But we are not at the north pole. Still we are in a position where the pole is well above the horizon, and from that fact we can determine the polar distance, although the absolute place of the pole is not pointed out by the pole star. Thus, if we suppose any star, A, Fig. 121, to be a certain distance from the pole, and the earth carrying the instrument to be in the centre of the circle H, Z, R, we can observe the zenith distance of that star, Z, A, when it transits our meridian above the pole, P; and we can then observe its distance, Z, B, when it transits below the pole; and it is clear that the difference between those two measures will give the distance A, B, or double the polar distance of that star, and the mean of the readings will give the distance, Z, P, the zenith distance of the pole, so that it is perfectly easy to determine the distance between the pole and the zenith, which, subtracted from ninety degrees, gives us the latitude of the place. It is therefore perfectly easy by means of this instrument to determine either the zenith or polar distance, and, knowing the polar distance, we get the declination, or distance from the equator, by subtracting it from ninety degrees. In our case it is the north polar distance or declination of any object in the heavens that we record; and if we take the precaution to do so with this instrument at the time given by the clock, when the object passes the meridian, we have the actual apparent place of that body in the sky; and in this way all the positions of the stars and other bodies, and their various changes, and the courses of the planets, have been determined. The transit circle is the most important instrument of astronomy, and such is the perfection of the Greenwich instrument that nothing could be more unfortunate for astronomy than that that instrument should be in any way damaged. And though many of us are admirers of physical astronomy, we have yet to find the instrument that is as important to physical astronomy as the transit circle at Greenwich is to astronomy of position. The room in which these transit circles are worked—the transit room—is required to be of special construction. A clear space from the southern horizon through the zenith to the north must at any time be available; this entails the cutting of a narrow slit in the roof and both walls, without the intervention of any beams across the room. This slit is closed by shutters or windows which are made to open in sections, so that any part of the meridian can be observed at pleasure. CHAPTER XVII. THE TRANSIT CLOCK AND CHRONOGRAPH. We have now to consider the way in which the transit instrument is used and the functions which both it and the transit circle fulfil.