Page images
PDF
EPUB

substances conducting, and not condncting electricity. It has lately been shewn by Sir R. Phillips, that there is no fluid per se, and that Electrical Phenomena arise from the decomposition of gases, whose re-union occasions the spark, report, &c.

The experiments and inquiries of Newton on the subject of optics, began in 1666, and soon made a vast addition both to the extent and importance of the science. Having admitted a beam of light into a dark chamber, through a hole in the window-shutter, and made it fall on a glass prism, he received the coloured spectrum on a board at the distance of about twelve feet from the first, and also pierced with a small hole. The coloured light which passed through this second hole was made to fall on a prism, and afterwards received on the opposite wall. It was found that the rays which had been most refracted, or most bent from their course by the first prism, were most refracted also by the second, though no new colours were produced. The true cause, therefore, of the length of the image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their in cidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall. It was also observed, that when the rays which fell on the second prism were all of the same colour, the image formed by refraction was truly circular, and of the same colour with the incident light. It was in this way that Newton measured the extent of each colour, and taking the mean of a great number of measures, he assigned the following proportions, dividing the whole length of the spectrum, exclusive of its rounded terminations, into 360 equal parts ; of these the Red occupied 45, Orange 27, Yellow 48, Green 60, Blue 60, Indigo 40, Violet 80.

Thus furnished with so many new and accurate notions concerning the nature and production of colour, be proceeded to applv them to the explanation of phenomena, and the subject which naturally offered itself the first to this analysis was the rainbow.

That two refractions and one reflection were at least a part of the machinery which nature employed in the construction of this splendid arch, had been known from the time of Antonio de Dominis ; and the manner in which the arched figure is produced had been shown by Descartes; so that it only remained to explain the nature of the colour and its distribution. As the colours were the same with those exhibited by the prism, and succeeded in the same order, it could hardly be doubted that the cause was the same. NEWTON showed the truth of his principles by caleulating the extent of the arch, the breadth of the coloured bow, the position of the secondary bow, its distance from the primary, and by explaining the inversions of the colours,

The theory from which the explanation of Refraction is deduced is, that light is an emanation of particles, moving in straight lines with incredible velocity, and attracted by the particles of transparent bodics. When, therefore, light falls obliquely on the surface

[ocr errors][ocr errors][ocr errors]

of such a body, its motion may be resolved into two, one parallel to that surface, and the other perpendicular to it. Of these, the first is not affected by the attraction of the body, which is perpendicular to its own surface; and, therefore, it remains the same in the refracted that it was in the incident ray. But the velocity perpendicular to the surface is increased by the attraction of the body, and, according to the principles of dynamics, whatever be the quantity of this velocity, its square, on entering the same transparent body, will always be augmented by the same quantity. But if there be two right-angled triangles, with a side in the one equal to a side in the other, the hypothenuse of the first being given, and the squares of their remaining sides differing by a given space, the sines of the angles opposite to the equal sides must have a given ratio to one another. This amounts to the same with saying, that, in the case of refraction the sine of the angle of incidence is to the sine or the angle of refraction in a given ratio.

Newton, after considering the reflection and refraction of light, proceeded to treat of its inflexion. Having admitted a ray of light through a hole in a window-shutter into a dark chamber, he made it pass by the edge of a knife, or, in some experiments, between the edges of two knives, fixed parallel, and very near to one another; and, by receiving the light on a sheet of paper at different distances behind the knives, he observed the coloured fringes which had been described by GRIMALDI, the Italian optician, and, on examination, found, that the rays had been acted on in passing the knife edges both by repulsive and attractive forces, and had begun to be so acted on in a sensible degree when they were yet distant by, ooo of an inch of the edges of the knives. The experiment enabled him to draw this conclusion, that the path of the ray in passing by the knife edge was bent in opposite directions, so as to form a serpentine line, convex and concave toward the knife, according to the repulsive or attractive forces which acted at different distances,

The attention of Newton was called to the subject of gravitation, by a letter from Dr. Hooke, proposing, as a question, to determine the tine in which a body let fall from a height descends to the ground, taking into consideration the motion of the earth on its axis. This induced him to resume the subject of the moon's motion, which he had before partially considered ; and the measure of a degree by Norwood having now furnished more exact data, he found that his calculation gave the precise quantity for the moon's momentary deflection from the tangent of her orbit, which wss deduced from astronomical observation. The moon, therefore, has a tendency to descend toward the earth from the same cause that a stone at its sur. face has; and if the descent of the stone in a second be diminished in the ratio of 1 to 3600, it will give the quantity by which the moon descends in a second, below the tangent to her orbit, and thus is obtained an experimental proof of the fact, that gravity decreases,

a

as the square of the distance increases. He then found, that the three great facts in astronomy, which form the laws of Kepler, gave the most complete evidence to the system of gravitation. The first of them, the proportionality of the areas described by the radius vector to the times in which they are described, is the peculiar character of the motions produced by an original impulse impressed on a body, combined with a centripetal force continually urging it to a given centre. The sscond law, that the planets describe ellipses, having the sun in one of the foci, common to them all, coincides with this proposition, that a body under the influence of a centripetal force, varying as the square of the distance inversely, and having any projectile force whatever originally impressed on it, must describe a conic section having one focus in the centre of force, which section, if the projectile force does not exceed a certain limit, will become an ellipse. The third law, that the squares of the periodic times are as the cubes of the distances, is a property which belongs to the bodies describing elliptic orbits under the conditions just stated.

But, says Professor Playfair, did the principle which appeared thus to unite the great bodies of the universe act only on those bodies ? Did it reside merely in their centres, or was it a force common to all the particles of matter? Was it a fact that every particle of matter had a tendency to unite with every other? Or was that tendency directed only to particular centres ? It could hardly be doubted that the tendency was common to all the particles of matter. The centres of the great bodies had no properties as mathematical points, they had none but what they derived from the material particles distributed around them. But the question admitted of being brought to a better test than that of such general reasoning as the preceding. The bodies between which this tendency had been observed to take place were all round bodies, and either spherical or nearly so, but whether great or small, they seemed to gravitate toward one another according to the same law. This alleged principle has, however, been often controverted; and within this year (1821) Sir RICHARD PHILLIPS has published a series of Essays, in which he asserts that the phenomena of planetary aggregation are owing to the two-fold motions, which direct all their parts towards their centres : that the orbicular motions of the planets arise from the sun's action on the elastic medium of space, diffused inversely as the squares of the distances; that the rotatory motions arise from the unequal action of the same medium on the near and remote extremities of planetary axes; that heat is atomic motion; and, in fine, that all force, and all phenomena arise universally from matter affected by or multiplied into motion. Matter multiplied by motion, says Sir Richard, produces force, change, or phenomena, and not matter multiplied by attraction, or by repulsion, or by gravitation; and it is therefore the business of philosophers to trace the special motions which in every case unite with matter and produce particular phenomena.

a

Newton now perceived that, in the earth, another force was combined with gravity, and that the figure resulting from that combination could not be exactly spherical

. The diurnal revolution of the earth, he knew, must produce a centrifugal force, which would act most powerfully on the parts most distant from the axis. The amount of this centrifugal force is greatest at the equator, and being measured by the momentary recess of any point from the tangent, which was known from the earth's rotation, it could be compared with the force of gravity at the same place, measured in like manner by the descent of a heavy body in its first moment of its fall.

The precession, that is, the retrogradation of the equinoctial points, had been long known to astronomers; its rate had been measured by a comparison of ancient and modern observations, and found to amount nearly to 50° annually, so as to complete an entire revolution of the heavens in 25,920 years. The honour of assigning the true cause of this phenomenon was reserved for the most cautions of philosophers. He was directed to this by certain analogy observed between the precession of the equinoxes and the retrogradation of the moon's nodes, a phenomenon to which his calculus had been already successfully applied. The spheroidal shell or ring of matter which surrounds the earth, in the direction of the equator, being one half above the plane of the ecliptic and the other half below, is subjected to the action of the solar force, the tendency of which is to make this ring turn on the line of its intersection with the ecliptic, so as ultimately to coincide with the plane of that circle. This, accordingly, would have happened long since, if the earth had not revolved on its axis. The effect of the rotation of the spheroidal ring from west to east, at the same time that it is drawn down toward the plane of the ecliptic, is to preserve the inclination of these two planes unchanged, but to make their intersection move in a direction opposite to that of the diurnal rotation, that is, from east to west, or contrary to the order of the signs.

By an analysis also of the force of gravity, Newton explained those inequalities in the elevation of the waters of the ocean to which we give the name of tides. The motion of Comets yet remained to be discussed. Newton shewed that the orbit of the comet must be a conic section, having the sun in one of its foci, and might either be an eclipse, a parabola, or even a hyperbola, according to the relation between the force of projection and the force tending to the centre.

For more than thirty years after the publication of these discoveries, the system of vortices kept its ground, and a translation from the French into Latin of the Physics of Rohault, a work entirely Cartesian, continued at Cambridge to be the text for philosophical instruction. About the year 1718, a new and more elegant translation of the same book was published by Dr. Samuel Clarke, with the addition of notes, in which that profound and ingenious writer explained the views of Newton on the principal objects of discussion, so that the notes contained virtually a refutation of the text; they

a

a

did so, however, only virtually, all appearance of argument and controversy being carefully avoided. Whether this escaped the notice of the learned Doctors or not is uncertain, but the new translation, from its better Latinity, and the name of the editor, was readily ada mitted to all the academical honours which the old one had enjoyed. Thus, the stratagem of Dr. Clarke completely succeeded, and the Newtonian philosophy first entered the university of Cambridge under the protection of the Cartesian.

The Aberration of the fixed stars is the discovery of Dr. BRADLEY ; he and his friend MOLYNEUUX, in the end of the year 1725, were occupied in searching for the parallax of the fixed stars by means of a zenith sector. The sector was erected at Kew; it was of great radíus, and furnished with a telescope twenty-four feet in length, with which they proposed to observe the transits of stars near the zenith, according to a method that was first suggested by HOOKE, and pursued by him so far as to induce him to think that he had actually discovered the parallax of y Draconis, the bright star in the head of the dragon, on which he made his observations. They began their observations of the transits of the same star on the 3d of December, when the distance from the zenith at which it passed was carefully marked. By the observations of the subsequent days the star seemed to be moving to the south; and about the beginning of March, in the following year, it had 20% to the south, and was then nearly stationary. In the beginning of June it had come back to the same situation where it was first observed, and from thence it continued its motion northward till September, when it was about 20" north of the point where it was first seen, its whole change of declination having amounted to 40". This motion occasioned a deal of surprise to the two observers, as it lay the contrary way to what it would have done if it had proceeded from the parallax of the star. The repetition of the observations, however, confirmed their accuracy; and they were afterwards pursued by Dr. Bradley, with another sector, of a less radius, but still of one sufficiently great to measure a star's zenith distance to half a second. It embraced a larger arch, and admitted of the observations being extended to stars that passed at a more considerable distance from the zenith. It then occurred to Bradley that the appearances might arise from the progressive motion of the earth in its orbit. He saw that RÖMER's observation concerning the time that light takes to go from the sun to the earth gave a ready expression for the velocity of light compared with that of the earth. The proportion, however, which he assumed as best suited to his observations was somewhat different; it was that of 10313 to 1, which made the radius of the circle of aberration 20', and the transverse axis of the ellipse in every case, or the whole change of position, 40%. It was the shorter axis which Bradley had actually observed in the case of y Draconis, that star being very near the solstitial colure, so that its changes of declination and of latitude are almost the same. In order to show the truth of his theory, he computed

« PreviousContinue »