Page images

observation, and were even, by astronomers, confounded with the millions of stars of the same apparent magnitude that occupy almost every point of the heavens. From their smallness it follows, that they have no sensible effect in disturbing the motions of the planets already known. Their orbits are considerably eccentric; and the plane of one of them has an inclination to the ecliptic greater than the inclinations of all the other planetary orbits put together. This great inclination and eccentricity will render the calculation of the disturbances produced in the motion of these bodies by the larger planets, (particularly of Jupiter and Mars, between which they are situated,) a matter of considerable difficulty, and may be the occasion, as Delambre remarks, of extending the science of analysis beyond its present bounds.

The first of these planets was discovered by Piazzi at Palermo, the third by M. Harding, the two others by M. Olbers of Bremen. The astronomer last named is of opinion, that these small bodies are the fragments of one large planet which an explosion, from some unknown cause, has burst in pieces; and hence he concludes, that all their orbits ought to cut one another in two opposite points of the heavens, which he found, by calculation, to be, one near the constellation Virgo, and the other near the Whale; and that, of course, they must pass through these points twice in every

[blocks in formation]

revolution; so that, in order to discover all the fragments, astronomers ought to examine these two places of the heavens very frequently. In effect, all the four have been found near these points; and the two last, after Olbers had suggested the idea just mentioned.

Since the year 1789, seventeen comets have been discovered; and, along with the names of Messrs Mechain, Olbers, &c. by whom they were observed, we are glad to see the name of Miss Herschel. The orbits of all these comets have been calculat ed. The comet that appeared so remarkable in the autumn of 1807, is thought by Olbers to revolve in a very eccentric ellipse, and to have a periodic time of no less than 1900 years.

Delambre concludes this article with Dr Herschel's description of the heavens, the double, triple, quadruple, and nebulous stars, together with those that have disks like planets, in some cases round, in others irregular. The discovery of the revolution of Saturn's ring by the same excellent astronomer, is also mentioned, as also the coincidence of the time of that revolution with the theory of gravity, and the prediction of Laplace. The observations of Dr Herschel on the figure of Saturn himself are not mentioned.

A rumour prevailed for some time, that Piazzi had discovered the parallax of the fixed stars; but as M. Delambre makes no mention of a discovery,

which, if real, would be no doubt one of the greatest in astronomy, we must suppose that M. Piazzi's observations have not yet led to any satisfactory result. The notes mention a work, founded mostly on Dr Herschel's observations, by Schroeter of Amsterdam, on the dimensions of the universe: it was crowned by the Royal Society of Haerlem in 1802; it cannot fail to be highly interesting; and we very much regret that it has not yet reached this country.

The next general head is that of Physique Mathématique, or what we would call experimental philosophy. Delambre begins with remarking, "That the revolution recently brought about in chemistry, could not happen without turning many experimentalists a little out of their ordinary course, when they saw in a neighbouring science, a road opened that promised more numerous discoveries. We shall nevertheless find, in the mathematical branch of physics, some curious researches and interesting inventions."

Among these, one of the most remarkable is the Balance of Torsion; which, by the twisting and untwisting of a thread or wire, affords a measure for forces that are too small to be appreciated by any other means. It was with this that Coulomb was so successful in determining the law of electric attraction and repulsion, and afterwards in showing that the phenomena of magnetism follow a law altogether similar, namely, the inverse of the square

of the distance. By help of the same instrument, he was able to measure the smallest effects of magnetism ; to find the temperature (considerably elevated) at which these effects entirely disappear; and to show that magnetism is not, as has been generally supposed, a property peculiar to certain bodies, but one that exists in all, even in those that appear most devoid of it. The same balance enabled him to measure the resistance which fluids oppose to motion, the law of which resistance is expressed by two terms, of which Newton found out only the first, the second not being sensible except in motions that are extremely slow.

The instrument by which Mr Cavendish determined the gravitation of balls of lead toward one another, is, as Delambre observes, no other than Coulomb's Balance, executed on a large scale. Mr Cavendish found from his experiments, that the mean density of the earth is five times and a half as great as that of water.

Here, however, we must be permitted to observe, that though Mr Cavendish's Balance does not differ in principle from that of the excellent experimenter quoted by Delambre, it was not copied from it. The experiments of Mr Cavendish were indeed made about the year 1798; and the first experiments of Coulomb with his balance are published as early as the year 1785, if we mistake not. The instrument that Mr Cavendish employed had, however, been invented before that pe

riod by the Rev. Mr Mitchell, F. R. S., and was purchased at his sale by Mr Cavendish. We are to consider the instrument, therefore, as originally an English invention, and re-invented in France by Coulomb, without any knowledge whatever of what was done in England.

We cannot help remarking too, when we reflect on the results obtained from this instrument in the hands of the English and of the French philosopher, that the gravitation measured by the former, may have been affected by the magnetism which the latter supposed he had discovered in all bodies. The effects of the one force may have been mistaken for those of the other, and a degree of uncertainty is thrown on the determinations of both. This observation, however, we only throw out loosely perhaps an accurate comparison of the experiments might determine how much is to be ascribed to the one cause, and how much to the other it is right, however, that this source of inaccuracy should be considered.

The application of the barometer to the measurement of heights, or, more properly the formula for determining heights by help of the barometer, deduced by Laplace, is mentioned among the discoveries of the latter. Laplace used in his formula the specific gravity of mercury, as it is commonly stated. The coefficient or multiplier of the logarithmic difference which he thus obtained, was

« PreviousContinue »