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120 and constant. You may ask whether we can prevent a constellation setting in this way: of course we cannot do that: a constellation will seem to remain steady as long as you can look at it: but the mirror is being turned round by clockwork,

and in course of time you will find that it will present its edge to you, and then its back! so that you can no longer see the constellation, which takes the opportunity to set below the horizon.

When a coelostat mirror is used in this way we can fix the telescope in any position we like within certain limits. One position which very naturally occurs to us is the horizontal one. If we lay the big telescope on the ground, there is no danger of