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prepared, I made an assistant strew some hair-powder with a puff into the beam of light, while I kept my attention fixed upon the screen. As soon as the hair-powder reached the beam of light, the screen was suddenly cohered with the most beautiful arrangement of concentric circles, displaying all the brilliant colours of the rainbow. A great variety in the size of the rings was obtained by making the assistant strew the powder into the beam at a greater distance from the mirror ; for the rings contract by an increase of the distance, and dilate on a nearer approach of the powder.
'This experiment is so simple, and points out the general causes of the rings which are here produced in so plain a manner, that we may confidently say they arise from the flection of the rays of light on the particles of the floating powder, modified by the curvature of the reflecting surface of the mirror.
'Here we have no interposed plate of glass of a given thickness between one surface and another, that might produce the colours by reflecting some rays of light and transmitting others; and if we were in. lined to look upon the distance of the particles of the floating powder from the mirror as plates of air, it would not be possible to assign any certain thickness to them, since these particles may be spread in the beam of light over a considerable space, and perhaps none of them will b,e exactly at the same distance from the mirror.
* I shall not enter into a further analysis of this experiment, as the only purpose for which it is given fn this place is to show that the principle of thin or thick plates, either of air or glass, on which the rays might alternately exert their fits of easy reflection and easy transmiason, must be given up, and that the fits themselves of course cannot be shown to have any existence.'
Dr. Herschel purposes, in a continuation of the present paper, to extend his speculations, in order to discover the immediate cause that produces concentric rings; and therefore we must reserve our opinion concerning the truth and sufficiency of the cause to a future time. Hitherto, we are favoured only with a glimpse of this cause. Yet, if Dr. H. intends to class the pbumomena of coloured rings with the phenomena of inflection,—rhat is to refer both to the same cause,—perhaps he is to be informed that he is anticipated in bis speculations. In the years 179V and 1800, two pamphlets by an anonymous author were published, one intitled, "The Observations of Newton concerning the Inflections of Light, actompanied by other Obsirvations dijfaent /torn his," &c* The second of these tractatt-s was "an Account of the lrides or Corona -which appear around and contiguous to the Bodies of the Sun, Moon, and other luminous Objects" f; and the explanation of these Corona: is given on pnn iples similar to those from
* See Rev. Vol. xxxii. N.S. p.12. f s«e Vol, Xxxt. p. 270.
5 which which Dr. H. in his 33d article suggests that we may account for the concentric circles seen in his powdered atmosphere. As we have already said, however, we must wait for the continuation of this subject. The tracts which we have just mentioned have probably never fallen in Dr.H.'s way. Otherwise, since he takes notice in his paper of works relating to the subject of his inquiry, he would not have passed over these in silence. form so laborious a compilation.' To this claim we shell of course refer in our examination of the book; and we shall endeavour to estimate the author's merit according to the standard which he has himself erected.
Observations and Measurements of the Planet Vesta, by John Jerome Schroeter, F.R.S. Translated from the German. The extreme smallness rather than the positive extent of the planet Vesta is to be inferred from the information contained in this short paper. Mr. Schroeter succeeded, he says, in measuring the disk by means of a thirteen feet reflector, with a power of ?88. The apparent diameter is not more than 0,488 seconds, and is only half of the apparent diameter of the fourth satellite of Saturn.
The planet Vesta, then, since it lies between Mars and Jupiter, must be exceedingly small. It is also remarkable that this planetary atom is found in the same region with Ceres, Pallas, and Juno, which are not bigger than the Island of Sicily*; and since it is in close union with them, Mr. Schroeler says that it has the same cosmological origin. Are we hence to understand that these four revolving bodies formerly belonged to the same earth, till they were separated by internal convulsion, or by the rude invasion of a Comet?
Observations on the new celestial Body discovered by Dr. Olbers, arid of the Comet which was expected to appear in January 1807 tn its Return from the Sun; by William Herschell, L.L.D. F.R.S. From the observations made on Vesta, Dr. H. does not draw the same inference that has been deduced by Mr. Schroeter.— The apparent disk, viewed with a power of 460, was about 5 or 6 tenths of a second: but then Dr. H. rightly concludes that this was a spurious appearance, because higher powers destroyed the proportion which it bore to a real disk when equally magnified. Different telescopes and different powers were employed, but they all gave the same result: or, rather, we ought to say that at present nothing is determined concerning Vesta's disk.
With regard to the Comet, this paper affords merely a brief notice of its situation, and a proof that it does not possess a nucleus. Of the sixteen Comets which Dr. H. has examined, fourteen have been ascertained to be without any visible solid body in their centre; and the other two had an illdefined small central light, which did not deserve the name of a disk.
Art. V. The CtJe ofHealth and Longevity, or a conct'je View of the Principles calculated for the Preservation of Health, and the Attaiument of long Life. Being an Attempt to prove the Practicability of condensing, within a narrow Compass, the most Material Information hitherto accumulated, regarding the different Arts and Sciences, or any particular Branch thereof. By Sir John Sinclair, Bart 8vo. 4. Vols. 2I. 8s. Boards. Edinburgh, Constable and Co.; London, Cadell and- Davies, and Murray. 1807.
Tt appear^ both from the title and the advertisement, that *■ the object of the author in the composition of this work Was two-fold; though his principal motive for undertaking it was in order to try a literary experiment, viz. to ascertain how far it was possible to reduce into a small compass the large mass of information that we may possess on any particular subject. He observes that the quantity of books now existing in the worid is immensely great, probably not far short o£ 500,000; that they are every day accumulating; and that it therefore becomes highly necessary to endeavour to extract their most valuable parts, and to arrange them in such a manner that the knowlegs which they contain on different topics may be rendered easily accessible. This idea he starts as if it were quite original; adducing arguments in its favour, and laying down directions for performing it, exactly as if. no person had ever before attempted any thing of the kind. . Our readers, however, need not to be told that some of the most valuable among the modern scientific treatises are not only virtually, but even professedly composed on the identical plan of selection and condensation which is here brought forwards with all the air of novelty.
Having determined on the nature of his experiment, Sic John Sinclair next looked out for a proper subject; and his choice seems to have been determined, not by any previous acquaintance with the science, but principally by the accidental circumstance of his own health having been in such a declining state, as to have rendered it necessary for him to pay particular attention to it. We also give him full credit for an additional motive of a less personal nature; we believe that he was influenced by the persuasion that he had it in his power to confer a great obligation on his countrymen, by affording them instruction on a topic in which every member of the community was deeply interested. He distinctly states the grounds on which he rests the merits of his performance ;«it can only consist,' he remarks, «in the value of the materials he has collected, in the manner in which they are arranged, and in the reasons which induced him to
This publication, then, consists of four Urge octavo volumes; the first being the only original part of the work, and containing * all the knowlege which he considers to be essentially necessary for the attainment of health and longevity:' the three other volumes are chiefly compilations, consisting of extracts from the writings of different authors who have treated on these matters. One of the most striking peculiarities of the author's method of writing is observable in the minuteness of his arrangement, and the numerous heads into which every •ubject is divided that falls under his consideration. His first and main division is formed into three parts, 1st, 'Circumstances which necessarily tend to promote health and longevity, independent or individual attention, or the observance of particular Tules.' 2dly, • Rules for preserving health and promoting longevity.' 3dly, < Regulations for the health of the community.' It will be immediately perceived that the subject of the first part, although affording many important and curious points for physiological investigation, is irrelevant to the direct object of the work, by referring to events which arc out of the power of the individual; such as parentage, natural constitution, sex, &c. The third part is entirely passed over, the reason for which omission is stated to be that < to do it ample justice would require a separate and very extended discussion :' but of the validity of this ground, the reader will probably form a different opinion from the author. The second part being in itself the most important, and also occupying the greatest portion of the volume, we shall consider it rather in detail j bearing in mind the particular object of the author, whicti is not merely to afford an useful body of information on the matter in debate, but also to give a correct specimen of the manner in which scientific subjects ought to be treated.
Sir John Sinclair commences by an introduction, in whifth he discusses at some length the point whether health is likely to be benefited by an attention to rules; and after having duly weighed the arguments that have been adduced on both sides of the question, it is ultimately determined in the affirmative. He next inquires, with equal assiduity, into the causes which in general render persons so indifferent respecting their health, and so little inclined to take the advice of those who have made it an object of professional attention. One of the principal circumstances is conceived im be the difficulty which is experienced in procuring,information 5 a difficulty, however, which we are to suppose will be entirely removed by the publication of this Code. Another cause here pointed out is that the means of preserving health have not hitherto been made the peculiar study of the physician; an allegation which, we confess, very greatly surprised us, and which every medical man will no doubt be disposed to repeL
We now enter more immediately on the subject, by a chapter on Air, which, according to the author's plan, is divided into five sections. « r. The nature of the atmosphere in general, and the substances of which it consists. 2. Its transparency, weight, and the other mechanical properties which it possesses. 3. The qualities by which it is distinguished. 4. Circumstances which render breathing or respira• tion necessary for the sustenance of life; and 5. the. rules which ought to be observed as connected with that important function.' The account of the atmosphere is detailed at considerable length, and is tolerably correct, but is by no means peculiarly appropriate to the investigation: it contains a number of particulars respecting the chemical properties of the air, which have no immediate relation to health, and which are put together without any merit either of selection or arrangement. In the second section, the mechanical and other useful properties of the atmosphere are classed under five heads; transparency, fluidity, weight, perpetual motion, and elasticity : each of which is separately considered, and, as in the last section, gives occasion to a number of common-place remarks, to which we have little to object farther than that they have scarcely any reference to the design of the work, and might have been delivered in a much shorter compass. -—The qualities, as distinguished from the properties of the air, next come tb be considered; of these, eight are enumerated, under the heads of airs, hot, cold, moist, dry, light, heavy, inland, and maritime. Adhering rigidly to his methodical arrangement, Sir John discusses each of these qualities separately, and seems anxious to communicate to his readers the sum and substance of the knowlege that he has been able to collect on this point: but, after all, we meet with very little that is to the purpose, and with much that is uninteresting. He informs us that the fibres are relaxed and lengthened by hot air, that they are contracted by cold air, that cold air prevents the saline and acrid parts of the perspirable matter from being evacuated, that moist air is peculiarly unwholesome, and that air which is very hot is still more pernicious.