London. Bristol. 86 log. 1·93450 70 log. 1·84510 25° tan 9·66867 30° tan 9·76144 ———- ———- 1·60317 = 40·1 1·60654 = 40·4
so that the observations accord very closely in fixing the height at a little exceeding 40 miles at disappearance, but a slight correction is necessary to allow for the Earth’s curvature. There are other methods of computing the heights, one of which is explained by Prof. A. S. Herschel in a paper entitled “Height of a Meteor” (‘Monthly Notices,’ vol. xxv. p. 251).
_Meteoric Observations._—A large number of meteor-showers still await discovery, and there are features even in connection with the best known streams which remain to be elucidated. Such doubts as now exist are only to be cleared away by assiduous observation made with the utmost accuracy possible both of the _directions_ and _durations_ of meteors.
This attractive field of investigation has certainly been neglected in recent years, and the reason of this may perhaps be found in the complications inseparable from it, in the need of great patience and scrupulous care in observation, and the necessity of gaining experience before the observer can feel a reliance on his work, and draw safe conclusions. Meteors are so fugitive, so diverse and erratic in their apparitions, as to be quite beyond the scope of instrumental refinements. They must necessarily be observed under many disadvantages. Positions have to be fixed from very hurried and often imperfect impressions. But these drawbacks, formidable as they at first appear, may be severally overcome by practice, by careful regard for the conditions under which meteors are displayed, and the marked differences of aspect induced by these conditions. When the observer has acquired a practical knowledge he will proceed with confidence in his work, and avoid many of the difficulties surrounding it.
In recording meteor-tracks for the purpose of discovering the radiant-points, the chief feature in which precision is essential is the _direction_ of flight. A perfectly straight wand, held in the hand for the purpose, should be projected upon the path of every meteor directly it is seen, and then when the eye has quickly noted the position and slope relatively to the fixed stars near, it should be reproduced on the chart or celestial globe. The time, mag., estimated duration, and details of appearance should be registered in a tabular form, with the R.A. and Dec. of the beginning-point and probable radiant. The end-point and length of path may be left until next day, in order to save valuable time. The wand is a great assistance to the eye in retaining the approximate directions and noting the places. If a meteor belongs to the slow, trained class, or if it belongs to the swift, streak-leaving order, the path may be very accurately noted, for the wand can be adjusted to its direction before the meteor or its visible offcome has died away. In the case of short, quick meteors, devoid of either streaks or trains, and generally shooting from radiants at high altitudes, they are more difficult to secure, as they vanish before one may turn, and the observer must rely upon the mere impression he received. But even these succumb to experience, and will be found to resolve themselves into a number of sharply defined radiants scarcely less certain than the positions derived from the streaked or trained meteors.