226 
NATURE 
[ Fan. 5, 1882 
of the dark masses on the great northern belt. The spot 
is also liable to become very faint. I have carefully 
noted these variations, and though the observations are not 
sufficiently full to determine the period, if any, they show 
that the spot becomes faint almost to invisibility at inter- 
vals of about 56 days, and that increased brightness of 
the spot is accompanied with accelerated motion. I 
believe this particular object is a permanent feature on 
the planet, and that it lies far below the level of the dusky 
belts. Mr. Marth has determined from a discussion of 
the observations of 1880 and 1881 (to November) that the 
mean motion of the spot has been uniform, and this is 
important as a proof of its stability. My own numerous 
observations have led me to conclude that : — 
1. It is self-luminous and light-emitting. 
2. That it is a part of, or projection from, the actual 
surface of the planet. 
3. That therefore it indicates the real rotation period of 
Jupiter, which is gh. 50m. 6°6s. (= daily rate 87848), as 
deduced by Mr. Marth. The motion of the red spot 
shows a decided slackening, so that we cannot accept it 
asa reliable and invariable indication of the motion of the 
Jovian sphere with which probably it has no material 
connection. 
These conclusions are supported by the fact that we 
cannot admit the idea of an object as permanent and 
conspicuous as the white spot, rushing on in advance of 
the already swift axial movement of the planet (as 
computed from the positions of the red spot) whereas 
we can more readily understand that atmospheric ob- 
jects, such as the belts and red spot (which are forms 
of identical phenomena), would show a tendency to lag 
behind the rapid motion of the sphere. We must allow 
that there will be a failure of objects on the extreme outer 
envelopes of Jupiter, to keep pace with the tremendous 
velocity of objects on his real surface. The dusky belts, 
the red spot, and similar markings, are probably openings 
in the Jovian atmosphere, and the slackening motion of 
these objects is simply the indication that they are be- 
coming more shallow than formerly, whence we may infer 
that the motion will continue to decrease until they are 
finally dissipated. 
A comparison of my recent observations with those 
made by Gledhill and Welb in the years 1869-72, show 
that many of the features which they described and de- 
lineated (in the Astronomical Register and Popular 
Science Review) are still visible or have reappeared after 
an interval of obscuration. The great red spot may be 
the same object as Gledhill’s ellipse of 1869-71. In many 
of the details visible then and now there is a remarkable 
similarity both in aspect and position, and the observers 
of Jupiter should further carefully investigate the physical 
appearance of the planet with a view to obtain more dis- 
tinct evidence on the question of periodic variations. In 
this connection I may quote a remark by the late Mr. 
Lassell (Wonthly Notices, vol. xxxiv. p. 310), where, in 
referring to round light spots he saw on Jupiter in March, 
1850, and March, 1874, he says: “I believe the appear- 
ance of these spots is very rare, as I have not seen them 
for many years, and the general similarity of the aspect 
of the planet now [1874] and then [1850] suggests the 
idea that the various phases return in cycles, which I 
think more probable than that absolute secular changes 
occur in the heavenly bodies within the limit of time of 
any human records.” W. F. DENNING 
LITTLE ELECTROMOTORS 
f hee probability that within a few months almost every 
large town and city will be supplied with electricity | 
on a large scale for the purpose of lighting, has brought 
into prominence the question of utilising the same supply 
for the purpose of producing power on a small scale for 
sundry domestic purposes. There are a number of objects 
for which machinery is employed, though on so small a 
scale that it would not be worth while to set up a steam- 
engine or gas-engine to drive it, to say nothing of the 
inconvenience of a steam- or gas-engine in a private 
house. To drive a sewing-machine, for example, or to 
work a light turning-lathe, requires a comparatively small 
power, and usually only for a limited time. It is natural 
then to think that when the power of electricity is avail- 
able in the wires which supply electric light, such a power, 
especially as it is so simply and readily controlled, might 
be economically employed for such purposes. 
But to drive machinery by electric currents necessitates 
the employment of the appropriate electric engine or 
“ electromotor,” which, as its name implies, is an engine 
which, by the expenditure of electrical energy, does 
mechanical work. Such engines have been known since 
1831, when Prof. Henry first constructed a rotating engine 
driven by electromagnets. Ritchie, in 1833, indepen- 
dently constructed an electromagnetic apparatus for pro- 
ducing continuous rotation. Fig. 1, which we borrow from 
Prof. S. Thompson’s “ Lessons in Electricity and Mag- 
netism,” shows a modification of Ritchie’s electromotor 
frequently found in collections of electrical apparatus. It 
consists simply of an electromagnet, CD, poised upon a 
ju 
ne) 
——— 
on 
Fic. 1. 
pivot between the poles, NS, of a steel horseshoe magnet, 
and fitted with an arrangement of mercury cups, A B, as 
a commutator, by means of which the current arriving 
through the wires is so directed through the coil as to 
produce motions, in one sense, only round the axis. The 
pole C of the electromagent is attracted round toward s 
until, just as it nears S the wire beneath C passes from 
one mercury-cup to the other, so reversing the current 
and causing C to be repelled from s and attracted to N. 
To speak of the further developments of these machines 
in the hands of Jacobi, Sturgeon, Froment, and others, 
would be to traverse ground too wide for the scope of an 
article like this. Paccinotti’s discovery of the ring-arma- 
ture, which in 1869 he applied to the construction of an 
electromagnetic motor which was also capable of being 
used as a generator of currents, dropped strangely out of 
sight. And the subsequent discovery of M. Gramme 
that his generator would work as a motor was only the 
beginning of a new epoch in the history of electromotors. 
We know that all the magneto-electric and dynamo- 
electric machines used to generate continuous currents of 
electricity, whether of Gramme, Siemens, Brush, or 
Edison are reversible. If we drive them by mechanical 
power they yield electric currents, and if on the other 
hand we supply them with currents of electricity, they 
can run backwards and do work for us. Sawing and 
ploughing are now done every day by this means. We 
have Siemens’ electric railway and tramway, and many 
other useful applications of the same principle, of which 
