Page:Scientific Memoirs, Vol. 2 (1841).djvu/41

Rh 2. In determining the values of the parts of the scale.

3. In observing the vibrations and elongations. (Vide seq. quieting bar.)

More accurate directions for the execution of all the measurements here mentioned will be given in the sequel.

For the measurements of Intensity measuring scales are required, by which the position of the deflecting bar is determined. These measuring scales may be laid horizontally and parallel to the magnetic meridian, on both sides of the box in which the magnetometer is included, in such manner that lines connecting the corresponding points of the two measuring scales shall be horizontal, and at right angles with the magnetic meridian. The scales should be placed at such height that the deflecting bar placed on them stand at an equal height with the vibrating bar. When this is not the case, the vertical distance between the deflecting bar situated on the measuring scales and the vibrating bar must be measured. The measuring scales must be about 5 to 6 metres in length, and should project an equal distance north and south beyond the magnetometer. If the width of the room allow, it is advantageous to add a third measuring scale horizontally and at right angles with the two former. It may pass under the box of the magnetometer, in such manner that it would be met by a plumb-line let fall from the middle between the centres of suspension and gravity of the vibrating bar. The measuring scales must be so arranged as to allow of displacing them longitudinally, in order to dispose them in such manner, that the deflecting bar, situated at corresponding points, in front and in rear, may produce the same amount of deviation. After these preparations, the measurement of the Intensity consists,

1. In determining the moment of inertia of the deflecting bar. (Vide seq. weights and weight-holder.)

2. In measuring the time of vibration of the deflecting bar.

3. In measuring the deflection of a suspended auxiliary bar produced by the deflecting bar, at two different distances of the latter, in a south and north, or east and west direction from the magnetometer.

To this general view of the arrangements of the magnetical observatory, and of the apparatus to be placed therein, may be added the following remarks on the separate parts of both.