If we express this view of Newton's in the language of the undulatory theory, which is now universally accepted, we obtain the results developed in the preceding pages. It is true that our theory does not accept a peculiar "substance" of light or of heat; nevertheless, according to it, the radiation of light and heat consists also in purely material processes, in a sort of motion, in the vibrations of ponderable resisting substances. Quiescence is darkness and death; motion is light and life.

An undulating motion proceeding from a point or a plane and excited in an unlimited medium, cannot be imagined apart from another simultaneous motion, a translation of the particles themselves; it therefore follows, not only from the emission, but also from the undulatory theory, that radiation continually diminishes the mass of the sun. Why, nevertheless, the mass of the sun does not really diminish has already been stated.

The radiation of the sun is a centrifugal action equivalent to a centripetal motion.

The calorific effect of the centrifugal action of the sun can be found by direct observation; it amounts, according to Chap. III, in one minute to 12,650 millions of cubic miles of heat, or 5.17 quadrillions of units of heat. In Chapter IV it has been shown that one kilogram of the mass of an asteroid originates from 27.5 to 55 millions of units of heat; the quantity of cosmical masses, therefore, which falls every minute into the sun amounts to from 94,000 to 188,000 billions of kilograms.

To obtain this remarkable result, we made use of a method which is common in physical inquiries. Observation of the moon's motion reveals to us the external form of the earth. The physicist determines with the torsion-balance the weight of a planet, just as a merchant finds the weight of a parcel of goods, whilst the pendulum has become a magic power in the hands of the geologist, enabling him to discover cavities in the bowels of the earth. Our case is similar to these. By observation and calculation of the velocity of sound in our atmosphere, we obtain the ratio of the specific heat of air under constant pressure and under constant volume, and by the help of this number we determine the quantity of heat generated by mechanical work. The heat which arrives from the sun in a given time on a small surface of our globe serves as a basis for the calculation of the whole radiating effect of the sun; and the result of a series of observations and well-founded conclusions is the quantitative determination of those cosmical masses which the sun receives from the space through which he sends forth his rays.

Measured by terrestrial standards, the ascertained number of so many billions of kilograms per minute appears incredible.

This centrifugal motion is perhaps the cause of the repulsion of the tails on comets when in the neighborhood of the sun, as observed by Bessel.

AM. JOUR. SCI.-SECOND SERIES, VOL. XXXVII, No. 110.-MARCH, 1864.

This quantity, however, may be brought nearer to our comprehension by comparison with other cosmical magnitudes. The nearest celestial body to us (the moon) has a mass of about 90,000 trillions of kilograms, and it would therefore cover the expenditure of the sun for from one to two years. The mass of the earth would afford nourishment to the sun for a period of from 60 to 120 years.

To facilitate the appreciation of the masses and the distances occurring in the planetary system, Herschel draws the following picture. Let the sun be represented by a globe 1 metre in diameter. The nearest planet (Mercury) will be about as large as a pepper-corn, 3 millimetres in thickness, at a distance of 40 metres. 78 and 107 metres distant from the sun will move Venus and the Earth, each 9 millimetres in diameter, or a little larger than a pea. Not much more than a quarter of a metre from the Earth will be the Moon, the size of a mustard seed, 21 millimetres in diameter. Mars, at a distance of 160 metres, will have about half the diameter of the Earth; and the smaller planets (Vesta, Hebe, Astrea, Juno, Pallas, Ceres, &c.), at a distance of from 250 to 300 metres from the sun, will resemble particles of sand. Jupiter and Saturn, 560 and 1000 metres distant from the centre, will be represented by oranges, 10 and 9 centimetres in diameter. Uranus, of the size of a nut 4 centimetres across, will be 2000 metres; and Neptune, as large as an apple 6 centimetres in diameter, will be nearly twice as distant, or about half a geographical mile away from the sun. From Neptune to the nearest fixed star will be more than 2000 geographical miles.

To complete this picture, it is necessary to imagine finely divided matter grouped in a diversified manner, moving slowly and gradually towards the large central globe, and on its arrival attaching itself thereto; this matter, when favorably illuminated by the sun, represents itself to us as the zodiacal light. This nebulous substance forms also an important part of a creation in which nothing is by chance, but wherein all is arranged with Divine foresight and wisdom.

[To be continued.]

ART. XVI.-Second Notice of Recent Researches relating to Nebulæ; by A. GAUTIER.'

Labors of Lord Rosse; First Memoirs.-Since 1827 Lord Rosse has been engaged in the construction of large specula for astronomical telescopes. In 1839 he finished his first telescope, which had a speculum three feet in diameter with a focal distance of 27 feet, and he described the method of constructing it in a me

› Translated for this Journal from the Bibliothèque Universelle, for June, 1863.

moir which was inserted in the Philosophical Transactions of the Royal Society of London for the year 1840. Afterwards, he undertook to produce specula of larger dimensions, and in 1844 he completed two, six feet in diameter, with a focal distance of 56 feet, for the employment of which he constructed a huge telescope, which he erected in the open air, between two walls to which it was attached, near his country seat at Birr (Castle) or Parsonstown in Ireland, twenty-five leagues southwest of Dublin.

Lord Rosse has been for several years President of the Royal Society of London. In 1850 he published in the Transactions of this ancient and illustrious Society, his first memoir of 15 pages quarto upon his observations of nebulæ, accompanied with four plates representing seventeen of these celestial objects. Before noticing his second memoir upon the same subject, presented to the same Society in 1861, I will mention some details extracted from the preceding memoir.

In the memoir of 1850, Lord Rosse first pointed out the spiral form which he had discovered in many nebulæ, a fact of great importance as throwing new light upon the constitution of those celestial systems.

The beautiful nebula, No. 51 of Messier's catalogue, (No. 1622 of the catalogue published by Sir John Herschel, in the Phil. Trans. for 1833,) is situated in the constellation Canes Venatici, near Bootes, in about 13h 23m of right ascension and 48° of north declination. It had been described by Messier as a double nebula containing stars; by Sir William Herschel as a brilliant nebula surrounded at a little distance by a sort of halo or glory, and having a companion near. Sir John Herschel had further observed the division of the southwest border of the annulus. into two branches. Lord Rosse, in 1845, was the first to dis cover the spiral structure forming many circumvolutions without curves returning regularly upon themselves. The sketch which he gave from many observations with the telescope of six feet diameter presents a dozen distinct circumvolutions more or less extended.

"We see," said this astronomer, "that every step of progress in optics exhibits a more complicated structure in this nebula, and it is more and more difficult to explain it by the dynamical laws which prevail in our solar system. The second nebula is evidently connected with the larger one, but the form of the nebula being such as is represented in the figure, this connection increases the difficulty of conceiving of any hypothesis to explain it. It appears in the highest degree improbable that such a system exists without interior movement. It is possible to unite to this idea that of a resisting medium, but the supposition of an equilibrium purely statical is not admissible. Some posi

tive measurements, whether of changes of brightness, or of form, or of variations of position, will therefore be most highly interesting, but they present great difficulties." Mr. Johnston Stoney, whom Lord Rosse associated with himself in these observations, meanwhile, in the spring of 1849 and 1850, made micrometric measurements, assigning in the present memoir the relative positions of different stars situated in the nebula, No. 51 of Messier, as referred to its central nucleus. The nebula No. 99 of Messier, (No. 1173 of Sir John Herschel's catalogue,) situated about 12h 10m of right ascension and 15° of northern declination, has also given opportunity for some similar measurements. This is the second nebula in which Lord Rosse has shown a very distinct spiral structure. He has described also, in his memoir of 1850, twelve fainter nebulæ of the same class, and he surmised that some others are of the same kind. He described and figured in this memoir five new annular nebulæ, in addition to the two already contained in the catalogue of Sir John Herschel; also some more stars called nebulous, and other nebulæ of an elongated lenticular form, three of which are represented in the plates attached to this memoir.

The Latest Memoir of Lord Rosse.-The new memoir presented by Lord Rosse to the Royal Society of London in June 1861, and which appeared in 1862, in Part III of the Philosophical Transactions for 1861, consists of 65 pages quarto. It is accompanied by seven plates, one of which is devoted to the telescope of six feet aperture, while the others represent 43 nebulæ. This memoir is the result of seven years' observations made with his great telescope; but, as Lord Rosse remarks, in a climate so foggy as Ireland the labor of a year, measured by the number of hours in which it is possible to make good observations of nebulæ, is not considerable.

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Here," he says, "in winter celestial objects are usually most distinct, and the sky is the darkest before eleven o'clock at night; the sky however soon becomes luminous and the details of the nebula which are less distinct disappear. In spring and autumn the change of light is not so prompt nor so decided, but the nights are shorter. Guided by the admirable catalogue of Sir John Herschel (containing the positions and a summary description of 2306 nebula) we have examined nearly all the more brilliant nebulæ known, with the exception of a few in the vicinity of the pole, and have observed a great part of the fainter ones. We have not especially sought for new nebulæ, but yet meanwhile many such have been accidentally discovered in the immediate neighborhood of nebulæ already well known, but they are for the most part very faint objects presenting little interest. In all cases where any peculiarity has been discovered, such, for example, as a spiral curvature, lines or dark spaces, a sketch has been made, and the most remarkable objects have been submitted to a detailed examination, on favorable nights, sometimes with the aid of a micrometer. In our eminently variable climate, when we em

ploy high magnifying powers and large apertures, vision is more or less altered, either by the tremor of the air, or by the vapor, and the state of the air varies enormously, in both these respects, from one night to another, and even from hour to hour. Neither is the performance of the speculum uniform. Sudden alterations of temperature in this humid climate produce deposits of dew, and the speculum gradually becomes tarnished. It is possible to remedy this by artificial heating, but that would produce other difficulties and we have not had recourse to that expedient. It is thus difficult to say that any celestial object has been examined under circumstances altogether favorable. Nevertheless as it is not probable that we should be able to add much to the details already obtained with the nebulæ, unless it be under conditions of the atmosphere particularly advantageous and very rare, I have decided not to defer longer the presentation of this memoir."

The author begins with numerous details of the processes adopted for casting and polishing large specula as well as the method adopted for mounting them. As I have previously had occasion to mention it in my Notice of 1845, I will not follow out this subject, which has more interest and importance for makers of instruments of this kind than for the public.

I will merely mention that the specula made by Lord Rosse are formed of an alloy of a little more than two parts of copper to one part of tin, the specific gravity of the alloy being 8.8. The mirrors of three feet in diameter weigh about 1200 French pounds, and those of six feet diameter weigh 4 tons or 8000 pounds. The mounting is not equatorial and the telescope does not move very much from the meridian. In order to be able to use it constantly it is convenient to have two specula to be used alternately. Lord Rosse has made use of the Newtonian form, employing a small reflecting mirror, inclined at an angle of 45°, which permits the observer to place himself at one side of the instrument with an eyepiece. He estimates that a linear magnifying power of about 1300 times is the largest that can generally be used with advantage in his telescopes, for the observation of nebulæ; but he has occasionally made use of a power of more than 2000 times for discovering small stars with the telescope of 3 feet aperture. On some occasions the 6 feet reflector has admitted even more than this; but in the climate of Ireland those occasions are rare and of short duration. The author thinks that telescopes of even greater dimensions may be constructed and employed with advantage in favorable climates, for studying the details of nebulæ of feeble light, as well as to recognize a great nuinber that are double or multiple. He also thinks that it would be well to employ silver for the second reflection. Lord Rosse says he has often experienced much difficulty in choosing between numerous observations, in view of the uncertainty which sometimes exists in regard to the reality of a fact, presented sometimes in one way and sometimes in another, accord