MAGAZINE OF SCIENCE AND ART. 
2G6 
ine the effects produced on their molecules by the action 
of" light. The molecular changes produced by elec¬ 
tricity are still more considerable though better known. 
“If letters are cut out of a sheet of paper placed 
between two clear surfaces of glass, and the whole 
turned into a Leyden jar by tinfoil on both sides and 
electrified, an imago of the letters is produced on the 
glass and rendered visible by breathing on it. If the 
glass be covered with a film of iodized collodion, treated 
as for a photograph, and exposed to uniform daylight, 
the invisible image is developed on the collodion, its it 
the paper letters had been originally applied to it. 
Here the invisible molecular change produced by elec¬ 
tricity in the glass was evidently extended to the colo- 
dion,* producing there a chemical change like that 
produced by light. 
“Similar results were observed by Mr. Babbage in 
the frame of an old looking-glass, where a pattern in 
gold leaf and plaster of Paris had impressed itself on 
the glass in a manner rendered visible by breathing on 
it while in the electric frame. The object of sll these 
illustrations was to show that agents, such as light and 
electricity, produce changes in the molecular consti¬ 
tution of bodies.” 
To the Editor of the Sydney Magazine of Science 
and Art. 
London, March 10, 1858. 
Per Tamar, R.M.S.S. 
Sin,—The great scientific event of the month is, of 
course, the launching of the monster ship, the Great 
Leviathan , Which is now securely moored on the oppo¬ 
site side of the river to that on which she was launched. 
Her appearance in the water is very remarkable Her 
height is not so very striking, perhaps, even now that she 
is empty, not much more than that of liO gun line of 
battle ship, but the length has a most extraordinary effect. 
Looked at from some distance off on the side, the notion of 
a ship is completely lost—she is a quay—a breakwater 
—a battery—a cliff—anything but a ship. Some notion 
of this may be formed, from the fact, that the space 
which her side occupies to the eye, when seeu in 
this way, is very nearly an acre; the cut of the prow 
is, to a landsman s eye, remarkably sharp ; she appears to 
taper for a third of her length, up to the perpendicular 
razor edged cutwater. 
The internal fittings arq progressing slowly enough ; it 
is said in consequence of want of funds, and hence. Lire 
directors are very particular about allowing visitors on 
board. The saloons and state-rooms are, however, mostly 
partitioned off, sufficiently to give an idea of their size. 
The first class saloon is, according to the new fashion, 
in the fore part of the ship; it is 110 feet long by 40 I 
broad and 15 high. This unusual height allows of two | 
stories of state rooms along the sides; the lower are devoted, j 
principally, to passengers* luggage, store rooms, Ac.; the 
upper are convenient little state-rooms, about 12 feet by 
6. with bath rooms and every convenience. Above this 
saloon and of the same size will be the first-class drawing¬ 
rooms. 
The amount of possible accommodation seems almost 
boundless, besides the second class saloon and cabins, 
which, though not quite so large, are sufficiently so 
aHd extremely comfortable Captain Harrison, who was 
on board, was talking of building on the main deck, 
(which is now quite flush, no quarter-deck at all a poop 
one hundred feet long, with state-rooms along the 
sides. The cost would be £ JO 000, but it was calculated 
to pay its cost in three voyages, if passengers could be 
got to fill it The total number of passengers which he 
undertakes to carry, with ease, is 4,500, or 10 000 troops, 
giving them one-fifth more accommodation than any troop 
ship now in use. To rule such a floating commonwealth 
demands a good head and strong will indeed, and there is 
no reason to suppose Captain Harrison wanting in either; 
a man evidently of great natural ability and determina¬ 
tion ; he has had many years experience as commander of 
Cunard steamers, and is universally allowed to stand in the 
first rank of his profession. 
You will get, from many other sources, more technical 
details in the construction of the ship and its engines. 
In the paddle-engine there is one no*el contrivance, 
which deserves a moment’s notice. Instead of the 
ordinary method of disconnecting the paddle-wheels and 
engines, an ingenious contrivance is used, called Dixon’s 
Friction* Clutch." The principle of which is. that connec¬ 
tion is made by means of a solid iron wheel being grasped 
by an iron outside ring, tightened by a strap, this can be 
instantaneously loosened and the paddles disconnected. 
Now that the launch is actually accomplished, it is 
interesting to remember how the predictions and theories 
of both professional men and bunglers have been falsified. 
After the first unsuccessful attempt the received explana¬ 
tion was, that the iron of the rails and cradle had under¬ 
gone a sort of welding, or interlacing of the fibres, 
through the enormous pressure, and that this would be 
sure to get worse so as to make the launch quite impos¬ 
sible ; others declared that the wags had settled, and the 
ship was lying in a trough parallel to the water, so that 
the ship had to be pushed up hill. All examination of 
the rails shows the hastiness of ail these conclusions, they 
are as straight as when first laid down ; the settlement 
I though the pressure lasted so much longer than expected) 
is inappreciable, and the surface shows nothing of the 
« welding” or interlacing so much talked of. Whether it 
really was a mistake to build it so far from the water, or 
to launch it on iron instead of wood, is hard for an unin¬ 
itiated to say; but we know that Mr. Brunei was quite 
supported by Mr. Ft. Stephenson, in fearing the responsi¬ 
bility of launching it on wood in the ordinary manner ; 
and when two such authorities agree what shall we (who 
are not shareholders) do, but take off our hats and bow to 
their united Judgment, 
The last number of the Chemical Society's Journal 
contains some interesting researches by Dr. Roscoe, on 
the ventilation and sanitary condition of dwellinghouses, 
undertaken at the instance of the commission of inquiry 
into the condition of soldiers’ barracks, which has just 
made such astounding disclosures. The results he has 
arrived at concerning the amoutit of air required per 
minute and the amount of impurity which is hurtful, 
do not differ essentially from those of previous observers. 
The interesting result is that the amount of ventilation 
afforded by the permeability of brick wails is very consi¬ 
derable. Dr. Roscoe’s experiments were made with an 
air-tight box, one side of which was formed by a brick. 
It was found that icet bricks are little, if at all, permeable, 
which explains the unhealthiness of new or damp houses. 
To many of your readers it may be important to consider 
that iron houses are quite destitute of this sort of venti¬ 
lation, 3rid hence can hardly be so healthy as brick ones. 
Another interesting result is that there is little or no 
difference between the upper and lower parts of the atmo¬ 
sphere of a room, except to some extent in temperature, 
that is. that hot air mostly collects towards the top, but not 
impure air. 
The well known Bunsen’s voltaic battery has lately 
received a useful modification by M. Grenet. a young 
French savan. The disadvantage in the use of this 
battery has been the gradual decrease of intensity observ¬ 
able as the action continued. This was shown by Poggen- 
dorf to be owing to deposition of oxide of chromium by 
the deoxidation of the bichromaof potassa used as the 
exciting liquid. This objection M. Grenet ingeniously 
removes by making a current of air pass through the 
liquid, the mechanical disturbance prevents the deposition 
of the oxide, while the oxygen of the air seems to reoxidise 
to some extent that which is already formed, reproducing 
bichromate of potassa. 
THE STEAM PLOUGH. 
To the Editor of the Sydney Magazine of Science and 
Art. 
Sjr— l presume th^t the columns of your valuable 
periodical are at the serviee of those who may have 
any information to contribute or fact to relate, that pro¬ 
mise to be interesting to the scientific world or your 
general readers I, therefore, avail myself of the oppor¬ 
tunity its pages afford, and venture to hope, you will give 
insertion to the following communication. 
