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the geologist. Let us now enter into the consideration of some of the elementary substances, which are of the most importance as ingredients of the crust of the earth.

At the head of this list stands oxygen, which combines with so many substances, and enters so largely into the composition of many minerals, and those the most common ingredients of rocks, that it may be said to form half of the ponderable matter of which the exterior parts of the globe consist. It constitutes 8 parts out of 16 of silica, 8 parts out of 18 of alumina, 8 parts out of 28 of lime, 8 parts out of 20 of magnesia, 8 parts out of 48 of potash, and 8 parts out of 32 of soda. It is an essential element of many acids. United with carbon, in the proportion of 16 to 6, it forms carbonic acid, which constitutes nearly half of carbonate of lime, which has been estimated as one-eighth of the crust of the globe.

Oxygen forms 8 parts out of 9 of water, and enters extensively into the composition of many of the ores of manganese, tin, lead, iron, and copper ; but it is never met with except in combination with some other substance.

A large portion of the atmosphere consists of oxygen, deprived of which, it loses its power of supporting animal life. Oxygen is not inflammable, but a supporter of combustion. Every substance that will burn in atmospheric air, burns with far greater brilliancy in oxygen gas. Even iron and steel undergo rapid combustion in it. Combustion is a combination of oxygen with the burning body, and the products of combustion in oxygen gas, or atmospheric air, are oxides.

Oxygen, in its elementary state, is an invisible, permanently-elastic gas, without taste or smell, a feeble refractor of light, and a non-conductor of electricity. When the compounds of oxygen are submitted to the action of a galvanic battery, the oxygen is always determined to the positive pole, and is therefore said to be negatively electric. Its combining proportion has already been stated to be 8.

HYDROGEN. HYDROGEN is the lightest of all known material substances, and is taken as the standard of comparison for the specific gravity of the gases, as water is for that of solids and liquids. It is by no means an important element in the composition of rocks. It is, like oxygen, an invisible and permanently-elastic gas. It is highly inflammable ; combined with sulphur it forms sulphuretted hydrogen, which, as well as hydrogen itself, is emitted from the earth in volcanic regions. Sulphuretted hydrogen, - when absorbed by water, communicates to it the peculiar properties of the Harrowgate and other sulphureous springs. That species of limestone called swinestone, is supposed to derive its fetid odour when rubbed, from the presence of sulphuretted hydrogen. Combined with carbon, hydrogen forms carburetted hydrogen, which issues in large quantities from between beds of coal, and produces those fatal explosions known to miners by the name of fire-damp. Hydrogen constitutes 1 part in 9 of water. Water is contained in most minerals as an accidental ingredient, either inclosed in cavities, as in rock crystal, calcedony, and flint, or absorbed by most earthy and porous minerals ; but it is also chemically combined with many minerals, both crystalline and massive, as an essential ingredient. These combinations with water are called hydrates; and

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when water forms an essential ingredient of a crystallized mineral, it is called its water of crystallization. Ilydrogen is a positive electric with respect to oxygen, chlorine, and that class of bodies, but negative with respect to the metals. It is a powerful refractor of light. It soon causes death to an animal that breathes it. Its combining proportion is 1.

NITROGEN. NITROGEN is likewise a permanently-elastic, invisible gas, without taste or smell. It is not combustible, but extinguishes burning bodies that are plunged in it. It is incapable, when respired, of supporting animal life; and yet, diluted with oxygen, in the proportion of 8 parts of oxygen to 28 of nitrogen, it forms atmospheric air, so essential to life. Air is not a chemical combination of these gases, but a mechanical mixture of them. A chemical combination of oxygen and nitrogen, in proportions which we have before stated, constitutes nitric acid, one of the most violent and poisonous of that class of bodies. Nitrogen combined with hydrogen forms ammonia, a volatile substance, which possesses all the properties of an alkali. The sulphate and muriate of ammonia are exhaled from volcanoes. Nitrogen, in the state of nitric acid, enters into the composition of nitrate of potash, which is an abundant production of the earth in various parts of France, Germany, Italy, Spain, Hungary, Persia, India, and America.

CHLORINE. CHLORINE is a yellowish-green vapour, of an astringent taste, and disagreeable odour. It is perfectly irrespirable, exciting spasms and irritation of the throat, even when much diluted with atmospheric air. It may be condensed by cold and pressure into a yellow liquid. It has a violent action on some of the metals, which, when introduced, in the state of powder, or in fine leaves, into chlorine, are inflamed and enter into combination with it. It is known in the mineral kingdom as one of the constituents of chloride of sodium, or rock-salt. We have likewise ores of lead, mercury, and silver, which are chlorides of those metals ; but it never occurs uncombined. With hydrogen it formis muriatic acid, which is one of the components of the salts called muriates. Muriate of soda, which by evaporation is converted into chloride of sodium, is the principal ingredient in sea water, which likewise contains muriate of magnesia. These two salts are also found in many saline springs. Muriate of ammonia has already been spoken of as a volcanic product. Chlorine is a negative electric; its combining proportion is 36. Its name is derived from its greenish colour (xopos).

FLUORINE. The substance to which this name has been given has never been obtained in an insulated form ; and there is a difference of opinion among chemists, whether Derbyshire spar should be considered as a compound of calcium and fluorine, or of oxide of calcium and an acid of which fluorine is one of the elements. To this acid the name of fluoric was given, and it was supposed to consist of fluorine and oxygen, but the experiment of Sir Humphry Davy, since repeated by other chemists, affords strong presumption that fluor spar is a direct combination of fluorine and calcium, and that the acid produced when these two elements are separated by means of aqueous sulphuric acid, consists of fluorine united to hydrogen, derived from the water of the sulphuric acid. It has therefore received the name of hydrofluoric acid. It possesses the property of dissolving silica, and consequently of corroding glass. Fluor spar is an abundant mineral production. The hydrofuoric acid is also found in the cryolite, and few other rare minerals. Fluorine is a negative electric, Its combining proportion is 18.

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REPORT OF MAGNETICAL EXPERIMENTS,

TRIED ON BOARD AN IRON STEAM VESSEL, BY ORDER OF THE RIGHT HONOUR

ABLE THE LORDS COMMISSIONERS OF THE ADMIRALTY. BY EDWARD J.

JOHNSON, ESQ., COMMANDER, R.N., F.R.S. The very extensive use of iron in the construction of modern vessels, and still more recently, the formation of steam vessels entirely of that material, has rendered the compass, notwithstanding its successive improvements, little more than a piece of useless lumber: or, more properly speaking; it is become unworthy of confidence, and, consequently, where it is trusted, a deceptive and frivolous apparatus. Indeed, the compass in its rudest forni, even the Chinese, or the early European, with ships built as ships were then built, was worthy of far greater confidence than the most improved compass is on board a vessel of modern construction. There is no doubt, indeed, that were the weather always clear, the compass might be advantageously dispensed with in maritime affairs; but, as during not only days but weeks of the most tempestuous weather, when not a single lunar can be taken, nor any kind of complete observation made, the vessel is driving about amongst known or unknown dangers, it cannot but be viewed as a most perilous condition, when the direction itself upon which she is sailing, is a matter of almost total uncertainty. If, indeed, she be out at sea, and far from land, she is safe, provided her strength be sufficient to ride out the storm : but every year, we are well assured that numerous cases of the heart-rending scenes of wreck and desolation would be escaped, had they the power to ascertain the course upon which they were bearing. This, however, is utterly

. impossible as ships are now built, by the use of the compass simply.

Mr. Barlow's correcting-plate is, on this account, one of the greatest boons conferred on modern navigation. This term, as most of our readers are aware, is something of a misnomer; since the plate, instead of correcting the error produced by the iron of the ship, doubles it: but we would not quarrel with names—as it is with things we have to deal. It enables us, by a very simple numerical process, to ascertain approximately the effect of the iron of the ship upon the direction of the needle, and to make allowance for it in our reckoning.

It is strange, however, to witness the apathy with which the reckless seaman, in time of security, looks upon the possible danger of a future, and not remote, period. A single hour in port would enable the master to find the effect of his iron with considerable precision : and yet this single hour he thinks it too much to give to his own and his crew's

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future safety. Strange infatuation !but infatuation almost always follows close upon the heels of familiarity with danger.

We do not require to be told that Barlow's plate is not perfect. This we are as fully aware of as any one: and we do not urge implicit reliance upon it, in any sense of the word, under all possible circumstances. Still, if it enable us under all conditions to ascertain the amount of the effect of iron approximately, and often within very narrow limits, surely we must be determined upon rushing into danger, if we do not avail ourselves of it to the degree in which it can assist us. We are utterly opposed to the use of the compass at all, in those cases where it can be dispensed with : but as cases so perpetually occur where it is our only guide, and those cases precisely those of the greatest danger, it is surely worthy of the most serious attention, from every practical navigator, as well as from men of science*.

At the period when Mr. Barlow proposed his plan of the correctingplate, he had in especial view the effect of the immense masses of iron which the guns on board men of war contained. Of course, cæteris paribus, the same circumstances would occur on board the smaller vessels in the merchant service, and require correction accordingly. The recent introduction of iron steamers has, however, given a new and important interest to this contrivance. These are chiefly designed for passengers, and, in some cases, more than five hundred are crowded on board a single steamer. We do not indeed, just now, know to what extent the iron steamers have been introduced: but as they have many advantages, in respect to security and convenience, over those of wood, they will most likely supersede them entirely, provided they can be rendered as safe for the purposes of navigation by the compass,-circumstances giving rise which must inevitably occur in all voyages

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considerable extent. The inquiry into this possibility, it was the main object of Captain Johnson's experiments to satisfactorily answer; and we proceed to give a brief analysis of them.

The Dublin Steam Navigation Company placed at the disposal of the Lords Commissioners of the Admiralty a fine new vessel, built en: tirely of iron, the Garryowen, for the purpose of investigating the effect of the vessel upon the indications of the compass, in any way that their Lordships may think propert. They appointed Captain Johnson to make the requisite experiments; and he repaired in her to the port of Limerick,

* In an early number we shall give a | All iron used in hull and bailers is thorough examination of the principles of malleable. Barlow's correcting-plate; and endeavour DIMENSIONS OF VESSEL, &c. to show the degree of theoretical evidence, combined with experiment, this method Length on deck, 130 feet; beam 21 ft. 6 in.

Do. has for its foundation,

keel, 122 3 ; depth llît. O in. + WEIGHT OF IRON.

38 double frames a-midships, of angle iron

2 in. wide x 3 deep x in Total weight of iron, including hull, machinery, anchors, cables &c.

3 x 3 x Š 180 tons.

17 single frames forward, of
22

do. aft, Weight of iron in hull

95
Diameter of chimney

3 feet. Do. engines Do.

ditto shafts and wheels

Height Do. boilers

Draught of water, forward 5 ft. 3in, aft 5ft. Do. chimney

1 16

Two ENGINES =85 HORSE POWER, Do.

anchors and cables 1 10 Stem, 14 feet long x 4 feet wide.

Diameter of cylinder

3 ft. O in, Beam, 4 in. dleep, 4 in wide, bound

Diameter of wheel

15 ft. 6 in. with iron plate3,

Engine makes 27 strokes per minute,

3X3 Xia

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28

40 12 30

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in the autumn of last year, to carry them into execution. The results have been printed for the Royal Society's Transactions ; but they are not yet published. We avail ourselves, however, of a copy of the Memorial, with which we have been favoured, to lay before our readers a succinct account of them, and a few reflections on the results they bring to light.

There being no wet dock in the port of Limerick in which the Garryowen could be conveniently swung round, to make the observations in different azimuths, point by point, he fixed upon a position in Tarbert Bay, well adapted to his purpose. His operations were commenced on the 19th of October, and continued, as circumstances permitted, till the 18th of November.

In order, however, to show the positions in the vessel at which the several observations were made, the following diagram is given. The line y z is the keel, y being the stern, and z the stem ; v is the place of the chimney, and w the axle of the paddle-wheel. The positions of the other points will be easily judged of, from the accompanying scale and table: and these designate the positions of the compasses named by the several letters themselves.

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A Quarter deck, 5 feet 9 inches above the deck.
B Forecastle, 5 feet 11 inches above the deck.
C Bowsprit, on glass-legs.
D On the fore-part of the temporary poop, above deck 8 feet 54 inches.
E On the after-part of the temporary poop, 8 feet 5 inches from the deck.
F On a stage level with the taffrail.
G On a plank 4 feet below the main-gaff end, and above deck, 20 feet 5 inches.
I On the centre of the temporary poop, above deck 13 feet 4 inches.
L On the poop projecting over the stern.
MMM Three stations on the stage over the stern, level with the taffrail.
O Between the paddle-boxes.
P Two-thirds up the fore-topmast, above the deck 40 feet 2 inches.
Q (On glass legs) in the fore-hold.
S In the iron sphere a-midships, above deck 7 feet.
T In the cabin.
All made in the middle of the vessel, 9 feet 11 inches from each side.

After the preliminary operations of fixing stations, &c. were gone through, Captain Johnson saw it to be necessary to put all the iron which the vessel carried, in the places which it usually occupied during the

voyage, such as the anchors, cables, &c. He then tried the effect of the whole in that direction of the vessel where, in the generality of cases, the deviation had been found to be a maximum, so as in some degree to guide him in the selection of a place for the principal observations,— or that, which his orders especially directed, in which to place a steering compass, and where the effect of the plate may be successfully tried. Vol. II.

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