A MANUAL OF SCIENTIFIC ENQUIRY. SECTION I. ASTRONOMY. By G. B. AIRY, ASTRONOMER ROYAL. THE science of Astronomy may occasionally derive benefit from the observations of navigators, in the following respects :— By contributions to Astronomy in general. By improvement of the methods of Nautical By accurate attention to Astronomical Geography. The remarks which follow will be arranged under these heads. General Astronomy. 1. The first point which calls for attention is the observation of the places of comets or other extraordinary bodies, especially those which can be seen only in low northern or in southern latitudes. In regard to these observations (and indeed to almost all others), one remark cannot be too strongly impressed on the observers-that a bad observation, or an observation B which is given without the means of verification, is worse than no observation at all. In order to make the observations good, the following cautions must be observed: The index-error of the sextant must be carefully ascertained. 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. 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 confusion 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 observations; 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 altitude. 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 indexerror, 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 corrected for depression, should also be stated. At some convenient 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. The time of making every observation should be entered exactly as it is read from the chronometer or hack-watch. By the side of this should be placed the error of the chronometer or hack-watch on Greenwich time, or on time at the ship (as may be most convenient); and, after this, the corrected time. At some convenient place, either at the beginning or the end, must be written out all the observations by which the error of the chronometer is ascertained. If its error on Greenwich time is given, the longitude of the ship must also be given, and the means and observations by which that longitude has been determined must be stated at length. If a hack-watch is used, the comparison of the hackwatch with the chronometer must be given. The last observations by which the latitude was determined, and the course and rate of sailing of the ship, must also be given. All the observations should be sent in this detail to the Admiralty or other body appointed to receive them. |