The telescope tube is firmly screwed at right angles to the flanged end of a massive shaft 16 inches in diameter, called the declination axis, extending through the cubical section D of the polar axis NDS, Fig. 2, through the declination sleeve E into the housing F. This declination axis is rotated, carrying the tube with it, on ball bearings in D and F, this rotation being effected by an electric motor with reduction mechanism, gearing into a large spur gear attached to the end of the declination axis, the whole being concealed within the declination housing F. Hence the tube can be turned at the rate of 45 degrees to the minute to any required position up or down, north or south. The position in the sky, the declination, corresponding to latitude on the earth, is read on a large circle graduated into degrees within F and subdivided into 5 minute intervals on the small auxiliary circle H. The Polar Axis As positions north or south are given by turning the tube on the declination axis, so positions east or west are given by rotation on the polar axis, so called because it points to the pole of the heavens and is exactly parallel to the axis of the earth. The Polar axis NDS Fig. 2, which is 21 feet long and weighs 9·5 tons, is built up of three steel castings, a central cubical section D and two conical end sections, all securely bolted together and turning in ball bearings on its ends. The upper, north, bearing is carried in an adjustable pillow block, by which the final parallelism with the earth’s axis is obtained, bolted on the curved cement pier shown at the left or north in Figs. 2 and 3. The lower, south, bearing is carried in a massive cast iron pedestal bolted to the south cement pier. The polar axis is rotated on these bearings, also at the rate of 45 degrees per minute, carrying the declination axis and tube with it to any position east or west in the sky by an electric motor and reduction gearing concealed within the south pedestal. The position east or west in the sky, the right ascension as it is called corresponding to longitude on the earth, is read by means of a graduated circle shown above G, Fig. 2, which is divided into 24 hours and each hour into single minutes. While longitudes on the earth are occasionally expressed as so many hours and minutes east or west of Greenwich, right ascensions in the sky are almost invariably given in hours and minutes rather than degrees. [Illustration: Fig. 3.--TELESCOPE FROM THE WEST] The Driving Clock It is evident, by rotation of the telescope on the declination and polar axes by means of the quick-motion motors, that the tube can be pointed in any direction in the sky, towards any star. But owing to the rotation of the earth on its axis from west to east, which is the cause of the apparent motion of sun, moon and stars from east to west, the telescope will be quickly carried eastward of the star which will only remain for an instant in the field.