592 
NATURE 
[| April 19, 1883 
fashion they are sure to do) from right and left, the bird can 
take advantage of their alternation to rise higher and higher, or 
at least to remain floating, without more effort than that which 
is required to give the due slore to its wings to make the most 
of every gust. 
Next suppose the whole air with its two alternate opposite 
currents (as above) to be travelling horizontally in the same 
direction as one of the two opposite currents. Whether this 
supposition represents a possible state of things I hardly know, 
but it would correspond in some measure with the commonly 
observed phenomenon of a succession of alternate gusts and lulls 
in the wind. Under these conditions, if the air-movement be 
all horizontal, it is difficult to see Eow the bird can tum the 
alternate gusts to advantage, uvless it can alternate its own 
direction accordingly, stemming the gust and wheeling round to 
fall back with the lull. The bird then would either circle or 
would fellow a wavy ccurse oblique to the direction of the wind. 
But I imagine that alternate gusts and lulls (as felt, say, at the 
top of an observatory) are generally caused by a succession of 
vortices, of which only one phase at a time is present to the 
cbserver. These vortices will be infinitely various in the direc- 
tion of their axes and currents, and it is useless to try and 
imagine their relative positions. Probably the sea-birds, with 
their ze ns of inherited experience, have acquired an instinctive 
perception of the probable sequences and correlations of air- 
streams and air-swirls, and are thereby guided so to steer their 
course, selecting the upward and avoiding the downward cur- 
rents, as to gain the greatest possible advantage of lifting force 
that those currents can afford, to the great eco omy of their 
muscular strength, which would otherwise have to be spent in 
the labour of the wing. 
In reading of the way in which albatrosses and other large 
sea birds will follow a ship at sea with Jittle or no flapping of 
the wings, it has occurred to me that the great obstacle which 
the ship herself offers to the wind must of necessity give the 
wind an upward throw and originate a vortex in the air, pos ibly 
large enough and persistent «nough to be useful to the birds. If 
the ship be a streamer, the drift of smoke from the funuel will 
indicate approximately the path of the retiring vortex. It is 
long since 1 have had any opportunity of observing, but I well 
recollect that the gulls used often to be seen in close relation to 
the smoke that drifted to leeward of the steamer. It is true that 
any chance norsels of biscuit, &c., thrown from the steamer 
would probably be thrown to leeward, and this might help to 
determine the position of the expectant gull. 
Again, at sea, the ocean waves themselves, such as roll in 
from the Atlantic to the Land’s End, must throw the wind into 
rolling vortices, which would afford slant upward currents. The 
slant, though very flat, might well be sufficient for the purpose 
of support to the long-winged sea-birds that know how to 
use it. 
On land, countless obstacles impede the lower wind and tend 
to throw the air into a roll. 
Bearing in mind, then, the perpetual variation in strength 
and direction of current in a high wind, the whirls and gusts, 
and veering flaws, and seeing how it is posible for the bird to 
utilise every such variation (except a downward current) to the 
purpose of its bodily support, we may, I think, obtain some 
insight into the agency whereby the birds accomplish their 
marvellous feats of soaring ana sailing, upborne upon stiff- 
strained, motionless wings. 
Further observations however are required for the {ull 
solution of the problem which I have here only tentatively 
approached. HUBERT AIRY 
Woodbridge, February 28 
SOME POINTS IN ELECTRIC LIGATING! 
ay HE science of lighting by electricity was civided by the lecturer 
into two principal parts—the methods of production of elec- 
trie currents, and of conversion of the energy of those currents into 
heat at such a temperature as to be given off in radiations to 
which the eye was sensible. The laws known to connect to- 
gether those phenomena called clectrical, were essentially 
mechanical in form, closely correlated with mechanical laws, 
and might be most aptly illustrated by mechanical analogues. 
For example, the terms ‘‘ potential,” ‘‘current,” and ‘‘resist- 
* Abstract of lecture delivered at the Institution of Civil Engineers on 
Thursday evening, April 5, by Dr. John Hopkinson, F.R.S., M.Inst.C E. 
ance,” had close analogues respectively in ‘“‘head,” ‘‘rate of 
flow,” and ‘‘ coefficient of friction” in the hydraulic transmission 
of power, Exactly as in hydraulics head multiplied by velocity 
of flow was power measured in foot-pounds per second, or in 
horse-power, so potential multiplied by current was power and 
was measurable in the same units. Again, just as water flowing 
in a pipe had inertia and required an expenditure of work to set 
it in motion, and was capable of producing disruptive effects if 
that motion were too suddenly arrested, so a current of electri- 
city in a wire had inertia: to set it moving electromotive force 
must work for a finite time, and if arrested suddenly by breaking 
the circuit the electricity forced its way across the interval as a 
spark. Corresponding to mass and moments of inertia in me- 
chanics there existed in electricity coefficients of self-induction. 
There was, however, this difference between the inertia of water 
in a pipe and the inertia of an electric current—the inertia of the 
water was confined to the water, whereas the inertia of the 
electric current resided in the surrounding medium. Hence 
arose the phenomena of induction of currents upon currents, and 
of magnets upon moving conductors—phenomena which had no 
immediate analogues in hydraulics. 
The laws of induction were then illustrated by means of a 
mechanical model devised by the late Prof. Clerk Maxwell. 
In the widest sense, the dynamoelectric machine might be 
defined as an apparatus for converting mechanical energy into 
the energy of an electrostatic charge, or mechanical power into 
its equivalent electric current through a conductor. Under this 
definition would be included the electrophorus aud all frictional 
machines; but the term was used in a more restricted sense, 
for those machines which produced electric currents by the 
motion of conductors in a magnetic field, or by the motion of a 
magnetic field in the neighbourhood of a conductor. The laws 
on which the action of such machines was based had been the 
subject of a series of discoveries. Oersted discovered that an 
electric current in a conductor exerted force upon a magnet ; 
Ampere that two conductors conveying currents generally ex- 
erted a mechanical force upon each other: Faraday discovered 
—vhat Helmholtz and Thomson subsequently proved to be the 
necessary consequence of the mechanical reactions between con- 
ductors conveying currents and magnets—namely, that if a 
closed conductor moved in a magnetic field, there would be a 
current induced in that conductor in one direction, if the number 
of lines of magnetic force passed thrcugh the conductor was in- 
creased by the movement ; in the other direction if diminished. 
Now all dynamoelectric machines were based upon Faraday’s 
discovery. Not only so; but however elaborate it might be de- 
sired to make the ai alysis of the action of a dynamo-machine, 
Faraday’s way of presenting the phenomena of electromagnetism 
to the mind was in general the best point of departure. The 
dynamo-machine, then, essentially consisted of a conductor made 
to move in a magnetic field. This conductor, with the external 
circuit, formed a closed circuit in which electric currents were 
induced as the number of lines of magnetic force passing through 
the closed circuit varied. Since, then, if the current in a closed 
circuit was in one direction when the number of lines of force 
was increasing, and in the opposite direction when they were 
diminishing, it was clear that the current in each part of such 
circuit which passed through »he megnetic field must be alternat- 
ing in direction, unless indeed the circuit was such that it was 
continually cutting more and more lines of force, always in the 
same direction, Since the current in the wire of the machine 
was alternating, so also must be the current outside the machine, 
unless something in the nature of a commutator was employed to 
reverse the connections of the internal wires in which the current 
was induced, and of the external circuit. There were then 
broadly two classes of dynamoelectric machines—the simplest, 
the alternating-current machine, where no commutator was used ; 
and the continuous-current machine, in which a commutator was 
used to change the connection with the external circuit just at 
the moment when the direction of the current would change. 
The theory of the alternate-current machine was then explained, 
and it was proved that two independently-driven alternate-current 
machines could not be worked in series, but that they might be 
worked in parallel circuit, and hence were quite suitable for dis- 
tribution of electricity for lighting without the necessity of 
[roviding a separate circuit for each machine. 
It was easy to see that, by introducing a commutator revolving 
with the armature, in an alternate-current machine, and so 
arranged as to reverse the connection between the armature and 
the external circuit just at the time when the current weuld 
