To the foregoing arguments, we may add the following, drawn from analogy. Dr. Herfchel fays, that among the great number of nebula which he has already feen, amounting to more than goo, there are many, in all probability, equally extenfive with that which we inhabit; and yet they are all feparated from each other by very confiderable intervals. Some, indeed, there are, that feem to be double and treble; and though with most of them it may be, that they are at a very great diftance from each other, yet he does not mean to fay that there are no fuch conjunctions; though there may be alfo fome thinly fcattered folitary ftars, not yet drawn into fyftems; their number cannot be very confiderable: a conjecture that is abundantly confirmed, in fituations where the nebulæ are near enough to have their ftars vifible; for they are all infulated, and generally to be seen upon a very clear and pure ground, without any ftar near them, that might be fuppofed to belong to them: and though they may be often feen in beds of ftars, yet from the fize of thefe ftars, we may be certain that they are much nearer to us than thofe nebula, and belong undoubtedly to our own fyftem.

THE ORIGIN OF NEBULOUS STRATA.

Dr. Herfchel thinks the nebula that we inhabit, has fewer marks of profound antiquity upon it than the reft; having previoufly fuppofed that the condenfation of clufters of flars is to be afcribed to a gradual approach; the number of ages that must have past before fome of the clusters could be fo far condenfed as they are at prefent, makes him naturally afcribe a certain air of youth and vigour to many very regularly fcattered regions of our fiderial fyftem. There are many places, where he afferts that there is reafon to believe, that the ftars,

if we may judge from appearances, are now drawing towards various fecondary centers, and will, in time, feparate into different clusters, fo as to occafion many fub-divifions. Our fyftem, after numbers of ages, may be divided fo, as to give rife to a ftratum of two or three hundred nebulæ.

AN OPENING IN THE HEAVENS.

Some parts of our fyftem feem to have already fuftained greater ravages from time than others: in the body of the Scorpion, there is an opening, or hole, which is probably owing to this caufe; it is 4 degrees broad.

A PERFORATED NEBULA, OR RING OF STARS.

Among the curiofities of the heavens, should be placed a nebula that has a regular concentric dark spot in the middle, and is probably a ring of ftars; it is of an oval fhape; in the northern fide 3 very faint stars may be feen, as alfo one or two in the fouthern: the vertices of the longer axis feem lefs bright, and not fo well defined as the

reft.

PLANETARY NEBULE.

These are so named from a fingularity of appearance, which renders it difficult to clafs them. Their light is fo uniform and vivid, the diameters fo fmall and well defined as make it probable that they should be common nebule: if nebulæ, they must be comprefied, and condenfed in the highest degree.

Though the words condenfation and cluster often occur in the foregoing extract, we are by no means to infer that any of the celeftial bodies, in VOL. IV.

Q

our

our nebula, are nearer to one another than we are to Sirius, whofe diftance is fuppofed to be not lefs than 38 millions of miles. The whole extent of the nebula being, in fome places, near 500 times this distance, must be such, that the light of a star placed at it's extreme boundary, fuppofing it to fly with the velocity of 12 millions of miles every minute, muft have taken near 3000 years before it could reach us.

Thefe immenfe fpaces, thefe numerous hofts of fyftematic univerfes, are probably connected the one with the other. Like fo many immenfe circufes, by the mutual contact of their circumambient fpheres, they prefs each other: thefe aereal atmospheres being alfo connected and interwoven together by an infinity of infertions, conftitute a celeftial sphere, which is again linked with others, till by an infinity of orbs they obtain a form, which is the origin and pattern of all forms, in which all the variegated fiderial revolutions harmonioufly concur to one and the fame end; that of mutually ftrengthening and establishing each other, and forming a celeftial union.

OF THE TELESCOPIC APPEARANCE OF THE PLANETS.

OF THE SUN.

"The observations which might with fullnefs of evidence confirm the opinion of planetary worlds, feem to be placed out of our reach, and we can scarce hope to make our optical inftruments fufficiently perfect, to render the inhabitants thereof vifible to us. All, therefore, that we can do, is to examine if the planets are accommodated with thofe things which we are used to confider as neceffary to animal exiftence. Lands, feas, clouds,

vapours,

vapours, and an atmosphere, or body of air, are objects that we may expect to find on the face of an habitable world."

By means of the telefcope, we are enabled, in fome measure, to afcend into the celeftial region, and view the fun, moon, and ftars, as they would appear to us if they were brought fo many times nearer to us as the telescope magnifies; the light proceeding from the luminary we are looking at, being diminished in the fame proportion.

The telescope is one of those discoveries, of which no idea could have been formed, previous to the period in which the Supreme Being was pleased to unveil to the human mind fome of the mysterious powers of glafs: the importance of this difcovery, and the extent to which it may be carried, still lie hid among the fecrets of infinite wifdom. It is by this inftrument more than by any other, that we have been led onward in our advances towards a perfect knowledge of the heavenly bodies, and that aftronomy has been raised from little more than a catalogue of stars, into a science.

When we look at the fun through a telescope even of moderate power, the eye being defended by a piece of coloured or fmoked glafs, nay, even by the naked eye, when guarded in the fame manner, we discover on his furface many black, or rather lefs bright fpots, of various fizes and fhapes. Sometimes thefe fpots will vanish in a very fhort time after their first appearance; fometimes they travel over his whole disk, or visible furface, from west to eaft, when they disappear, and in twelve or thirteen days they appear again, fo as to be known, by their magnitude and figure, to be thofe that had disappeared before. Thofe, however, which are of the longest continuance, do not appear to have much folidity of confiftence, for in a little time

Q 2

they

228

LECTURES ON NATURAL PHILOSOPHY.

they also vanish, and become bright like the rest of the furface.

The fpots are more frequent at fome periods than at others; in fome years, the fun's disk has for many months been perfectly free from them; in others, he has for months been more or lefs obfcured by fpots: the most remarkable phenomena of thefe fpots, as obferved by Schenier and Hevelius, are as follow: 1. Every spot, which has a nucleus, or dark part, hath alfo an umbra, or fainter fhade, furrounding it. 2. The boundary betwixt the nucleus and umbra is always diftinct and well-defined. 3. The increafe of a fpot is gradual, the breadth of the nucleus and umbra dilating at the fame time. 4. In like manner, the decrease of a spot is gradual, the breadth of the nucleus and umbra diminishing at the fame time. 5. The exterior boundary of the umbra never confifts of fharp angles, but is always curvilinear, how irregular foever the outside of the nucleus may be. 6. The nucleus of a fpot, whilft on the decrease, often changes it's figure, by the umbra incroaching irregularly upon it, infomuch that in a small space of time new incroachments are difcernable, whereby the boundary between the nucleus and the umbra is perpetually varying. 7. It often happens, that by thefe incroachments the nucleus of a spot is divided into two or more nuclei. 8. The nuclei of the fpots vanish before the umbra. 9. Small umbræ are often feen without nuclei. 10. A large umbra is feldom feen without a nucleus in the middle of it. 11. When a fpot, which confifted of a nucleus and an umbra, is about to disappear, if it be not fucceeded by a facula, or spot, brighter than the reft of the difk, the place it occupied is in a very little time not to be perceived.

In the Philof. Tranf. vol. Ixiv. the reader will find feveral curious obfervations on these spots, by Profeffor Wilfon, and the Rev. Mr. Wolafton. The

latter