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of the astronomer, who wants to know the time down to the tenth or even to the hundredth of a second. Moreover, no meridian circle can have its axis set so exactly east and west that the instrument shall not deviate a little from the meridian. The astronomer must therefore make allowances for the error of his clock and for the deviation of his instrument; and these require much careful observation and calculation. Even when he does the best he can, a single observation will always be liable to little errors which he wishes to make as small as possible. He does this by repeatedly determining the position of every star which he puts upon his list. He generally has to be satisfied with three or four observations on the great mass of the stars, but on the more important stars he makes them by scores or hundreds.

To determine the declination of a star, a graduated circle is necessary. This consists of a brass or steel circle, much like a carriage wheel, of which the axis is the same as that on which the telescope of the meridian instrument turns. The circle is firmly attached to the axis so that it must turn with the telescope as the latter sweeps along the celestial meridian. The graduations of the circle consist of very fine marks or lines all round its circumference. The latter being divided into three hundred and sixty degrees, every degree is marked by such a line. Between these it is common to mark thirty intermediate lines, which are therefore two minutes apart. Attached to one or both the stone piers which support the instrument are four microscopes, so fixed that the graduations on the circle are seen through them. When