Page:Scientific Memoirs, Vol. 2 (1841).djvu/48

36 when measuring the duration of vibrations, to make the commencing arc no greater than 2 or 3 degrees, and in observing changes of direction, to make the arc as small as possible, never allowing it to exceed 2 or 3 minutes. This end is attained with the quieting bar, in the use of which every observer must practise himself. It is a magnetic bar half the length and breadth, and four times lighter than the principal bar. When this bar is held by the observer behind the theodolite in a horizontal position, and at a right angle with the magnetic meridian, it will cause at this distance (about 5½ meters), if it is strongly magnetized, a deviation of about one minute, westerly if its north pole is held easterly, and vice versâ. This deviation becomes smaller in proportion as the bar is removed from the horizontal position, and disappears entirely with its approach to the vertical position. No inconvenience is therefore occasioned by such a bar standing by the wall or near the clock-case (as in Plate II. and III.), till wanted. The use of the quieting bar in magnetic measurements is manifold; and it is important, in order to attain perfect and skilful facility in the performance of these experiments, to become accurately acquainted with its mode of operation. A separate article will therefore be allotted subsequently to the explanation of the rules and laws for its various uses and modes of action.

Finally, the building may be situated in the neighbourhood of other buildings without any injury to the observations. The magnetic observatory in Göttingen, for instance, could not, without causing many difficulties, be situated far from the astronomical observatory. The magnetometer is stationed about 60 metres westward of the astronomical observatory. At this distance moderate magnetic forces exercise so small an influence on the magnetometer, that it has been found unobjectionable to erect in a room of the astronomical observatory an auxiliary magnetic apparatus, which is of very essential service in absolute measurements.

More accurate directions will be given subsequently for determining the influence of a distant magnet, according to its force and position relatively to the magnetometer; and will especially serve this purpose, that when several magnetical apparatus (for instance, a principal magnetometer, an auxiliary magnetometer, and an inclinatorium) are to be fixed in neighbouring