A manual of scientific enquiry : prepared for the use of officers in Her Majesty's Navy, and travellers in general / originally edited by Sir John F.W. Herschel.

Date:: 1886

Credit: A manual of scientific enquiry : prepared for the use of officers in Her Majesty's Navy, and travellers in general / originally edited by Sir John F.W. Herschel. Source: Wellcome Collection.

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$to make the observations good, the following cautions must be observed:— The index-error of the sextant must be carefully ascer- tained. If it has not been found a short time before the observations, it must be found as soon as possible after them. The distance of the comet from three conspicuous stars in different directions must be measured with the sextant. {See Appendix No. 3.) The point of the comet which is observed with the sextant should be precisely described. It is desirable that the navigator should be possessed of some star-maps or star-charts, by means of which he will be able at once to give the proper names to the stars, and much con- fusion and loss of time will be avoided.* If the time at the ship and the latitude are very well known, there will be no occasion to make further observa- tions ; but if these are not well known, some attempt must be made (by the use of Becher's horizon, or by any equivalent method) to ascertain the altitude of the stars and the comet. The lower these objects are, the greater must be the care in the determination of their altitudes. For affording means of verification, these rules should be followed:— The observations of distance with the sextant should be entered in the book precisely in the manner in which they are made. The reading of the sextant, uncorrected^ should be written down : in a column by the side of this should be written the correction for index-error, with a statement whether it is to be added to, or to be subtracted from, the sextant-reading : in the next column should be written a reference to the observations by which the index-error was determined: and in the last column should be written the distance as corrected. For the altitudes, the height of the eye, the depression of the horizon, and the altitude cor- rected for dperession, should also be stated. At some con- venient place, either at the beginning or at the end of all, should be written out all the measures by which the index- error was ascertained, exactly in the manner in which they were made, and so that any other person can deduce from them the value of the index-error. * Keith .Johnston's Atlas of Astronomy, edited by J. E. Hind, will suffice for most purposes, but Dien's Atlas Celeste, Paris, 1877, con- tains a much more complete set of star-charts.—(K, S. B.)$