226 
for the induced current are similarly electrically con- 
nected. In constructing the secondary coils, they are 
fixed together between two insulating surfaces by bolts 
and nuts, the projections by which the several conduct- 
ing disks are connected projecting helically or spirally 
around the coil (the projections of the primary alternating 
with those of the secondary coil), and form convenient 
means for connecting up any number of conyolutions as 
required. 
The end disks of one of the helices thus formed are 
connected to the leads of the primary circuit by binding- 
screws, and the end disks of the other helix are similarly 
connected to the leads for the induced or secondary cur- 
rent. Inthe centre of the disks is a hollow cylinder of 
paraffined cardboard or other suitable insulating material, 
around which the helices are arranged, and in this cylin- 
der is a core of soft iron, or of soft iron wires, which is 
capable of being automatically raised and lowered in the 
cylinder, so as to regulate as required the current passing 
through the coil. 
The main wire from the dynamo is connected up in 
series to the primary helices of a group of secondary 
generators, and, in passing through the primary helices, 
induces a current in the secondaries, the tension of which, 
according to the experimental investigations of the in- 
ventors, increases first with the intensity or quantity of 
the primary current, and, secondly, with the rapidity of 
the interruptions or alternations, or the variations of its 
potential. Each secondary generator forms a complete 
installation, and can be put in or out of circuit at pleasure. 
The secondaries may be connected up in series, in mul- 
tiple arc, or in multiple series, as desired, the connections 
being readily altered by means of a switch-board ; tension 
or quantity is thus obtained according to the nature of 
the current required. The lamps or other receivers fed 
from the secondary generator can be connected at will 
to their respective circuits, and are also independent of 
one another. 
These generators are made to work in connection with 
alternate-current machines, because the latter can be 
constructed up to almost any power, as no two parts of 
the machine having great difference of potential need be 
in close proximity, and the alternation of current may be 
made as quickly as desired. The generating dynamo is 
so constructed and operated that the quantity of current 
is preserved constant, and the tension is varied to carry 
this current through the primary conductor against the 
varying counter electromotive force due to variations in 
the work done in the secondary circuits of a number of 
secondary generators. If /V represents work, C current, 
£& electromotive force, and & resistance, and if either of 
these factors be changed, the others must be altered in 
the same ratio, according to the formula— 
9 ee 
VWi— GE — Gk — Rp 
if uniform effects in the secondary circuits are to be 
desired. 
One of the chief characteristics of this system is that if 
the primary current be kept constant the loss due to 
resistance remains fixed, no matter what energy is trans- 
mitted—so that if an increase of energy is desired, the 
only factor that has to be increased is the electromotive 
force, which bears no ratio to the loss in the conductor. 
This circumstance is of importance in any house-to-house 
lighting scheme, where a conductor may be laid down to 
supply a certain area, and if the lights are not taken up 
at once, the necessary current can be supplied later within 
the limits of the dynamo, by increasing the electromotive 
force, without increasing the size of the conductor, the 
strength of the current, or the loss in the line. 
As regards the very high potential required upon the 
secondary generator system, the danger is limited to the 
supply station, as between the two poles of the main 
NATURE 
[Fuly 9, 1885 
dynamo there is an unbroken metallic circuit, which 
maintains the continuity of the flow of current ; and as 
regards each secondary circuit the work done is repre- 
sented by a secondary generator, and the only danger 
would be in grasping both primary terminals at once, 
which may be made impossible of performance. It will 
be necessary as regards the dynamo that it shall be insu- 
lated from the earth, and also that such parts of the 
circuit as carry high tension electricity shall be so pro- 
tected that it shall be impossible to make contact between 
them and the earth. 
In comparing this system, in which there is a loss in 
the transformation of the energy by the secondary gene- 
rator, with the direct system, this loss will have to be 
balanced against that caused by resistance due to distance, 
whilst as regards the regulation of the supply of energy, 
this is effected by means of a regulator working the 
exciting machine of the dynamo at the station ; by its 
means, when a secondary generator is cut out of circuit, 
a proportionate amount of power is saved. The secondary 
generators also regulate the energy absorbed, so that a 
perfect control of power is obtained, which is especially 
important for domestic supplies of electricity, as, when a 
suitable current measurer has been designed, consumers 
will be able to pay simply according to the amount 
consumed. 
At present the extreme northern end only of the East 
Arcade at the Exhibition is being lighted on this system ; 
it is proposed, however, to extend it to the full length of 
the East Arcade and to the concert-room. 
THE AFGHAN DELIMITATION COMMISSION 
V E are indebted to the courtesy of the Kew authorities 
for the opportunity of publishing the accompanying 
letter from Surgeon-Major Aitchison, C.I.E., F.R.S., which 
gives the most recent account of his work as naturalist to 
the Expedition :— 
Camp Tir-Phul, Northern Afghanistan, 
6 miles from Khusan 
DEAR SIR JOSEPH HOOKER,— 
I am now able to write to you with some pleasure, as I 
have been able to put together this year some 300 species 
in all. The last 100 I obtained on a ten days’ trip that I 
made from this camp. I left this on April 25 under very 
bad auspices, as it had blown all night and was blowing 
a terrible gale with every chance of a heavy fall of rain 
from the north. But I started and got as far as Khusan, 
in the vicinity of which, beside the ruins of an old “ serai,” 
I halted. 1 picked up a few odds and ends, the chief 
attraction was the Rosa margerita (if a new sp.) m/z. It 
covers the whole country in localised patches, and being 
very dwarf in habit, not above 2 feet, the flowers are seen 
to perfection ; they open out expanding almost flat, when 
the brilliant eyes, formed by the claret colour of the bases 
of the petals, gives it quite a character. Amongst my rose 
hips sent to you last year this was one of the species. I 
hope to be able to supply you with a lot more, it would 
make a lovely flower border. 
I marched next to a place on the right bank of the Hari- 
rid River opposite Toman-agha, fifteen miles. Our route 
lay over a plain that had once been the bed of the river 
where the river had made a great bend ; the river, after 
silting up this bend, had left it. The most characteristic 
plant here was a Rhubarb, usually with 3-root leaves of 
immense proportion for the size of the flowering stem ; 
these leaves are so pressed flat to the ground that it 
reminds one more of the I’%cforia regia leaves (without 
the margin), and this is the habit of the plant; the plant 
was fruiting, having large winged fruit of a most brilliant 
scarlet ; it will make a grand thing in gardens. The 
