The surfaces of the cylinders determined by very careful measurement were, the sheet iron, 28,54; the chest plate, 30,77; and the solid cylinder, 28,94 inches.

Reducing the deviations to the same extent of surface, viz. that of the solid cylinder, they become respectively 141, 184, and 184 minutes.

These last results perfectly coincide with the deductions of Mr. BARLOW, that the effect of iron on a ship's compass is as the surface, and is wholly independent of the mass; but that a certain degree of thickness of the iron (about two tenths of an inch) is necessary to the complete developement of this effect.

The following are the principal inferences which may be drawn from the experiments I have detailed.

That the best material for compass needles is clock spring; but care must be taken in forming the needle to expose it as seldom as possible to heat, otherwise its capability of receiving 'magnetism will be much diminished.

That the best form for a compass needle is the pierced rhombus, in the proportion of about five inches in length to two inches in width, this form being susceptible of the greatest directive force.

That the best mode of tempering a compass needle is, first to harden it at a red heat, and then to soften it from the middle to about an inch from each extremity, by exposing it to a heat sufficient to cause the blue colour which arises again to disappear.

That in the same plate of steel of the size of a few square inches only, portions are found varying considerably in their

capability of receiving magnetism, though not apparently differing in any other

respect.

That polishing the needle has no effect on its magnetism. That the best mode of communicating magnetism to a needle, appears to be by placing it in the magnetic meridian, joining the opposite poles of a pair of bar magnets (the magnets being in the same line), and laying the magnets so joined, flat upon the needle with their poles upon its centre; then having elevated the distant extremities of the magnets, so that they may form an angle of about two or three degrees with the needle, they are to be drawn from the centre of the needle to the extremities, carefully preserving the same inclination, and having joined the poles of the magnets at a distance from the needle, the operation is to be repeated ten or twelve times on each surface.

That in needles from 5 to 8 inches in length, their weights being equal, the directive forces are nearly as the lengths.

That the directive force does not depend upon extent of surface, but in needles of nearly the same length and form, is as the mass.

That the deviation of a compass needle occasioned by the attraction of soft iron, depends, as Mr. BARLOW has advanced, on extent of surface, and is wholly independent of the mass, except a certain thickness of the iron, amounting to about two tenths of an inch, which is requisite for the complete developement of its attractive energy.

XI. Notice respecting a volcanic appearance in the Moon, in a Letter addressed to the President. By Captain HENRY KATER F.R. S.

DEAR SIR,

Read February 8, 1821.

London, February 8th, 1821.

Ir r may perhaps be interesting to the Royal Society to be informed, that on Sunday evening, the 4th instant, I observed a luminous spot in the dark part of the moon, which I was inclined to ascribe to the eruption of a volcano.

The telescope used was an excellent Newtonian of 6 inches aperture, with a power of 74. The moon was exactly two days old, and the evening so clear, that I was able to discern the general outlines in the dark part of her disk. Her western azimuth was about 70°, and her altitude about 10 degrees.

In this position at 6 hours 30 minutes, the volcano was situated (estimating by the eye) as in the accompanying sketch. [See Plate X.] Its appearance was that of a small nebula subtending an angle of about 3 or 4 seconds.

Its brightness was very variable; a luminous point, like a small star of the 6th or 7th magnitude, would suddenly appear in its centre, and as suddenly disappear, and these changes would sometimes take place in the course of a few seconds.

On the evening of the 5th, having an engagement which prevented my observing it myself, I arranged the telescope

for two friends, who remarked the same phenomena as the night before, but in an inferior degree, partly perhaps in consequence of the evening not being so favourable.

On the 6th I again observed it; it had certainly become more faint, and the star-like appearance less frequent. I could see it very distinctly with a power of 40. As the moon approached the horizon, it was visible only at intervals when the star-like appearance took place. On the same evening I had the pleasure of showing it to Mr. HENry Browne, F.R.S.

I regret that I had no micrometer adapted to my telescope; but I have reason to believe the distance of the volcano from the edge of the moon was about one tenth of her diameter, and the angle it formed this evening with a line joining the cusps was about 50°.

I remarked near the edge of the moon, a well known dark spot, from which the volcano was distant, as nearly as I could estimate, three times its distance from the edge of the moon.

In a map of the moon published by Dr. KITCHENER (and which is the best small map with which I am acquainted), there is a mountain sufficiently near the situation of the volcano, to authorize the supposition that they may be identical.

On the 7th I could still see the volcano, and the occasional star-like appearance; but I do not think it was sufficiently perceptible to have been discovered by a person ignorant of its precise situation. I am inclined however to think, that the difficulty of seeing it is rather to be attributed to the

increased light of the moon, than to the diminished action of

the volcano.

I have the honour to be,

Dear Sir, &c. &c.

HENRY KATER

To Sir Humphry Davy, Bart.

P. R. S. &c.

P. S. Since the preceding letter was written, I have ascertained that the spot in which I observed the volcanic appearance is that named ARISTARCHUS. This spot was particularly examined by HEVELIUS, who calls it Mons Porphyrites, and who considers it to be volcanic. If his drawings are to be relied upon, it has undergone a considerable change in its appearance since his time.

Sir WILLIAM HERSCHEL has recorded in the Philosophical Transactions an observation of three volcanoes, which he perceived in the moon, April 19th, 1787, at 101. 36TM, sidereal time. One of these, which he says showed " an actual eruption of fire or luminous matter," was distant from the northern limb of the moon g'. 57",3, the diameter of the burning part being not less than g". I find that this observation was made about 9 o'clock in the evening, when the moon was not quite two days old; and from the situation of the spot described by Sir WILLIAM HERSCHEL, I have no doubt of its being the same that I have noticed.