The first four or five days over hard clay, and very barren ground; for 11 days over sandy hills and valleys; then for two months over more hard ground, with some hills, but not high ones, crossing some running waters of brackish quality, and passing a salt-mine and a brimstone-mine; then, after going for two days through a wood, came again to sand. Saw no town, but met occasionally with other parties of Arabs. This sandy district took about a month to pass, and it was terminated by arriving, without any previous distant view, at a large sea or lake. The day was very clear; and mountains were discernible on the opposite shore, but nearly resembling clouds. On the bank of this sea they came to an assemblage of huts called El Sharrag, belonging to the tribe of Orgaeleet. Here all the camels, goats, and sheep, with two persons of each family, were left. The remainder of the party were taken across the sea in a large boat built of a hard red wood, and without any iron in its construction, rowed by six blacks who were slaves, setting out at sun-rise, going the whole of the day; a little before sun-set the anchor, a large stone, was let down, and the boat remained stationary until sun-rise next morning, when they again set off, and proceeded till sun-set on the second day, when they anchored as before. Soon after day-break on the third day went on again, and about two o'clock in the afternoon arrived at the shore, having gone straight across, making for the mountains, at the foot of which they landed, in a district called El Hêzh. The water of this sea was smooth, had a deal of weed upon its surface, was clear under the weed, perfectly fresh, and had no appearance whatever of a current. Scott did not particularly look for a termination of this lake at the higher or lower end as he crossed, but is perfectly satisfied he did not see either end of it. Its length is from N.E. to S.W., and the boatmen called it the Bahar El Tee-eb. They also told him that to the southward there was a large salt sea, which they called the Bahar El Kabee-er, to which there was no end, and that the one they were upon run into it, where there were many Saffina el Ka-bee-er, or large ships, and a harbour called Bambarry. On the eastern shore of the Bahar Tee-eb, where the party landed, was a number of huts, built of wood, and covered with rushes. The name of the country was El Hêzh; of the particular place, El Tarsee Mahomet; and of the tribe inhabiting it, El Tarsee del Hêzh. Here were many pilgrims, accompanied by some of whom the party with whom Scott was set out at sunrise the morning after their arrival to go into the country, taking Scott with them, saying, they were going to Hez el Hêzh to Seedna Mahomet, where Scott must change his religion, or if he did not Mahomet would rise up and kill him. They proceeded through a crooked mountain route till about three o'clock, in about a south direction, when they arrived at a valley,

in which stood a building about the size and shape of a common barn, the ends being placed to the north and south: at the south end was a door: but there were not any windows in the building, nor any chimney. This place Scott was told was the grave of Seedna Mahomet, meaning of a great man connected with or related to Mahomet, whose name amongst the Arabs is Ur-soell. Here the party prayed, kissed a stone near the door, and told Scott he must now become a Mahometan, or be killed. He, however, steadily refused, and was not again urged, but was not taken any more to the grave, though the party frequently visited it during their stay at El Hêzh, which was about a month, when they left it in the same boat which took them there, and had also during their stay carried many other parties for the purpose of visiting the grave. On their return there was more wind: a mast was therefore constructed of two oars placed across each other, and a blanket was used for a sail; so that, leaving the shore about the middle of the day, and sailing all night, they arrived at El Sharrag about six o'clock the next morning. The whole party then set out on their return, traversed the sandy district for about a month; then came to the wood, where they had a conflict with some black people, called Bambarras, who were quite naked, tattooed, and armed with bows and arrows. Leaving the wood, they travelled for about a month and a half over hard ground; came to a valley where there was some vegetation; stayed there about six moons; then went three days' journey to another place, where they remained two or three months; then set off again for El Gibbla, and for a week or two went over hard ground; then came to sandy valleys, which took them a little more than a week to cross; and in about a week more they arrived at El Gibbla. They avoided going too far to the northward, being afraid of the Moors. He was detained with the tribe wandering about until the end of July, or beginning of August, 1816, when he escaped and got to the neighbourhood of Wadnoon, from whence he communicated with Mr. Wiltshire, the British Consul at Mogadore who sent for him: he arrived at that place on Aug. 31, left it Nov. 11, and got to London Dec. 9, 1816."

Upon the narrative, we may remark that the number of days which Scott and his party occupied in their journey, was more than 100, which at the rate of only 10 miles a day would make a distance of 1000 miles. In rowing across the lake, they spent about 30 hours, which at the rate of four miles per hour would give a breadth of 120 miles. It may be conjected that the route which they took was somewhat to the northward of the Niger, and probably to the N.W. of the supposed situation of Tombuctoo.

ARTICLE XI.

Remarks on Mr. Daniell's Theory respecting the Specific Gravity of Crystals. In a Letter to the Editor of the Annals of Philosophy. By Pat. Addle, Esq.

SIR,

I AM one of those who have always derived considerable pleasure from philosophical inquiries; but living in a very retired situation, and having little communication with philosophers, I have no means of becoming acquainted with the progress of scientific discovery except through the medium of the public journals. In Number VII. of the Journal of the Royal Institution, published in October last, I have been surprised and delighted with a very extraordinary, a very ingenious, and, as far as I know, a very novel theory of Mr. Daniell on the specific gravity of crystals. The character of this journal, published under the auspices of so learned a body, and conducted by Professors as eminent for their caution as for their talents, is a sufficient gua rantee that nothing trivial or absurd will ever be found in its pages, and I entered upon the examination of the theory with the confidence that such authority must naturally inspire.

The formation of crystals from spherical atoms, as detailed by Dr. Wollaston in the Phil. Trans. had previously attracted my attention; and though I have lived long enough to know that the convulsion of a frog's leg led to the decomposition of the alkalies, I was not prepared to expect, that the theory of Dr. Wollaston (pretty and amusing I admit) should have been but a link in the great chain of discovery, and should have furnished Mr. Daniell with the foundation of a doctrine that bids fair to overturn most of the received opinions in natural philosophy.

The theory of Mr. Daniell possesses the characteristic of all great discoveries-simplicity, and cannot be more concisely or more beautifully stated than in his own words. After referring to figures representing a tetrahedral and octohedral pile of balls, both composed of triangular faces, the bases of which are constituted of four particles, he says, "The tetrahedron is contained by four of these similar and equal planes, and the octobedron by eight, so that the whole superficies of the latter is exactly double that of the former. Now it is obvious that solids so constructed must differ in their specific gravities, unless the number of elementary particles in the octohedron be exactly double the number in the tetrahedron; that is to say, unless the number of atoms in a given space be equal in both arrangements. But it will be found that the tetrahedon is composed of 20 spheres, and the octobedron of 44, the latter containing more than double the number of particles under a double surface. Tha

specific gravity of the latter solid must, therefore, be greater than that of the former."

Here is a mathematical demonstration that requires only the Q. E. D. to give it the air and stile of the early geometricians; it is logical, and, I will venture to say, unanswerable. Adopting the old formula, it may be stated thus: the specific gravities of bodies vary directly as the weights, and inversely as the surfaces. There are many familiar facts (not the less striking on that account) that illustrate and confirm this principle. Gold in its more solid form is one of the heaviest metals; let it be beaten into gold leaf, i. e. increase its surface, and it will swim upon water; nay, upon ether. A dry sponge (which, including all its pores, possesses, perhaps, the greatest quantity of surface of any known body of the same size) will swim upon our lightest fluids. Saturate it with water; i. e. destroy as it were the surface of its pores, and it immediately sinks. Mr. Daniell, however, has adopted a more scientific mode of bringing his theory to the ordeal of experiment. He says, a mass of fluor spar will divide either into octohedrons or into tetrahedrons: "the question, therefore, seems to resolve itself into this: Is the specific gravity of a mass of fluor, split into the form of an octohedron, greater than the specific gravity of the same mass split into the form of a tetrahedron ?" To determine this, he proceeded to try the experiment "with all the care and attention which the delicacy of the investigation required;" and the specific gravity of the octohedron he found to be 3.037, while that of the tetrahedron was 2.909-he adds, "the result of this experiment, therefore, was perfectly satisfactory." Most assuredly it was, but perfectly unnecessary, at least for my convinction; for when once I have satisfied myself of the truth of an hypothesis by mathematical demonstration, I never suffer my confidence to be shaken by facts.

:

There are many men who would cavil at this result, and observe that, according to the theory, the specific gravity of the tetrahedron ought to have been 2-760, for 20 :: 3037: 2·760.An error of two units in the first place of decimals must never be allowed to overturn a theory founded upon a priori mathematical deduction. And after all, notwithstanding his precaution, the difference may have arisen from inaccurate manipulation. I, therefore, determined (for proselytes are generally enthusiasts), by experiments more decisive, and conducted, if possible, with still greater delicacy than those of Mr. Daniell, to place our theory (if he will allow me the use of the plural possessive) beyond the reach of controversy. Unfortunately, Sir, not being a mineralogist, I had no fluor spar to recur to, excepting, indeed, two vases, and an obelisk of Blue John, which graced our drawing-room chimney-piece; and, independently of the difficulty of detaching from them either an octohedral or a tetrahedral portion, it would have been unfeeling to propose such a sacrifice to

my wife, who, good soul, has no enthusiasm in these matters. It occurred to me, however, that if true of fluor spar, it must be equally so of every body in nature, fluid as well as solid. If from the same mass of fluor spar I obtain a different specific gravity according as I detach an octohedral or tetrahedral crystal, it follows, that by confining the same fluid in a vessel of an octohedral or a tetrahedral form, I shall so alter the surfaces of the mass of fluid compared with the number of elementary atoms, that its specific gravity must vary with every modification in the form of the vessel which contains it..

By the help of a rough model, an intelligent tinman in the neighbouring market-town, constructed for me an octohedral and a tetrahedral vessel, consisting of equal triangular faces, one of which was left open for the purpose of pouring in the fluid. The octohedron held two gallons, for it would have been absurd to risk the success of the experiment by conducting it on a small scale; and, to say the truth, I abhor (what it has been the fashion of late to call) microscopic chemistry. Besides, the beer can in my servants' hall held exactly two gallons, and to the eye was cylindrical. A large china punch-bowl, in which all my children had been christened, though not perfectly a hemisphere, was so near an approximation that I had no apprehension of error on that score, and a large kitchen funnel furnished me with a cone. The octohedron was laid upon one of its triangular faces, with the open side uppermost, and the tetrahedron was supported upon its apex in a large mass of clay, so that the plane of the open side should be perfectly horizontal. Thus prepared, I had the vessels carefully filled with water, all taken from the same pump, and successively plunged a beautiful hydrometer of Nicholson's into the octohedron, tetrahedron, funnel, beer-can, and punch-bowl. Now, Sir, I leave you to judge of my mortification on finding the instrument indicate identically the same specific gravity in all.

I varied the experiments by using distilled, instead of pump water; I tried Atkins's and Sykes's hydrometer, and the areometer of Beaumé, repeatedly, and always with the same result. After recovering from the temporary shock this disappointment occasioned, I soon, upon reflection, discovered that the pressure of the fluids upon the sides of the vessels being exactly equivalent to the pressure of the sides upon the fluid, I had no chance of ascertaining the difference of their specific gravities, unless they could be freely retained in their forms by their own specific attractions, and any attempt to obtain them in this state being hopeless, I abandoned these experiments as anomalies that would be explained when I had time to calculate the necessary allowance for pressure..

Not long after this, on my return one moonlight night from dining with a neighbour after I had been thinking of a mode of