THE SCOTTISH GEOGRAPHICAL MAGAZINE. THE ART OF OBSERVING.1 BY JOHN COLES, F.R.A.S., Map Curator, and Instructor in Practical Astronomy and Surveying, Royal Geographical Society. IN the remarks that I am going to make on the art of observing, I intend to confine myself entirely to observations which can be taken with portable instruments, such as the sextant, transit-theodolite, telescope, plane table, prismatic compass, and instruments for determining heights by measuring the pressure of the atmosphere. I am well aware how difficult, in the time at my command, it will be to do anything like justice to my subject, and at the same time to deal with it in such a manner as to command your attention. I trust, however, that I may succeed to some extent in doing so. Now, the art of observing with such instruments as I have mentioned is by no means difficult to attain. It consists of an acquaintance with each of the instruments and its adjustments, a knowledge of what to observe, and how and when to take observations, so that the best results may be obtained, and errors eliminated. If an explorer, provided with a sextant, artificial horizon, and a good watch, were travelling in an unexplored country, the first thing he would want to know would be his position on the earth's surface, and to do this it would be necessary that he should find his latitude and longitude. Unless he succeeded in this, he might indeed describe the country he passed through, and in this way furnish some information to cartographers, but he would not be able to do more than give a rough approximate of the positions of important places, and characteristic features of the country. Now, with the aid of a diagram (Fig. 1), I think I can show you that finding the latitude by meridian altitude is a very simple operation. VOL. VII. 1 Read at Meeting of British Association, 1891. 2 Y In the first place, we have the fact that a heavenly body attains its greatest altitude when on the meridian above the pole. We also know that the elevation of the pole above the horizon is equal to the latitude of the place. These facts are capable of easy proof, but I must ask you to accept them as facts in the same manner as you would a statement that the distance of the sun is about 93,000,000 miles from the earth. Now, as we cannot see the pole of the heavens, and therefore cannot observe its altitude, we must find some other angle equal to its elevation above its horizon that we can measure, which we do in the following manner:-You will observe that the horizon is at right angles to the zenith, and that the equator is also at right angles to the pole. Therefore these two angles are equal one to the other. Then we may say that the angle ZCQ is equal to the angle NCH, and we therefore strike out ZN the co-latitude, which is common to both. Again we see that the angles RCQ and ECH are equal to one another because they are vertical and opposite; we may therefore also strike them out, and we have the angle ZCE equal to NCR, which is equal to the latitude of the place. Now, with a sextant or other angular measuring instrument we measure the elevation of the sun, or other heavenly body, above the horizon when it is on the meridian. If we take the altitude from ninety degrees, we get its distance from the zenith. Then by turning to the Nautical Almanac, at the proper page, we can find its declination, which is its distance north or south of the equator. In the case before us you will see that this declination, added to the zenith distance, will give the whole of the angle which is equal to the elevation of the pole above the horizon, and therefore equal to the latitude of the place. I must here mention that there are certain corrections to be applied to the observed altitude before we get the true altitude. These are index error, refraction, semi-diameter, and parallax. In the case of a star, the only correction necessary beside index error is for refraction, and these refraction corrections are given in tables in all works dealing with practical astronomy. Now, the result obtained from one observation of the meridian altitude is likely to be somewhat in error, and it is here that the art of observing comes in. For if we observe the meridian altitudes of two bodies on opposite sides of the zenith, and at equal, or nearly equal altitudes, all errors of altitude of whatever kind will be removed. I have several times mentioned the horizon, but away from the sea, unless in the neighbourhood of a large lake, we could not of course find any natural horizon that would answer our purpose. We must, therefore, make use of a substitute, which is in name, as it is in fact, an |