Jupiter's Satellites [pp. 266-267]

266
JUrirBR's SATBIIThL'S. [SEPTEMBER ~,
staircase which descends through the middle
of the column and terminates in the ground.
A still more striking instance of the value of
lightning-rods is a church on the estate of
Count Orsini, in Carinthia. This building was
placed upon an eminence, and had been so
often struck by lightning that it was deemed
no longer safe to celebrate divine service within its walls. In I~3O, a single stroke of lightning destroyed the entire steeple; after it had
been rebuilt, it was struck, on an average,
four or five times a year, without counting
extraordinary storms, during which it was
struck from five to ten times in a single day.
In l~~8, the building was reconstructed, and
furnished with a conductor; and, according
to Lichtenberg, up to 1~83-that is to say,
during the space of five years-the steeple
had been struck only once, and this stroke
had fallen upon the metallic point without
producing any damage. In short, no doubt
exists in the minds of intelligent and wellinformed men in regard to the efficiency of
well-constructed lightning-rods. Of course,
in this, as in every other department of applied science, we find men who exclaim against
them, and men who think that such appliances
are worthless, unless some particular notions
of their own are embodied in their construction; but, on the other hand, we find that the
great bulk of our scientific men are unanimous
in regard to their efficiency, as well as in respect to the best methods of constructing them;
and it is a curious fact that the rod which
now receives the greatest favor from those
who are most competent to form an opinion
in the matter is substantially the old rod described by Franklin. The country is at the
present moment overrun by so-called lightning-rod men, who palm off worthless and
expensive articles upon their customers, and
in many cases ar~, in addition to this, guilty
of downright swindling. It may be well,
therefore, to say that, by attending to a few
essential points, any ordinary carpenter or
house-builder can easily erect a rod that will
give perfect protection. The points to be
attended to are three: I. The rod should be
of sufficient size-a solid-iron rod, half an
inch square, or a copper rod, one inch wide
and one-tenth of an inch thick, are the sizes
recommended by the best authorities. Thin
copper strip is most easily handled; but a
copper wire, No. 1 or 0, may be more easily
procured, and will answer quite as welL The
light tubular and fancy rods sold by itinerant
venders are almost always deficient in metal.
There is no advantage in tubular, star-shaped,
twisted, or other rods. Harris, the great authority, says: "Provided the quantity of metal be present, the form under which we place
it is evidently of no consequence to its conducting powers" (Harris, "On Thunderstorms," p. lO~). Becquerel, Ponillet, Faraday, Noel, and every electrician of note that
we have consulted, agree with him.
2.In arranging the rod, carry it along the
ridge and along the corners of the gables.
Connect it with all tin roofs, gutters, waterspouts, etc.; and Jo not insulate ii. Points
are of comparatively small consequence, and
it is not best to disfigure a house by a row
of bayonets stuck on it. The crestings and
finials of Mansard roofs are as good lightning~
rod points as can be desired. The rod may
be painted the same color as the house; but
do not allow the paint to destroy the metallic
connection between the rod and the masses
of metal that we have mentioned. The best
mode of attaching the rod to the building is
by small staples for wires, and tacks or nails
for flat strips.
3. Beyond all question, the most importsnt
point in the construction and erection of light
ning-rods is the ground-connection. Careful
and extensive observation has convinced us
that, in regard to this matter, fully two-thirds
of the lightning-rods in existence are defective: and it is here that the itinerant vender
generally manages to cheat his customer most
thoroughly. There is no safety, unless the
rod is carried into the ground to a depth below the level of the wells in the locality.
This is the rule laid down by the commission
appointed by the French Government, and, if
space permitted, it would be easy to show
that it is based on sound principles.
Whether or not the rod should be connected with the gas- and water-pipes, is an
open question. We should say not. The
joints of gas- and water-pipes are generally
formed of insulating substances; and in several well-authenticated cases these joints have
been ruptured by the discharge, and the water allowed to flow out, and the gas set on
fire. In this case, as the least of two evils,
we would violate the rule which directs us to
connect the rod with all metallic substances,
and make no connection with the gas- and
water-pipes.
These are not the vague notions of an
isolated dabbler in science, but the conclusions arrived at by commissioners appointed
by the Governments of France and Britain,
for the special 7;1565]purpose of investigating this
subject. They may, therefore, be received
with the most implicit confidence.
But, while it is easy to protect buildings
and ships, it is not always possible to protect
isolated human beings. All that we can do
in this case is to avoid, as much as possible,
the vicinity of those objects that are likely to
attract the lightning. A few years ago some
wiseacre invented a portable lightning-rod,
consisting of an umbrella, having a metal
stem, to the lower end of which was attached
a chain that was allowed to drag along the
ground. Such a contrivance would be not
only useless, but dangerous, as it would be
utterly impossible to give such a portable rod
a good ground-connection.
The old directions about feather - beds,
glass-windows, etc., are all nonsense. One
of the safest places is a house furnished with
a good rod; one of the most dangerous is a
barn filled with new hay. Another very dangerous place is beneath a tree, and the middle of an open field is nearly as bad. A distance of fifteen to thirty feet from a tall tree
is a tolerably safe position. In a house unfurnished with rods, the most dangerous
places are near the fireplace or chimney, and
those corners down which the water~spouts
descend. The carrying of large metal bars
or rods is, of course, dangerous, because
every mass of metal tends to open up a line
of least resistance of which it will form a
part; and, if the human body should form
part of this line, serious consequences may
ensue. But it is not probable that small articles, like keys, watches, knives, etc., exert
any appreciable influence.
Cases have frequently occurred where
persons in small boats have been struck 1
killed. An instance of this kind occurred
within a few weeks. In such cases, if the
thunder-cloud is very near, it will be advisable to lie down in the boat, even at the risk
of getting thoroughly drenched. Any object
elevated above the surface of the water incurs
great risk of being struck. Every small boat
carrying a mast should be provided with a
lightning-rod. It may be easily and cheaply applied, and will prove a perfect protection.
JOHN PRiNN.
JUPITER'S SATELLITES.
UPITER'S satellites are fast becoming
important members of the solar system,
and recent observations upon their movements seem to confirm the theory that the
great primary around which they revolve
gives out heat and light as the sun does, only
on a vastly smaller scale.
We have alluded, in a former article on
"The Planet Jupiter," to Mr. Browning's
brilliant picture of the coloring of Jupiter's
belts, and the tremendous commotions which
are agitating his surface. Mr. Lassell, the
late president of the Royal Astronomical
Society, and Father Secchi, of the Roman
Observatory, by still more recent observations, have confirmed the former discoveries.
They have also examined, with great care,
the transits of Jupiter's satellites over his
disk. This phenomenon is a delicate test of
the planet's luminosity, compared with that
of other orbs in similar conditions. For, as
the satel]ites cross the disk, it can be easily
seen if they are darker or brighter than the
surface they transmit. The result bears`out
the theory so ingeniously elaborated by Mr.
Proctor, in his recent interesting papers on
tbis subject. The comparative blackness of
the four satellites has been fully established.
Mr. Lassell gives the following account of the
transit of the fourth satellite, witnessed under favorable atmospherical conditions, on
the night of the 30th of last December: "On
its first entrance, it was scarcely to be distinguished from the edge, not appearing at all
as the others do-as a round, bright spot.
As it advanced, it grew gradually manifestly
darker than the surface of the planet; and,
by the time it had advanced a fourth of the
way across it, had become a very dark if not
a black spot; so dark that, if I had looked
at Jupiter without knowing any thing of the
positions of his satellites, I should have said
that a shadow of a satellite was passing. I
remember having seen the like phenomenon
many years ago; but my impression is, that I
had never seen the disk of the satellite so near
to absolute blackness before. Of course, it is
only by contrast that it can possibly so appear; and we have in this fact a striking
proof of the exceeding brilliancy of the surface of the planet. In the same way the solar
spots, if not surrounded by the marvellous