more and more southerly at every transit over the meridian till the beginning of March, when it was found to pass twenty seconds more southerly than at the time of the first observation. About the middle of April it appeared to be returning towards the north, and at the beginning of June it passed the meridian at the same distance from the zenith as in December, when it was first observed. From that time it appeared more and more northerly at every transit till September following, being then near twenty seconds more northerly than in June, and no less than thirty-nine seconds more northerly than in March. From September the star returned towards the south, till it arrived, in December, at the same situation in which it was found a twelvemonth before.

The result of these observations, so different from what was expected, was a matter of great surprise to the observers ; for it appeared that the star was thirty-nine seconds more northerly in September than in March, just the contrary to what it ought to appear by the annual parallax of the stars. This may be illustrated by the opposite figure:

Let A B C D represent the orbit of the earth, and A and C the place of the earth at two opposite periods of the year; then a fixed object at E will be seen from the earth at A, in the line A E, which will point out its apparent place at G in the concave expanse of the sky. But at the opposite period of the year it will be seen from the earth at C, in the line C E, which will project its place in the heavens at F; so that, while the earth has passed from A to C, the object will appear to have moved from G to F, through the space G F, provided there be any sensible parallax. Now, in the case of the observations stated above, the observers who in September saw the star at F, did in March following observe it at K, in the right line A K, parallel to C F, and not at G, where it ought to have appeared by the parallactic motion; so that, instead of finding a parallax, they found a result directly opposite to what they expected, which exceedingly perplexed the observers, and one of them, Mr. Molyneux, died before the true cause of it was discovered.

Some time afterward, Dr. Bradley repeated the same observations with an instrument of great accuracy, to which was appended a telescope twelve and a half feet long. With this instrument, which was so nicely adjusted that he could depend upon it even to half a second, he continued his observa

tions for more than two years, not only on the bright star in Draco above alluded to, but on many other stars, and always F

In

observed the same appearances and arrived at the same results. At last, after many reflections and conjectures on the subject, he arrived at the following conclusion, namely, that the phenomena he had observed was owing to "the progressive motion of light, and the sensible proportion which its velocity bears to the velocity of the annual motion of the earth." other words, that the motion of light, combined with the progressive motion of the earth in its orbit, causes the stars to be seen in a different position, from what they would be if the eye were at rest. This position, after it was explained and demonstrated, was considered as one of the most brilliant discoveries which had been brought to light during the last century. It agrees with the velocity of light which had been deduced from the eclipses of Jupiter's satellites, and it amounts to a sensible demonstration of the annual motion of the earth. The observations which led to this discovery likewise prove the immense distance of the stars from the earth; for Dr. Bradley assures us, from the accuracy with which they were conducted, that if the annual parallax had amounted to so much as one second, he should have discovered it.

:

star.

If, then, the greatest annual parallax of the nearest stars does not amount to one second, their distance must be immense. Supposing the parallax to be exactly one second, the distance of a star having this parallax will be found by the following trigonometrical proportion: As the sine of 1" is to radius: so is the semidiameter of the earth's orbit to a fourth number, which expresses the distance of the Now a parallax of one second determines the object to be 212,000 times farther from the earth than is the sun. The distance of the sun is 95,000,000 of miles, which, multiplied by 212,000, produces 20,140,000,000,000, or more than twenty billions of miles. This distance is absolutely certain : it follows, as a matter of course, if the annual parallax were determined to be one second. It is the very least distance at which any of the fixed stars can be situated from our globe; but as the parallax does not amount to this quantity, their distance must be much farther than what is here stated, perhaps not less than double or treble that distance. We may acquire some faint idea of the immense distance stated above by considering that a cannon ball, flying with uniform velocity 500 miles every hour, would require four millions, and five hundred and ninety-five thousand years before it could reach an

object at the distance we have stated. Such are the ample and inconceivable dimensions of the spaces of the universe.

Several other methods have been resorted to by astronomers, in order, if possible, to determine the distance of the stars, but most of them are founded upon assumptions which have not yet been proved. The celebrated Huygens, as recorded in his "Cosmotheoros," despairing of being able to find an annual parallax, resorted to the following method: supposing that the star Sirius, one of the brightest fixed stars in the heavens, to be equal in lustre and magnitude to the sun, he endeavoured to diminish the apparent diameter of the sun to the eye, so that it should appear no larger or brighter than Sirius appears to a common observer. For this purpose he closed one end of a twelve feet tube with a very thin plate, in the middle of which he made so small a hole, that a very minute glass globule being put into it, so very small did the sun appear to the eye placed at the other end of the tube, that the light transmitted to the eye seemed not more splendid than that which we behold transmitted from Sirius with the naked eye. Having calculated, on the principles of optics, the quantity of diminution of the sun's apparent diameter, he found it to be only the 1-27664th part; or, the light and diameter of the sun appeared 27,664 times smaller than what we daily see. Hence he concluded that, were the sun at 27,664 times his present distance from us, he would appear as small as Sirius; and, consequently, if Sirius be of the same magnitude as the sun, the distance of that star must be 27,664 times greater than the distance of the sun from the earth, or 2,628,080,000,000; that is, two billions, six hundred and twenty-eight thousand and eighty millions of miles. This method of determining the distance of the stars depends upon two assumptions: 1st, that the sun and Sirius are equal in magnitude; and, 2d, that the eye judged correctly of the equality of the small intercepted portion of the sun to Sirius; both of which must be considered as uncertain. But it corroborates the. general position of the very great distance of the stars.

"This

On a principle somewhat similar, but by experiments conducted with far greater accuracy, Dr. Wollaston endeavoured to determine the same problem in relation to the stars. gentleman," Sir J. Herschel remarks, "by direct photometrical experiments, open, as it would seem, to no objections, has ascertained the light of Sirius, as received by us, to be to that

of the sun as 1 to 20,000,000,000. The sun, therefore, in order that it should appear to us no brighter than Sirius, would require to be removed to 141,400 times its actual distance. We have seen, however, that the distance of Sirius cannot be so small as 200,000 times that of the sun. Hence it follows that, upon the lowest possible computation, the light really thrown out by Sirius cannot be so little as double that emitted by the sun; or that Sirius must, in point of intrinsic splendour, be at least equal to two suns, and is, in all probability, vastly greater.".

The late Sir William Herschel proposed another method of determining the annual parallax by means of double stars, which he supposed would be free from the errors of other methods, and of such a nature that the parallax, even if it should not exceed the tenth part of a second, may still become visible. The following figure and description will convey a general idea of this method:

Fig. 8.

D

E

B