Page:Scientific Memoirs, Vol. 2 (1841).djvu/65

Rh observations; it is, however, not requisite, when a small change is found, to bring back the scale to its former position; it is sufficient to note down in the registry the point of division corresponding to the plumb-line.

It may probably not be superfluous to draw attention to one or two points of comparatively minor importance. It has been supposed, that the magnetometer and telescope are so arranged that the mean position of the magnetic declination corresponds to about the centre of the scale. However, at times of considerable variation, this centre frequently gets entirely out of the field of view, and then the above method of verification will no longer answer. If at such a time the verification appears necessary, the quieting bar must be made to perform an exactly opposite office to that which it generally serves; namely, to give the magnetometer a vibration of sufficient extent to reach, and even to go rather beyond, the spot required, and thus to allow the plumb-line to appear in the middle of the field, at that part of the vibration where the motion is slow, and where consequently the corresponding division of the scale can be determined with accuracy. It is obvious that if such cases occur in the course of a periodical series, the magnetometer must be again quieted in time for the next observation, and, consequently, skill in the use of the quieting bar is of great moment.

When the declination falls very nearly in the centre of the scale, unpractised observers must be on their guard not to confound the plumb-line with the vertical line of the telescope. In our apparatus both resemble one another so much, that with a very quiet state of the needle, a mistake is very possible, and did, indeed, once occur. When there is danger of such a mistake, it may be expedient temporarily to remove the plumb-line.

With respect to the form of communication, some persons are accustomed to send in the observations in full, others the partial and final results only, and several merely the latter. The last may be sufficient, if the calculations have been revised, and the communicated numbers collated; but the observations themselves should be preserved, in case a reference should be wished; and when unusually great changes occur, communication, in full detail, is most desirable. Besides the results of the observations, it is always proper to notice, in connection, the value of the parts of the scale (or the measurements on which the determination is founded), the time of vibration, the correction and rate of the