or seventy miles across the crater-many times larger than those on the earth. But it is just because it is a very small body that it has very large volcanoes. The force of gravitation is much less at the moon, and hence the ejective force is able to expel greater masses, and to act more uninterruptedly. The crater of Etna is but a spot compared with some of those in the moon. The proportional size he illustrated by a plaster model. The smallness of the moon, its mass being to that of the earth as 1 to 64, and its surface as 1 to 16, was the cause of the multitude as well as magnitude of these volcanoes. As the outer crust cooled, the moon would become, so to speak, hide-bound; and, by the contraction of the surface upon the liquid mass beneath, the latter would be spurted out in every direction, while the former would also exhibit those radiating striæ or cracks which are so very marked a feature in its appearance. Some of the cracks were upwards of seven hundred miles long. In illustration of the mode in which it was probable this appearance was produced in the moon, he showed portions of glass globes, which, after being filled with hot water, and then hermetically sealed, were broken by the application of cold to the outside, producing cracks arranged in precisely the same order as the corresponding cracks on the surface of the moon. In all his inquiries he had endeavoured to put the moon herself into the witness-box, and obtain her answer: we might not, however, always translate that answer aright. Besides the craters and these cracks, there are also many trachytic domes, where the force of ejection has not been sufficiently great to form craters ; and, lastly, there are great ranges of hills, the formation of which appears to have taken place in this way :-As contraction goes on, the liquid nucleus separates itself from the solid crust; the latter is bent inwards by the force of gravitation, and a cracking and crushing action takes place, with further eruptions in mountain-crests, or two adjacent surfaces are raised edge on edge, as in fields of ice. A shrivelled apple afforded a familiar illustration of this appearance and its cause; and applying the same rule to the earth, it appeared to him to give a most simple and beautiful explanation of mountain-ranges.

PROFESSOR NICHOL said, he was sure he was giving utterance to the opinion of every one present, when he said that the drawings which had been exhibited by Mr. Nasmyth were the most beautiful and faithful representations of the surface of the moon that had ever been constructed. He had little doubt that Mr. Nasmyth's investigations would ultimately lead to the most important results, especially in relation to the science of geology.

PHOSPHORESCENCE OF POTASSIUM.

This interesting property of potassium has not, MR. PETRIE believes, been hitherto observed, nor even suspected. It was not accident that led him to observe the fact; but, while speculating on the consequences of the dynamical theory of heat, he was led to the conclusion that cold potassium ought to be found luminous, and farther, that it ought to be only about a tenth part as luminous as phosphorus.

On testing this, experimentally, as soon as an opportunity allowed, with the precautions for sensitive vision, which the anticipated feebleness of the light indicated to be necessary, the result was, that on dividing a bit of potassium, (which was quite dry, being protected only by a coating of bees'wax,) the halves showed two distinctly luminous sections; the light being about a tenth part of that from a similar surface of phosphorus. The light diminished, naturally, as a protecting coating of oxide was formed, but

remained just perceptible to the most sensitive sight as long as half an hour. The paper concluded with the considerations which led theoretically to this fact.

COMETS.

PROFESSOR SMYTH treated of the physical nature of comets. He had connected the facts of observers, and had endeavoured to arrange them for ocular inspection in the following table of cometary axioms :— 1. A comet consists of a nucleus, and one or more gaseous envelopes. 2. The nucleus, if solid material, is exceedingly small.

3. The nucleus eccentrically situated in the gaseous body.

4. Comets of the longest periods have the largest bodies.

5. The more eccentric the orbit, the more eccentric the body of a

comet.

6. A comet rotates on its shorter nucleoid axis in the same time that it takes to revolve round the sun.

7. This axis is not always at right angles to the plane of the orbit. 8. There is also a quicker rotation round the longer axis.

9. A comet shines by reflected light, and shows a sensible phase. 10. In proportion to the eccentricity of the orbit, a comet increases in density, and decreases in size, in approaching the perihelion, and vice versâ.

11. The longer axis of a comet is straight at perihelion and aphelion; but, between these points, is concave towards the latter: the curvature being inversely as the eccentricity of the orbit.

12. (Sir J. Herschel's.) The component molecules of a comet are only held together by their mutual gravitation, each constituting almost a separate projectile, and describing its own parabola round the sun.

Professor Smyth directed attention particularly to axioms nine and ten, as perhaps most important; and in regard to which, as he thinks, his results are decidedly new. The grounds upon which these results are based were described and illustrated by diagrams.

PHENOMENA OF MIRAGE.

The REV. C. F. LYON had noticed the Red Head at Montrose, distant twenty-five miles from St. Andrew's, assume a square form, then notched, then double-notched. The outlines of the sea had risen up with angular corners, and pieces of the sea seemed raised up as if seen through unequal glass.

DR. SCORESBY had seen many such peculiarities. They occurred chiefly at seasons when the temperature changes suddenly from cold to heat, thus giving rise to inequalities in the density of atmospheric strata. Two mirages of the same object were frequently seen, as noticed by the late Dr. Young. I (said Dr. Scoresby) once saw a remarkable case of this kind, the land appearing raised up like basaltic cliffs. I then took a different elevation, by ascending to the top of the house, and was delighted to observe that the phenomenon had wholly disappeared. I came down, and now saw it as formerly. Again I descended to the bottom of the rising ground, and saw nothing unusual.-MR. HOPKINS thought that proper attention had not been paid to the influence of reflection, as well as refraction, in such cases.

TISSUE SPUN BY CATERPILLARS.

MR. DENNISTOUN exhibited a veil of the consistency of gossamer, fortytwo inches by twenty-four inches, made by caterpillars ascending an

incline, their course being directed by lines of olive-oil to produce the tissue. He had found an account of a similar product by caterpillars by the directive agency of parsley-paste, but he had only seen two besides his own; one of which, seven inches by five inches, obtained at Munich, with a picture drawn on it, was also shown.

CHOLERA IN THE INDIAN ARMIES.

The scope of DR. C. FINCH's paper was to show that the disease was neither so frequent nor so fatal in the East as is generally believed in this part of the world. A summary of these per-centages shows that, of the European force, 11,429 strong, stationed in the Madras Presidency during the year 1847, there were attacked by cholera only 271 per cent.,-little more than 1 man in 500; and of whom died 192 per cent.,-less than 1 man in 500. Of the European troops, 8,756 strong, serving in the Bombay Presidency in 1847, there were sufferers from cholera 515 per cent., or 1 man in 200; of whom died ·274 per cent., or about 1 man in 400. Of the Madras Native Army, consisting of 67,950 men, those attacked by cholera were 334 per cent., or about 1 man in 300; of whom died only 114 per cent., or 1 man in 900. Of the Bombay Native Army, comprising 43,930 sepoys, the ratio of sick to strength was 575, or little more than 1 man in 200; but the loss occasioned by the disease did not exceed 227 per cent., not amounting to 1 man in 400. These results demonstrate that, though epidemic cholera is still a frequent and fatal disease in the Indian armies, it is neither so prevalent nor so mortal as it is generally believed to be; and show that military service in India does not necessarily entail so great a risk of life from this disease as is generally supposed in this part of the globe.

ON THE POWERS OF MINUTE VISION.

MR. W. PETRIE stated results from experiments for determining the best sort of station-marks, and the errors liable, in observing with optical instruments that measure on the principle of bringing two reflections together. The experiments were performed in bright daylight, (but not sunshine,) being light of the maximum of advantage for perceiving black against a white ground. The general circumstances of the experiments were arranged rather to determine the facts of common practice, than the theoretic powers of vision. The author then detailed the various distances at which circular spots, lines, &c., white on black as well as black on white, could be seen, the distances being given in terms of the breadth of the object seen. An arrangement of lines was described, by which an alteration of their position to the extent of only one millionth part of the distance of the observer was made visible. One result of the experiments would be to show what should be the proper proportions of parts to be observed in forming letters to be read with the greatest distinctness at a distance,-a subject of much practical use in the present day, and admitting of a strictly scientific system, although generally left to the fancy of incompetent persons. White letters on a black ground should have their component lines of only half the breadth that black letters should have on a white ground. The direction of the eye, while appearing to gaze steadily at any object, does in reality keep wandering to an imperceptible distance on every side of the object looked at, but very rapidly. This wandering is not accidental, or an imperfection of sight, but an essential feature of vision; because it is not the continuance of an impression that is perceived, (by any of the animal

nerves,) but its commencement and termination, or, more strictly speaking, its increase and decrease. This principle is probably analogous to that by which a magnet creates an electric current in a neighbouring wire, not by its constant presence, but by the increase or diminution of its influence, either by a variation of its power, or of its position. This wandering propensity of the eye was shown to account for the relative facility with which different sorts of marks were seen at great distances: it takes place, apparently, in a minimum case, to the extent of an angle of 1 in 2,500. A dislocated line, (as in a vernier,) its falt being half its breadth, can be perceived to be so at a distance of 10,000 times its falt, if black on a white ground; and at 12,000 times, if white on a black ground. It shows itself, however, by giving the line a less steady appearance, than a perfectly even line would have, when narrowly watched, by running the eye along the line, at about half as far again. Experiments were then described, on the visibility of the positions of the ends of lines, and of hiatuses in lines, and of square dots as compared with round. But the last conclusion of practical importance was in respect of observing the angular position of station-marks, or of stars, by reflection, as in a sextant. From these experiments it appeared that the position of two closely adjacent dots or images, in sensible parallelism to a given direction, while it affords one of the simplest kinds of observation, is more accurately observable than their actual coincidence, or even than the junction of two lines, as if in a vernier.

PHENOMENA OF THE POLARISATION OF THE ATMOSPHERE.

SIR D. BREWSTER stated that by the aid of a polariscope, which he had formerly described, formed with two wedges of rock crystal cut in a peculiar way, he was enabled to determine, by an examination of the parallel bands, the neutral points of the sky, and the plane of polarisation. This he illustrated by a diagram; and added, that he had now convinced himself that not only was the light from the blue sky polarised by reflection in one plane, but under certain circumstances the refraction caused by some of the clouds polarised the light in quite a different plane; and he had actually in this instrument observed clouds by the action they exerted on the light, which were quite imperceptible to the eye unassisted by it.

PROFESSOR STOKES observed that heretofore it had been universally supposed that the clouds always exerted a depolarising action on the light which they reflected or transmitted to the eye.

METHODISM IN FORMER DAYS.

No. XLI.-A MISSIONARY'S QUALIFICATIONS.

MR. CH, a pious young man who had offered himself as a Missionary for the West Indies, was received on trial by the Conference held in Manchester, 1799, and appointed to the islands of Nevis and Montserrat,* where we had then five hundred members. Dr. Coke, being General Superintendent of the Missions, overruled the appointment; as appears by the following Ms. letter, addressed to Mr. Mather, Superintendent of the London Circuit:

* Minutes, vol. ii., pp. 4, 13.

VERY DEAR AND RESPECTED FRIEND,-I have desired brother H— to preach to-morrow morning, if it meet with your approbation, before as many of the Preachers as will be so kind as to attend. I have in a great measure told him my mind on the subject.

He ought not to go to the West Indies, unless he has abilities sufficient for the smaller Circuits in England.

My love to Mrs. Mather and the Preachers.

Your most obliged

Т. Соке.

The Preachers who heard him the following morning concurred with Dr. Coke; and the candidate's appointment was superseded. Nor does his name appear on the Minutes of the following year among those remaining on trial. (See Minutes, vol. ii., p. 42.)

City-Road, October, 1850.

THOMAS MARRIOTT.

VOLTAIRE AND GIBBON, AS WRITERS OF HISTORY.

THE following judicious and temperate remarks are from the pen of a writer who has himself acquired deserved celebrity as an historian,—Mr. Prescott, author of "Ferdinand and Isabella," "Cortes," and "Pizarro." They are taken from a volume recently published with the title of, "Biographical and Critical Miscellanies." (8vo., pp. 565. Bentley.) The Essays which the volume contains were originally, with a single exception, contributions to the "North American Review." That from which our extracts are taken, was founded on "Irving's Conquest of Granada,” and appeared in the above Review in October, 1829. Two or three observations suggested themselves while we were engaged in the task of copying ; but, instead of interrupting the reader by placing them in separate notes at the bottom of the page, we have placed them, in one note, at the conclusion of the extracts,-having previously marked, as the reader will perceive, the suggesting sentences.-EDITS.

THE personage by whom the present laws of historic composition may be said to have been first arranged into a regular system, was Voltaire. This extraordinary genius, whose works have been productive of so much mingled good and evil, discovers in them many traces of a humane and beneficent disposition. (1.) Nowhere is his invective more keenly directed than against acts of cruelty and oppression, above all, of religious oppression. He lived in an age of crying abuses both in church and government. Unfortunately he employed a weapon against them, whose influence is not to be controlled even by the most expert hand. The envenomed shaft of irony not only wounds the member at which it is aimed, but diffuses its poison to the healthiest and remotest regions of the body.

The free and volatile temper of Voltaire forms a singular contrast with his resolute pertinacity of purpose. Bard, philosopher, historian, this literary Proteus animated every shape with the same mischievous spirit of philosophy. It never deserted him, even in the most sportive sallies of his fancy. It seasons his Romances, equally with his gravest pieces in the Encyclopædia, his familiar letters and most licentious doggerel, no less than his Histories. The leading object of this philosophy may be defined by a single cant phrase,-" the abolition of prejudices." (2.) But in Voltaire, prejudices were too often confounded with principles.