battery, and that a shot is fired at the ship, and I remarked that the direction which the shot takes through the ship is not a direction exactly corresponding to that in which it is fired, but has an inclined direction, which inclination depends upon this, that after the shot has entered the first side of the ship, and before it comes out at the second side, the ship has advanced sensibly. The magnitude of the inclination depends therefore on the proportion of the velocity of the ship to the velocity of the shot. From this it is plain that if we know the extent to which the apparent direction of that line of the motion of the shot was changed when passing through the ship, we shall have the means of computing the proportion of the velocity of the ship to the velocity of the shot. Now this is a case strictly analogous to the motion of light. The earth is travelling along, and whilst it is so travelling along, light comes upon it from different objects, for instance from the stars. And the effect is the same as in the case of the ship; that in consequence of this motion of the earth, the light appears to come, not from the real place of the star, but from an ideal place of the star, which is in advance, as estimated by the direction of the earth's motion. If we know in what direction the earth is moving, the light of the star appears to come from a point more in that direction than it should.

I then endeavoured to point out to you the influence which this would have on the apparent places of the stars. We have an earth revolving in an orbit round the sun. The place of the star then will not appear always the same, but will always be found in a circle, whose centre is the true place of the star, the line from the true place to the apparent place being always in the direction in which the earth is moving. If

we can observe the star in different seasons of the year, we can infer from our observations how much the place of the star is perverted by this effect of aberration; we shall see how much the apparent path of the light is inclined to the true path of the light, as in the analogous instance of the breach made through the ship. Thus we have the means of comparing the velocity of the ship with the velocity of the shot, or the velocity of the earth with the velocity of light. And the result of the observation is this: that the place of the star is disturbed one way or the other in different directions at different seasons of the year, twenty seconds and one-third. The inference from this is, that the velocity of light is ten thousand times as great as the velocity of the earth in its orbit. The velocity of light is perhaps the most inconceivable of all things; the velocity is so enormous, 200,000 miles in a second.

But these are things which we must often look at with suspicion. What I have stated seems at first an indirect way of getting at these results. Even by a person properly conversant with these matters, such results are hardly received without additional confirmation. There are phenomena which give confirmation, which I will now explain. Jupiter has four satellites. Their orbits are, in proportion to his diameter, comparatively small. Our moon is at such a distance from the earth that she is not eclipsed very often; her distance being about thirty times the breadth of the earth. Jupiter's satellites are comparatively close to him; so close that three out of the four are eclipsed every time they go round.

On

watching the appearances of Jupiter, one of the most remarkable things observed is, the eclipses of the satellites, (first seen by Galileo). When the earth is

in one position with respect to Jupiter, we see the satellites go into the shadow; that is, we see them disappear without any apparent cause. In another position we see them come out of the shadow; that is, we see them begin to appear in the dark space a short distance from Jupiter. Figure 55 is adapted

at

E

FIG. 55.

to the supposition that the satellites are coming out of the shadow. Suppose that C is the sun; E', E", the earth in two positions; J', J", Jupiter, in two corresponding positions. The time which is most favourable for the observation of Jupiter's satellites is that when the earth is nearly between the sun and Jupiter, as at E', because then Jupiter is seen nearly the whole night. In a short time after the invention of telescopes, Galileo and other astronomers observed the satellites, and found that their eclipses could be observed with great accuracy, and registered them with great care. They were able in no long time to form tables and calculations of the eclipses of Jupiter's satellites. These occurred principally when the earth

and Jupiter were in such a position as E'J'. The earth went travelling on in its orbit, and came to such a position as E". Jupiter, who is very slow in his motions, travelled perhaps as far as J" in his orbit. And now came the remarkable thing: it was found that, when the earth came to such a position as E", the tables and preliminary calculations upon which had been founded the predictions of the eclipses of the satellites would not apply. The eclipses of the satellites invariably occurred later than they ought to have done.. This occurred year after year, and it was a long time before people could guess at the cause. Every time the earth came to that part of its orbit in which it is nearest to Jupiter, the eclipses of the satellites happened as predicted: every time the earth approached the part of its orbit furthest from Jupiter, the eclipses of the satellites occurred later than predicted. At last a very celebrated man, a Dane, of the name of Römer, gave the explanation, that in these latter observations the earth was further off from Jupiter than at the time when those observations were made on which the tables and calculations were founded and therefore, the light from Jupiter had to travel over a path longer by very nearly the breadth of the earth's orbit. Upon this calculations were made, and the result was this: that the time occupied by the passage of light across the semi-diameter of the earth's orbit is 8m. 18s.; and therefore the time occupied by the passage of light across the whole breadth of the earth's orbit is 16m. 36s. Upon applying corrections, proportionably to the distance, to the observations made in other positions, it was found that they all harmonized perfectly well, and no doubt was left of the truth of the result, that the time the light occupies in travelling from the sun to the earth is 8m. 18s.

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The question for us now is this: does this determination of the velocity of light agree with the deduction made from the aberrations of the stars? We found that the light travels ten thousand times faster than the earth moves in its orbit; if the light occupy 8m. 18s. in coming from the sun to the earth, does that imply a speed ten thousand times as great as the speed of the earth? The fact is, that the two calculations, though perfectly independent, support each other with the greatest nicety; and there is no doubt of the correctness of the measure of the velocity of light.

The subject on which I then proceeded at the lecture yesterday was the measure of the distances of some of the fixed stars; and I observed in the first place, that it was necessary for me to premise these various things, namely, the explanation of precession, nutation, and aberration, and for this reason: that the apparent places of the stars are disturbed by them to a very sensible degree, both in right-ascension and in North Polar distance; and that the very utmost accuracy is necessary in everything relating to the observations upon which the measure of the distances of the stars are to be founded.

I assume that we now know the meaning of the term "parallax." Between the apparent places of the moon, as seen at one point of the earth and as seen at another point of the earth, there may be a difference of a degree and half, or more. Now when we have a degree and half of difference, an error of a second is of no particular consequence. The parallax of the sun, as found in the way described in a former lecture, is a very much smaller quantity, between 8 and 9 seconds; that is, there is a difference of 8 or 9 seconds in the sun's places, as seen at the centre of the earth and on the

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