204 
IiIE. C. V. WALKEE ON MAGNETIC CALMS AND EAETH-CIJEEENTS. 
the earth on the Dover side of Tonbridge, I obtain the other limiting line, London- 
Tonb ridge, Nos. 23 — 24; or if the -wires are connected with the earth on the London 
side of Tonbridge, I obtain the Dover-Tonbridge line. This is not given in my 
Table XI., but is very nearly identical with the Ashford-Tonb ridge line there given, 
and numbered 32 — 31. It makes an angle of 118° E. with the magnetic meridian, a,nd 
therefore falls, as shown in fig. 1, Plate III., intermediate between the other two, and 
incidentally is very useful in this investigation. 
The earth-currents which form the subject of the present communication are not 
detected by the ordinary telegraph galvanometer. I have therefore prepared a small 
horizontal galvanometer. The coil is 2^ inches long, f of an inch wide, and f of an inch 
high, and is filled with silk-covered copper wire, one yard of which weighs 5 grains ; it 
is No. 35 of the Birmingham iron-wire gauge, corresponding to a diameter of -g^ inch. 
It is placed in the magnetic meridian. The needle is 1 inch in length, and carries a 
light index projecting from the E. side of the coil. The range of the index is about 
55° on either side of 0°. The whole is covered by a glass shade. Earth-currents that 
attract no attention on the telegraph needles, produce on this instrument a deflection 
of 40° or 50°. It is placed in proper connexion with one of the Dover-London wires, 
and can be at any instant placed in circuit by merely pressing a spring, and thrown 
out of circuit by removing the pressure. Possession is obtained with equal facility of 
the London-Tonbridge or the Dover-Tonbridge section, by inserting a brass plug in one 
or other of two holes made in a divided brass disc. This is all the apparatus required. 
It is fixed on a slab within arm’s reach from my chair. At any moment, when I see by 
the telegraph needles that the wires are unoccupied by telegrams, I can take the three 
complete observations in a few seconds. The word “ up ” 
is engraved in face of the N.E. quadrant, and the word 
“ down ” in face of the S.E. 
The arrangement of this miniature observatory will be 
readily understood from the following diagram (fig. 9). 
L is the telegraph wire, entering the office from London ; 
D the wire from Dover; S the contact spring, and s the 
stud on which it rests. When the spring and stud are in 
contact, the ordinary telegraph signals pass along them 
between D and L without being visible on the galvano- 
meter G. The needle n, with its index ^, and the circle of 
the galvanometer G, are given in one-third size ; a and b 
are the terminals of the galvanometer wire. When the 
spring S is depressed or removed from contact with the 
stud s, as shown in the diagram, no current can pass be- 
tween D and L without passing through the galvano- 
meter G. 1, 2, and 3 show in one-third size a brass disc divided into three parts, and 
fixed upon a block of mahogany, insulated each from the others. The middle piece of 
the disc is connected by a wire with the gas- and water-pipes, and is therefore in con- 
