The observer has then to read the heads of all these seven microscopes on the pier side, and also two positions of the horizontal wire on the screw-head at the eye-piece. The following morning he will also read off from the chronograph-sheet the times when he made the ten taps as the star passed each of the ten vertical wires. There are, therefore, nine entries to make for one position of a star in declination, and ten for one position of a star in right ascension. The observer will also have to read the barometer to get the pressure of the air at the time of observation, and one thermometer inside the transit room, and another outside, to get the temperature of the air. In some cases thermometers at different heights in the room are also read. A complete observation of a single star means, therefore, the entry of two-and-twenty different numbers. It may be asked, What is the use of reading the barometer and thermometer? The answer to the question can only be given by contradicting a statement made above, that the true pole lay midway between the position of the telescope when pointing to the pole-star at its upper transit, and its position when pointing to it at its lower transit. The pole being very high in the heavens in this country, there are a great number of stars that, like the pole-star, cross the meridian twice in the twenty-four hours--once when they pass above the pole, moving from east to west, once when they pass below it, moving from west to east As the real distance of a star from the true pole does not alter, it follows that we ought to get the position of the pole from the mean of the two transits of any of these stars, and they ought all to exactly agree with each other. But they do not. So, too, I said that the stars all appeared to move as in a single piece. If, then, we constructed an instrument with its axis parallel to the axis of the earth, and fixed a telescope to it, pointing to any particular star, if we turn the telescope round as fast from east to west as the earth itself is turning from west to east--if we built an equatorial, that is to say--we ought to find that the star once in the centre of the field would remain there. As a matter of fact, when the star got near the horizon it would soon be a long way from the centre of the field. Sir George Airy, the seventh Astronomer Royal, makes, with reference to this very point, the following remarks: 'Perhaps you may be surprised to hear me say the rule is established as true, and yet there is a departure from it. This is the way we go on in science, as in everything else; we have to make out that something is true, then we find out under certain circumstances that it is not quite true; and then we have to consider and find out how the departure can be explained.'