stone, and at a point a little north there appear to be solid masses of compact rock, which look like currents of lava. From Sandy Point to Deep Bay, the rocks which occur are those mixed with cinders of a black colour, and full of glassy or transparent crystals.

St. Eustatia is formed of two hills that appear to have been both craters of volcanoes; the western one is more ancient, and is filled up with earth, &c.; the eastern one is higher, and appears to be more recent, the crater being only partially filled. The space between these two hills is filled with cinders, forming a plain with a bay on each side; the one to the leeward is the harbour, on the edge of which stands the town.

sea.

On the southeast side of the large hill, towards St. Christopher, there is a stratification of madrepore limestone, alternating with beds of shells, similar to those found at present in the The whole of this marine deposition dips to the southwest at an angle of upwards of 45 degrees from the horizon, resting upon a bed of cinders, full of pumice and other volcanic rocks, and is immediately covered by a bed of madrepore, sand, and cinders, mixed together with blocks of volcanic rocks, so disseminated that there can be no doubt of the volcanic origin of the substances above and below the madrepore rock, which may be from five to six hundred yards thick. Part of this madrepore rock is changed into silex, having the part that surrounded the animal converted into chalcedony. A considerable quantity of gypsum is found near the same place, in a crystalline state.

Saba. This little island seems to finish the volcanic formation, and consists of one mountain, rather rougher and more rugged than St. Eustatia, but apparently of nearly the same kind of rocks.

The foregoing description of the volcanic islands may per-. haps authorize the following general remarks.

1st. That there is a great similarity in the substances ejected, which are marked by a family feature running through all the rocks, cinders, &c. of the different islands; and it is to be observed that the proportion of cinders, pumice, and other

light substances, is much greater than of the solid lavas, which are but thinly scattered; also that the cinders are always the lowest stratum on a level with the sea; and the masses of solid lava, near that level, repose on a bed of cinders, in every place where I had access to them.

2d. The madrepore and coral rocks, mixed with shells, partly similar to those found at present in the sea, are found in many places alternating with the cinders, and other volcanic rocks, presenting much the appearance of the whole having been ejected from the bottom of the ocean.

3d. The direction of the islands, running from north to south, a little easterly, corresponds with the direction of the strata of those stratified islands, lying to the eastward: such as Barbadoes, St. Bartholomew, &c. which should seem to support the supposition, that the seat of combustion occupies a stratified substance, running parallel to the general stratification of the surrounding rocks.

4th. In all the islands there are one or more soufrieres, all of which form alum rocks, and deposit sulphur, proving that sulphur is one of the ingredients that support the combustion; and perhaps giving strength to the supposition, that whatever may have been the original cause of the combustion, that cause is uniform, and the same through all the islands.

5th. In the late eruption of cinders, there was a great quantity of stones thrown out, exhibiting no appearance of . having ever been in a state of fusion, but only roasted by a considerable heat; most of these rocks have every appearance of belonging to the primitive class, by their crystalline structure, and the position of their component parts: from which it would appear reasonable that the following conjectures may be hazarded.

1st. That the islands were probably thrown up from the bottom of the ocean.

2d. That the seat of combustion is more probably in a substance stratified, and that sulphur is one of the combustible ingredients.

3d. That the substance so stratified is most probably

primitive, and that consequently the combustion, is in the primitive region covered by the transition, which forms the islands of the eastern group.

ART. XVI. On the original Formation of the Arabic Digits.

ADMITTING the probability of the conjectures of the author

of the third article in No. II, respecting the original formation of the Arabic digits, 1, 2, 3, 4, 5, 6, 8, 0, founded on the data there premised, may we not suppose it more likely that the Arabians would have formed the remaining digits, from different compounds of the inferior numbers, rather than that they would have gone aside (according to the hypothesis of the author of Art. 18, No. III.) and have had recourse to the Greeks for the formation of 7, and 9, or have substituted the Greek figures for their own, on account of the supposed awkwardness of their composition? For as the conjectures of the author of Art. 3, respecting the formation of 2, 3, 4, 5, 6, and 8, cannot but be admitted as very ingenious, and the remaining two digits may be formed after a similar manner, is it not much more reasonable to assume, that they were all formed after the same method? otherwise we are left to institute the inquiry, on what sufficient grounds the Arabians should have borrowed from the Greeks the formation of those two digits, when they could have formed them themselves with as much ease as they had formed any of the others.

Having but lately become acquainted with the Journal of Science, &c. after the perusal of Art. 3, No. II, and Art. 18, No. III, it occurred to me, that the following would be the original formation of the digits in question, 7, and 9, following the plan proposed in Art. 3.

7, is first a perpendicular 1, which with a straight line making a right angle with this perpendicular, becomes and with the addition of a line parallel to the first line is

and this hurried and rounded in writing, is at length 7; it being much easier to write a line inclined, than a perpendicular line 1.

9, is first a square, which, by producing the side on the right hand, becomes and this finally, when rounded, be

comes 9.

May 15, 1818

ART. XVII. History of Dr. Brewster's Kaleidoscope, with Remarks on its supposed resemblance to other combinations of plain Mirrors.

As

S this instrument has excited great attention, both in this country and on the Continent, we have no doubt that our readers will take some interest in the history of the invention. In the year 1814, when Dr. Brewster was engaged in experiments on the polarization of light by successive reflections between plates of glass, which were published in the Philosophical Transactions for 1815, and honoured by the Royal Society of London with the Copley Medal, the reflectors were in some cases inclined to each other, and he had occasion to remark the circular arrangement of the images of a candle round a centre, or the multiplication of the sectors formed by the extremities of the glass plates. In repeating, at a subsequent period, the experiments of M. Biot on the action of fluids upon light, Dr. B. placed the fluids in a trough formed by two plates of glass cemented together at an angle. The eye being necessarily placed at one end, some of the cement which had been pressed through between the plates, appeared to be arranged into a regular figure. The symmetry of this figure being very remarkable, Dr. B. set himself to investigate the cause of the phenomenon, and in doing this he discovered the leading principles of the kaleidoscope. He found that in order to produce perfectly beautiful and symmetrical forms, three conditions were necessary.

1. That the reflectors should be placed at an angle which was an even or an odd aliquot part of a circle, when the object was regular or the even aliquot part of a circle when the object was irregular.

2. That out of an infinite number of positions for the object both within and without the reflectors, there was only one position where perfect symmetry could be obtained, namely, by placing the object in contact with the ends of the reflectors.

3. That out of an infinite number of positions of the eye, there was only one where the symmetry was perfect, namely, as near as possible to the angular point, so that the circular field could be distinctly seen and that this point was the only one out of an infinite number at which the uniformity of the light of the circular field was a maximum.

Upon these principles Dr. B. constructed an instrument in which he fixed permanently across the ends of the reflectors pieces of coloured glass, and other irregular objects, and he showed the instrument in this state to some Members of the Royal Society of Edinburgh, who were much struck with the beauty of its effects. In this case, however, the forms were nearly permanent, and a slight variation was produced by varying the position of the instrument, with respect to the light. The great step, however, towards the completion of the instrument, remained yet to be made, and it was not till some time afterward, that the idea occurred to Dr. B. of giving motion to objects, such as pieces of coloured glass, &c. which were either fixed or placed loosely in a cell at the end of the instrument. When this idea was carried into execution, the kaleidoscope in its simple form was completed.

In this state, however, the kaleidoscope could not be considered as a general philosophical instrument of universal application; for it was incapable of producing beautiful forms, unless the object was nearly in perfect contact with the end of the reflectors.

The next, and by far the most important step of the invention, was therefore to remove this limitation by employing a draw tube and lens, by means of which beautiful forms could