or a curve in which the accelerating power shall be in a compound ratio of the distance and resistance; by which the so much desired isochronal property of the pendulum may

be obtained.

The striking part is less complex than any other method that has been devised, there being employed but two moving parts, one wheel and a hammer or balance.

The improvements in the watch consist in employing the same geering as in the clock, with a more perfect and simple escapement, and a more simple method of equalizing the main spring than the fusee and chain. It is confidently anticipated that these improvements will greatly contribute to the more perfect going of all timekeepers, there being so many sliding surfaces converted into rolling ones, and the number so much diminished.

DESCRIPTION.

Of the geering.-Let there be a wheel of the same number of angular teeth as it is required the revolutions of the arbor should exceed those of the wheel which it is to be geered into; and of such a width that the teeth of the wheel shall not be of too great an angle when a line drawn across the circumference shall pass the opposite extremes of two teeth. On the arbor there are no teeth, but a groove cut in and around it; so that one turn of the groove shall be equal in length to the width of the wheel. The distance from the point of contact on the circumference of the wheel, to the centre, should be to the distance from the point of contact on the arbor, to the centre, as the velocity of one, is to the velocity of the other; so that if a line be wound round the arbor, at the point of contact, it will be equal in length to one tooth of the wheel. The two surfaces passing over equal spaces in equal times, and in the same direction, will have a perfectly rolling action. This geering has the property of admitting the tooth of the wheel indefinite near to the centre of the arbor, still preserving sufficient strength, and thereby being enabled to give almost any difference of relative velocity between a wheel and an arbor.

The pendulum is suspended from a straight line that rolls upon a circle, or from a circle that rolls upon a straight line. The pullets are reversed; and at the centre, on the lower part, there are two planes that make an angle with the sides less than ninety degrees, which constitute the straight line that rolls upon a cylinder. The two planes are admitted into a corresponding groove in the cylinder, which prevents

Paged my from Lew.

them from slipping off; and to prevent a slip sideways, there is placed a small conical pin between the two planes, that is so fitted into an oblong hole that the two can roll together without friction; when the pendulum is so detached that the just accelerating power cannot be given without making the cylinder of an inconvenient size, a segment may be used, or the planes of the pullets that roll on the cylinder may be concave.

The striking part consists of a wheel of any convenient number of sharp pointed teeth, made to play into a cylinder, after the manner of a cylindrical escapement to a watch; on the cylinder there is confined a balance with two arms, the one a little longer that the other, so that when at the point of rest the groove in the cylinder will be at the right place for a tooth of the wheel to fall into, and give an impulse to the balance, one arm of which strikes a bell (operating as a hammer) and returns to the point of rest again, when another tooth passes in, operating as before.

To make a clock run a year with but three wheels, or a week with two, and to shew the hours, minutes, seconds, and fractional parts, and if for a year, the days of the week: let the teeth of the wheels be of such a number that the first will revolve once in five minutes, the second in six hours, the third, if for a year, in a week. The fractional parts of the second, are pointed by a hand from the pendulum, the days of the week by the last wheel. The hours, minutes, and seconds are pointed by hands which revolve with the two first wheels. The six hour wheel is placed in the centre of the dial, round the circumference of which there is two rows of figures beginning at one and ending at twelve; these spaces are subdivided into five minute spaces, which point the time to five minutes; the remaining minutes are found at the smaller circle, round which the five minute hand revolves, which spaces are divided into seconds.

Reference to the Plate.

A, fig. 1, a wheel with angular teeth. B, the arbor that is geered into the wheel A.

A, fig. 2, the pullets that have a concave surface to roll on the cylinder B. Fig. 3, the striking part; A the bell, B the wheel, C the cylinder, D the balance or hammer.

Fig. 4, a view of a two wheel clock, to run eight days, and to shew the hours, minutes, and seconds on the dial represented in fig. 5, the time to five minutes, on the outer circles, and under five on the small circle under the centre.

The watch escapement and substitute for the fusee and chain, is not represented in the plate.

ART. XIII.-Notice of M. Brongniart's Memoir on the Lignites. [J. W. W.]

We have received from M. Brongniart, a copy of his late Memoir on the Lignites, or those carbonaceous combustibles, to which a vegetable origin is to be ascribed, and which have hitherto been confounded with the coals. This memoir is particularly valuable from the nature and importance of the substance of which it treats, and from the masterly manner in which it is drawn up. It is one of the best models for geological reports with which we are acquainted. It presents us with all the important facts in the history of this mineral product, points out the distinguishing characters of each variety with singular perspicuity, and contains a full account of all the most important localities, together with some highly interesting speculations, in regard to the origin of these combustibles. In every page we discern the hand of one, who comes to his undertaking familiar with all that has been written and done in the science of geology, whose great object it is to communicate to others, in a simple intelligible manner, the results of careful observation, and exemplary patient research. M. Brongniart does not aim to astonish his readers, with an enumeration of unimportant minute shades of difference in rocks and minerals, and a long list of substances, with new names, with which we are fatigued in too many of the geological memoirs of the present day, and which instead of advancing the science, have a contrary effect.

The remark of a distinguished European geologist, that he who is not familiarly acquainted with mineralogy, is not prepared to engage in geological investigations, cannot be too fully impressed on those who find an interest in these pursuits. The belief that an acquaintance with a few of the most common rocks and minerals, is sufficient preparation for undertaking the geological description of a country, is continually leading to gross error. How often are we overwhelmed with a host of new names, which are afterwards found to have been applied to minerals, that have been long known, and which were evidently resorted to, from a want of sufficient acquaintance with the labours of others? Not only are the greatest errors in the description of rocks continually occurring, but their relative position, one of the most important circumstances in regard to them, is judged of, as has been so justly remarked by Humboldt, according to imperfect analo

gies, and to the circumscribed ideas acquired in limited tracts of country. The views of the geologist are too grand and extensive to be shackled by microscopical differences in the composition of rocks, on which new names are so often founded, and which so far from authorizing the multiplication of sub-species, should lead to more general and enlarged views.

The work of M. Brongniart was written for the Dictionary of Natural Sciences, now publishing in Paris, which is enriched with the contributions of so many distinguished scientific men. It is divided into three parts; the first relates to the mineralogical characters, varieties, properties, &c. of lignite; the second, to its geological position; and the third, to its geographical distribution, and the circumstances therewith connected.

The term lignite is applied to no other variety of combustible mineral, than such has evidently had a vegetable origin. Under this name are included not only all the varieties of brown coal of the Germans, but likewise many other kinds of bituminous fossil coal. The distinction between fossil coal, strictly so called, and lignite, was first pointed out by M. Voigt.

The lignites are distinguished mineralogically, by a deep black colour, a brilliant lustre, a resinous fracture, which is sometimes conchoidal. Their texture is homogeneous, in which the ligneous structure is not always discernible, but in some specimens where the mineral has not lost its black colour, it is sufficiently distinct. It occasionally however passes through different shades of brown, still retaining the fibrous character. Its vegetable and ligneous origin can scarcely be mistaken, although it sometimes has quite an earthy aspect.

When exposed to the fire, all the varieties burn with a clear flame. The most compact varieties, after distillation, exhibit the ligneous structure, and those which contain no pyrites give out a pungent fetid odour, devoid of any aromatic smell; this is also the case with coal and bitumen when equally pure. In other combustibles, the pyrites occasion a sulphureous odour, different from that peculiar smell of the lignites.

After combustion, there remains a pulverulent cinder, resembling that from wood, but often more abundant and earthy, more ferruginous, and of a reddish colour, containing in some cases nearly three per cent. of potash.

M. Brongniart remarks, that we have no analysis which can lead to a knowledge of the essential differences between the lignites, coal and bitumen.. The three following analyses, however, of lignites from places very distant from each other, exhibit sufficiently the constituent principles.

These analyses prove the existence of an acid analogous to the pyroligneous or pyro-acetous, and confirm the supposition of Vauquelin, that this acid is formed during the combustion of the lignite. This opinion is also confirmed by the experiments of Dr. Macculloch, on the properties of the products of the distillation of wood, peat, and the lignites called jet, bovey coal, and surturbrand. All these products contain more or less pyro-acetic acid, while it is not found in the products of the distillation of bitumens; which, on the contrary, contain ammonia, &c. These analyses show that the lignites and the coals are composed of the same ultimate principles, carbon, hydrogen, &c.; but that in the products they are combined in a different manner by heat, and lead to a true mineralogical distinction between them.

The specific gravity of the true lignite, (lignite piciforme jayet,) is from 1.10 to 1.50.

The proportion of the eminently combustible part, whether it be carbon or hydrogen, to the apparent mass of the combustible, appears to be one of the distinctive characters between the lignites and the coals. The real combustible seems to be much less condensed in the first than in the second; this is not deduced from the relations of the specific gravities, which are nearly the same in the two combustibles, but from the results obtained by M. Voigt; from which it appears that one volume of lignite will give as much heat as