them paper. Those calculations indeed are so full of inaccuracies, that it ould be necessary to go over them again, in order to compare rictly with my own, for which at present there is no leisure. I ought also to take notice here, that the same author, it seems, as taken measures of the horns of Venus by an instrument which, in his publications, he calls a projection table, and describes as his own; of which however, those who do not know its construction may have a very perfect idea, when they read the description of my lamp, disk, and periphery-micrometers, joined to what I have mentioned above, of using the disk micrometer without lamps when day-light is sufficiently strong; or even with an illumination in front, where the object is bright enough to allow of it, such as the moon, &c. I remember drawing the picture of a cottage by it, in the year 1776, which was at three or four miles distance; and going afterwards to compare the parts with the building, found them very justly delineated. I have also many times had the honour of showing my friends the accuracy of the method of applying one eye to the telescope, and the other to the projected picture of the object in view; by desiring them to make two points, with a pin, on a card fixed up at a convenient place, where it might be viewed in my telescope; and this being done, I took the distance of these points from the picture I saw projected, in a pair of proportional compasses, one side of which was to the other as the distance of the object, divided by the distance of the image, to the magnifying power of the telescope; and giving the compasses to my friends, they generally found that the proportional ends of them exactly fitted the points they had -made on the card. All which experiments are only so many different ways of using the lamp-micrometer. As to the mountains in Venus, I may venture to say that no eye, which is not considerably better than mine, or assisted by much. better instruments, will ever get a sight of them; though from the analogy that obtains between the only two planetary globes we can compare, (the moon and the earth) there is little doubt but that this planet also has inequalities on its surface, which may be, for what we can say to the contrary, very considerable. The real diameter of Venus, I should think, may be inferred with great confidence, from the measures I took with the twenty-feet reflector, in the morning of the 24th of November, 1791; which, when reduced to the mean distance of the earth, give 18′′.79 for the apparent diameter of this planet. This result is rather remarkable, as it seems to prove that Venus is a little larger than the earth, instead of being a little less as has been supposed; yet, on the nicest scrutiny, I cannot find fault with the measures. The planet was put between the two wires of the micrometer, which were outward tangents; and they were, after each measure, shut so as to meet with the same edge, and in the same place where the planet was measured. In this situation the proper deduction, for not being central measures, was pointed out by the index plate. The transits of the 25th were corrected for a small concavity of the wires, which being pretty thick and stubborn, were not strained sufficiently to make them quite straight, the amount of which was also ascertained by an examination of the division where the wires closed at the ends, and where they closed in the centre. The zero was, with equal precaution, referred to a point at an equal distance from the contact of the wires on each side; for they are at liberty to pass over each other, without occasioning any derangement. The shake, or play, of the screw is less than three-tenths of a division. The two planets, however, are so nearly of an equal size, that it would be necessary to repeat our measures of the diameter of Venus, in the most favourable circumstances, and with micrometers adjusted to the utmost degree of precision, to decide with perfect confidence that she is, as appears most likely, larger than the earth. The remarkable phenomenon of the bright margin of Venus, I find, has not been noticed by the author we have referred to; on the contrary, it is said, "this light appears strongest at the outward limb, from whence it decreases gradually, and in a regular progression, towards the interior edge or terminator." But the luminous border, as I have described it, in the observations of the 9th, 16th, 20th, and 22d of April, does not in the least agree with the above representation. With regard to the cause of this appearance, I believe that I may venture to ascribe it to the atmosphere of Venus, which, like our own, is probably replete with matter that reflects and refracts light copiously in all directions. Therefore on the border, where we have an oblique view of it, there will of consequence be an increase of this luminous appearance. I suppose the bright belts, and polar regions of Jupiter, for instance, which have a greater light than the faint streaks, or yellow belts, on that planet, to be the parts where its atmosphere is most filled with clouds, while the lat ter are probably those regions which are free from them, and ad mit the sun to shine on the planet; by which means we have the reflection of the real surface, which I take to be generally less lumin If this conjecture be well founded, we see the reason why spots on Venus are so seldom to be perceived. For, this planet having a dense atmosphere, its real surface will commonly be enve Joped by it, so as not to present us with any variety of appearances. This also points out the reason why the spots, when any such there are, appear generally of a darker colour than the rest of the body. [Phil. Trans. Abridged, 1793.] CHAP. XIX. AN ACCOUNT of three VOLCANOES IN THE MOON, It will be necessary to say a few words by way of introduction to the account I have to give of some appearances upon the moon. The phænomena of nature, especially those that fall under the inspection of the astronomer, are to be viewed, not only with the usual attention to facts as they occur, but with the eye of reason and experience In this we are however not allowed to depart from plain appearances; though their origin and signification should be indicated by the most characterising features. Thus, when we see, on the surface of the moon, a great number of elevations, from half a mile to a mile and a half in height, we are strictly entitled to call them mountains; but, when we attend to their particular shape, in which many of them resemble the craters of our volcanoes, and thence argue, that they owe their origin to the same cause which has modelled many of these, we may be said to see by analogy, or with the eye of reason. Now, in this latter case, though it may be convenient, in speak, ing of phænomena, to use expressions that can only be justified by reasoning upon the facts themselves, it will certainly be the safest way not to neglect a full description of them, that it may appear to others how far we have been authorised to use the mental eye. This being premised, I may safely proceed to give my observations. April 19, 1787, 10h 36' sidereal time. I perceive three volcanoes in different places of the dark part of the new moon. Two, of them are either already nearly extinct, or otherwise in a state of going to break out; which perhaps may be decided next lunation. The third shews an actual eruption of fire, or luminous matter. I measured the distance of the crater from the northern limb of the moon, and found it 3' 57",3. Its light is much brighter than the nucleus of the comet which M. Mechain discovered at Paris the 10th of this month. April 20, 1787, 10h 2′ sidereal time. The volcano burns with greater violence than last night. I be lieve its diameter cannot be less than 3", by comparing it with that of the Georgian planet; as Jupiter was near at hand, I turned the telescope to his third satellite, and estimated the diameter of the burning part of the volcano to be equal to at least twice that of the satellite. Hence we may compute that the shining or burning matter must be above three miles in diameter. It is of an irregular round figure, and very sharply defined on the edges. The other two volcanoes are much farther towards the centre of the moon, and resemble large, pretty faint nebulæ, that are gradually much brighter in the middle, but no well defined luminous spot can be discerned m them. These three spots are plainly to be distinguished from the rest of the marks upon the moon; for the reflection of the sun's rays from the earth is, in its present situation, sufficiently bright, with a ten-feet reflector, to show the moon's spots, s, even the darkest of them; nor did I perceive any similar phænomena last lunation, though I then viewed the same places with the same instrument. The appearance of what I have called the actual fire or eruption of a volcano, exactly resembled a small piece of burning charcoal, when it is covered by a very thin coat of white ashes, which frequently adhere to it when it has been some time ignited; and it had a degree of brightness, about as strong as that with which such a coal would be seen to glow in faint day-light."" All the adjacent parts of the volcanic mountain seemed to be faintly illuminated by the eruption, and were gradually more obscure as they lay at a greater distance from the crater. This eruption resembled much that which I saw on the 4th of May, in the year 1783; an account of which, with many remarkable particulars relating to volcanic mountains in the moon, I shall take an early opportunity of communicating to this Society. It differed, however, considerably in magnitude and brightness; for the volcano of the year 1783, though much brighter than that which is now burning, was not nearly so large in the dimensions of its eruption; the former seen in the telescope resembled a star of the 4th magnitude as it appears to the natural eye this, on the contrary, shows a visible disk of luminous matter, very different from the sparkling brightness of star-light. [Phil. Trans. 1787.] CHAP. XX. OF THE TWINKLING OF THE FIXED STARS, BY MR. MICHELL. HAVING AVING never yet seen any solution of the twinkling of the fixed stars, with which I could rest satisfied *, I shall offer the following, which may not perhaps be found an inadequate cause of that appearance; at least it has undoubtedly some share in producing it, especially in the smaller stars. It is not, I think, unreasonable to suppose, that a single particle of light is sufficient to make a sensible impression upon the organs of sight. Upon this supposition, a very few particles of light, arriving at the eye in a second of time, will be sufficient to make an object visible, perhaps not more than three or four; for though the im pression may be considered as momentary, yet the perception, occa * Some astronomers have lately adopted, as a solution of this appearance, the extreme minuteness of the apparent diameters of the fixed stars, which, they suppose, must, in consequence of this, be intercepted by every little mote that floats in the air; but, that an object should be able to intercept a star from us, it must be large enough to exceed the apparent diameter of the star by the diameter of the pupil of the eye; so that, if the star were a mathe matical point, it must still be equal in size to the pupil of the eye. |