone hand in hand with 
observation, and those most able to undertake the charge 
are now only waiting a more complete array of facts, in 
order to present us with a theory of terrestrial magnetism 
based on solid foundations, and equal in interest, in com- 
pleteness, and in the utility of its practical applications, 
to any of its sister sciences, The required facts may be 
ranged under these two heads, First the actual values of 
the magnetic elements at all parts of the earth’s surface, 
with the secular variations of these elements; and 
secondly, the daily, monthly, and yearly range through 
which these quantities vary, and the irregular pertur- 
bations to which they are subject. To meet the first 
demand, a complete set of magnetic observations must be 
taken in different countries, the stations of observation 
not being too far distant from each other ; and a repeti- 
tion of these at intervals of from ten to twenty years will 
supply an accurate record of the secular variations of the 
required elements. The second need can only be satis- 
factorily provided for by the establishment of permanent 
magnetic observatories, supplied with self-registering 
magnetographs, by which every change of the magnetic 
force can be correctly traced. 
Fortunately much has already been effected, and more 
is actually being carried out for the furtherance of this 
latter object. This country alone possesses three such 
observatories, Lisbon another, Florence a fifth, and four 
others are at present incourse of erection, at St. Petersburg, 
Bombay, Melbourne, and the Mauritius. The fact, now 
clearly established, that any disturbance of the magnetic 
needle traced by our magnetographs in England has a 
corresponding perturbation recorded at Lisbon and at 
Florence, is an earnest of what we may expect from a 
careful comparison of magnetograms from all quarters of 
the globe. 
But the other branch of observation, on which the 
determination of the absolute values of the magnetic 
elements solely depends, has scarce kept pace with the 
giant strides of the fixed observatories. Doubtlessa series 
of monthly absolute readings is being taken at each 
magnetic observatory to serve as a basis for the differential 
curves traced by the magnetographs ; but such observa- 
determination of the magnetic elements of these few 
points, however useful it may be, will scarcely suffice to give 
us a complete knowledge of the magnetic condition of the 
earth’s surface, The survey of the ocean we may well 
trust to the devoted and persevering enterprise of the 
naval officers, who are adding so much to our store 
of meteorological and magnetic knowledge ; but for the 
land surveys we must mainly depend on the accurate 
observations of private observers. A very limited 
number of scattered observations made at stations remote 
from each other, and the careful survey of scarcely half a 
dozen countries of Europe, is all we can point to as at 
present accomplished. If it be true that labour and 
expense are the great obstacles to be overcome when an 
important enterprise has to be undertaken, we may well 
wonder that the more laborious and by far the most 
expensive of the observations needed for the science 
of terrestrial magnetism are being amply supplied by the 
establishment of fixed observatories, whilst the magnetic 
surveys, which may be now made an agreeable pastime, 
and need scarcely add perceptibly to our usual expenses, 
are nearly entirely neglected. Most men who are daily 
engaged in intellectual occupations, require yearly a few 
weeks of repose, or at least of mitigated application, For 
many, some interesting work that does not too much 
engross their minds, and that differs considerably from 
their usual routine duties, offers a means of relaxation far 
more enticing than a complete cessation from labour, 
For men of this class, who have had some little experience in 
the use of instruments of precision, a magnetic survey 
might offer the greatest attraction during a summer 
vacation. A brief sketch of a simple method of carrying 
this into effect, which has been found to work well during 
two successive vacations, may not therefore be without its 
utility and interest. 
The country chosen for a rapid survey should be one 
that offers considerable facilities for railway communica- 
tion ; anda previous study of the direction of the lines will 
prevent much unnecessary expense and loss of time from 
travelling twice over the same ground. Should the rail- 
way arrangements of the country resemble those of 
France, a companion for the journey becomes an absolute 
necessity ; for the instruments used are of too delicate 
a make to be entrusted to the tender mercies of foreign 
railway officials of the registered baggage department. 
The instruments required for the survey are a dip 
circle, an unifilar, a small transit theodolite, and a good 
chronometer, Two tripod stands are nearly a necessity 
if time be any consideration, Three days might con- 
veniently be allotted to each station ; one for travelling, 
another for the observations, and the third for visiting the 
objects of interest in the vicinity. Twenty sets of 
observations would thus be completed in the course of 
two months; and two vacations so employed would furnish 
data sufficient for the accurate study of the magnetic con- 
dition of an extensive tract of country. But should the 
time at the observer’s disposal be more limited, and he 
feel equal to the task, the day of travelling might reckon 
as a day of rest, and the public monuments be visited 
during the leisure hours of the day of observation, 
Arrived at a station, the first thought should be about 
the choice of a fit place of observation. The garden of 
any large public institution will perhaps in general be 
found the most convenient and accessible, unless an ex- 
tensive and somewhat retired garden be attached to the 
hotel. Inquiry must next be made about the position of any 
considerable mass of iron, such as large pipes, which serve 
not unfrequently to convey gas or water at a few feet below 
the surface of a gravel walk or a grass plot, For the 
observation of the dip and the intensity, a good shady 
spot is required ; whilst, for the declination, a position 
well exposed to the sun, at the early hours of the morning, 
is the best possible. A tent may sometimes be required 
tories will always be few in number, and the accurate | to supply the want of trees; but when a large canvas 
