The Popular Science Monthly Volume 6 - Softcover

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1875 edition. Excerpt: ...reduced to the same epoch, gives the relative equation of A and B. Now if, instead of A registering his own observations on the chronograph (for example), we could have the star register its own transit, then B's observation, compared with this, would give at once an absolute equation. We cannot use the real star for this purpose; but several attempts have been made to construct an apparatus which should register the transit of an artificial star, which star could, at the same time, be observed. The principle of all of these machines is, in general, the same, and we will merely give a brief account of one which is now under trial by the Coast Survey. The artificial star is produced by lamp-light falling upon a small hole in a blackened plate; this plate is given a motion laterally, and the small point of light passes from one side to the other of a plate of ground glass, upon which lines are ruled to represent the spider-lines of the reticle. As the artificial star passes each wire, an electric signal is recorded on the chronograph, and the observer can also record his signal; and thus on the same chronograph-sheet many observations of absolute personal equation can be permanently recorded. Any velocity can be given to the star, so that it may pass through the field of view as slowly as the pole-star, or as rapidly as a star at the equator.1 1 The chief objection to this apparatus is, that there is a constant error in its indications; i. e., it can never be adjusted so as to give its signal at the exact moment of transit, but it is always too soon or too late. This is sought to be eliminated by allowing the artificial star to travel first from right to left, and then from left to right, and using the mean of the two determinations. It is...