Page:Scientific Memoirs, Vol. 2 (1841).djvu/62

50 are made at every 2½ minutes, as long as it may appear desirable to do so. An example is added: Observers in general are requested to pursue the course here pointed out whenever occasion may require it; and, in such case, it cannot be doubted that, whenever changes of such magnitude occur, a body of corresponding observations in close detail will be collected, and will furnish interesting conclusions respecting these remarkable phænomena.

If observers, instead of a clock beating seconds, are furnished with time-pieces marking other divisions of time, they must arrange their observations in an analogous manner, corresponding to the beats of the time-piece. The observations with a chronometer are more difficult than with a clock, particularly if the second hand is not truly centred, as is sometimes the case.

It may be well to add some general precautions for unpractised observers.

It is of the first importance that the movement of the needle should be perfectly free. Spiders sometimes get into the box, and attach their web to the needle. This may be so fine as possibly to escape observation with the eye. Previously to each term, therefore, the finger should be passed carefully round the needle on every side. Any impediment which may exist to free motion will diminish the time of vibration of the needle. The most minute spider's thread has a very considerable effect in this respect, of which a curious example will be related in its place.

In night observation it is necessary to illuminate the scale, which, at Göttingen, during the term-observations, is done by means of two Argand lamps. There is always an upward current of heated air above the flame, and, therefore, if one of the lamps is placed near and below the telescope, such a current passing before the object-glass will impair the distinctness of vision, and cause the divisions of the scale to appear tremulous and undulating. This inconvenience frequently occurred