264 
NAAT REE Te 
[| Fan. 6, 1870 
-the speed may be altered from sidereal to lunar rate by 
merely raising the bob of the pendulum through a small 
space, so as to diminish the time of oscillation, _ 
For special purposes, still greater accuracy might be 
obtained if the escapement were worked by the observa- 
tory clock by means of a small electro-magnet connected 
with the pendulum of the latter. Conversely, the escape- 
ment train might, with slight modifications and the addi- 
tion of a dial, be made to serve as a journeyman-clock, 
and show sidereal time with sufficient accuracy to be very 
useful in finding stars during two or three hours’ work. 
There is, by the way, another important modification 
well worthy of notice. When astronomers wish to deter- 
mine the position of a star, the diameter of a planet, &c., 
with rigorous accuracy, they employ a micrometer with 
spider webs, which in the daytime are visible in the field 
of view. At night, however, they, or the field itself, 
require to be lit up. This is managed by a lamp out- 
side and a reflector inside the tube, and to make this lamp 
perform effectively in every position of the telescope is a 
difficult matter; so difficult, in fact, on the old arrange- 
ment, that Messrs. Cooke and Sons, with their wonderful 
ingenuity, have entirely superseded it in this their latest 
instrument. Their exquisite contrivance will be seen from 
the annexed woodcut. 
THE COOKE ILLUMINATING APPARATUS. 
Listhe lamp. P, a prism to reflect the light on to the tube. D, a dise with 
diaphrams to regulate the quantity of light. B,a disc with glasses to 
regulate the colour of the light. S, spring catches to clamp these discs. 
C, counterpoise of lamp. G, Gravity poise. 
The telescope is furnished with a sufficient battery of 
eye-pieces, of powers ranging from 30 to 400, and also with 
abifilar micrometer. The position circle is permanently 
attached to the lower end of the main tube. 
The observatory is erected on the roof of the western 
tower of the New Schools. It is square, and surmounted 
by a revolving dome. It is obvious that an instrument 
erected on a tower cannot be wholly free from vibration ; 
but the latter is reduced to a minimum by supporting the 
telescope on two massive trussed iron girders stretching 
across the tower. ‘The floor of the observatory is sup- 
ported quite independently. 
Owing to the unfavourable weather of late, the final 
adjustments of the telescope have not been completed ; 
but it is hoped that before long it will be in a condition 
for food and accurate work, such as will justify the en- 
lightened liberality which has placed it where it is. 
H. G. MADAN 
REMARKS ON TERRESTRIAL MAGNETISM 
(Being the substance of a paper read at the Royal 
Astronomical Society, on Friday, Dec. 10) 
GOME years since I was led to the belief that earth 
currents and aurore are secondary currents pro- 
duced by rapid, though small, changes in the earth’s 
magnetism. 
In this hypothesis the earth was viewed as similar to 
the soft iron core of a Ruhmkorff’s machine, and the 
upper and rarer strata of the atmosphere and the moist 
upper surface of the earth as conductors in which 
secondary currents would be generated whenever any 
change took place in the magnetism of the core. 
This hypothesis is, I think, confirmed by the very 
interesting and valuable photographic traces of earth 
currents obtained by Mr. Airy, at Greenwich, in which, 
during times of great magnetic disturbance, the earth 
currents are seen to be very strong, and to vary alter- 
nately from positive to negative, lying about equally on 
both sides of the zero. 
It has occurred to me that this method of viewing 
things is capable of extension, and that it ought to be 
borne in mind that secondary currents are produced, not 
only in a stationary conductor such as that of the Ruhm- 
korff’s machine, where the magnetic core is made to vary, 
but also in a conductor which moves in the presence of a 
magnetic core of constant strength. 
Have we not in the earth such conductors in constant 
motion? We have the return trades constantly proceeding 
at a high elevation from the equator to the poles, the 
upper strata of which, from their tenuity, may no doubt 
be considered to be conductors; in their journey they 
cross the lines of the earth’s magnetic force: ought they 
not, therefore, to be the vehicles of electrical currents? 
My friend Mr. Lockyer has lately impressed upon me 
that the zodiacal light may possibly be a terrestrial phe- 
nomenon, and, therefore, that it may be connected in 
some way with the phenomena of terrestrial magnetism. 
May it not be the return trades rendered luminous 
through electric currents in the higher regions of the 
atmosphere, and may there not also be two species of 
aurore, the one occurring in stationary conductors, when 
the earth’s magnetism changes, and the other, in moving 
conductors, when the earth’s magnetism is constant ? 
But again, it must be allowed that these conductors 
conveying currents must react on the magnetism of the 
earth, and we might therefore expect that at those periods 
of the year at which the system of currents, viewed as 
meteorological phenomena, change most abruptly, the 
earth’s magnetism would also be particularly liable to 
change. May not this be an explanation of the excess 
of magnetic storms about the times of the equinox ? 
But besides these great terrestrial currents, we have the 
daily convection currents caused by the sun, which, when 
they reach the upper regions of the earth’s atmosphere, 
we may imagine to be conductors ; and as they also pass 
across lines of magnetic force, we may suppose them to 
convey currents. May not these, to some extent at least, 
account for the diurnal variations of terrestrial magnetism ? 
If this be the case we should have a ready explanation of 
the likeness observed by Mr. Baxendell between the wind 
curves and those of the declination. 
I have hitherto alluded only to atmospheric currents, 
but there are also oceanic currents, and more especially 
there is the tidal wave, which occurs twice every lunar 
day. No doubt the influence of the tidal wave, as a 
moving conductor, must be very small: but may it not 
help to account for the lunar-diurnal variation, which is 
very small likewise? 
But if there is an electric current of this kind in the 
ocean, it ought to be detected by the system of earth cur- 
rent wires which Mr. Airy has at Greenwich, inasmuch as 
the surface of the earth and the ocean are in electric 
communication with each other. Mr. Airy has, if I am 
not mistaken, detected indications of lunar-diurnal in- 
equalities in the results of his observations. On the other 
hand, he has detected no current with a single daily period 
that would account for the diurnal variation—a result in 
accordance with these views, since the currents producing 
such would be in the upper regions of the atmosphere. 
These views are given in order to invite criticism and 
