Author Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1856 Excerpt:...minutes from the truth. To this mean the index error and other corrections, with the exception of that for the depression of the horizon are to be applied. In observing the sun, it is preferable to take the centre instead of the limb. When altitudes of stars are taken, the level and white line must be illumined by a lanthorn held by an assistant; the light employed should be no more than sufficient to render the level and line visible. An instrument for the same purpose was contrived some time since by Captain Kater for the arctic expedition, and was found to answer very well, though the use of it was attended with some difficulties. This instrument is made by Mr. Dollond. It consists of a circle six inches diameter divided into degrees, and is furnished with a vernier reading off to minutes. At right angles to the index, and near the vernier, a level is fixed; and upon the centre of the instrument is placed a small plane mirror, which is rnoveable about an axis, the mirror continuing perpendicular to the plane of the circle. To the back of the circle a telescope is fixed, furnished with cross wires, and a skreen is so placed as to shield the eyes from the sun. To Astronomy, use this instrument, direct the telescope to the sun or s--V--' star; (if to the former, previously putting on a dark glass,) and keep the intersection of the cross wires on the centre of the object. Move the index till the level is nearly horizontal, and turn the mirror till the image of the bubble is seen in it by the left eye, whilst the sun or star is viewed with the right. Clamp the index, and move it by its tangent screw till the bubble of the level which is very active, is seen in the mirror to fluctuate backwards and forwards, whilst the cross wires are kept as steadily as possi...