* LORD ELLENBOROUGHMy Lords, I raise to call the attention of the House to the means that are generally adopted for ascertaining the errors of sextants both in the Royal Navy and in the Mercantile Marine; and to ask the First 223 Lord of the Admiralty if there is any probability of his causing a table of angular star distances to be published for the purpose of facilitating the necessary calculations as described in an article which appeared in the September number of the Nautical Magazine of 1901. This matter may appear somewhat technical, but there are several Members of your Lordships' House who have had sea experience, and others who are in the habit of assisting in the navigation of their own yachts, and they, at any rate, will have no difficulty in understanding the subject. It is almost needless to remark that the supposed position of a ship at sea, when out of sight of land and out of soundings, depends entirely upon observation taken by a sextant, whose error ought to be correctly known. The calculations for ascertaining some of these components are very simple, while others are very complicated. The index error can easily be found. It is the centring error that baffles the mariner. So much so, that Captain Lecky, the well-known writer on navigation, has declared that—
To test the centring errors is practically beyond the power of the navigator.It is customary, in consequence, to send sextants to be corrected for centring and other errors to Kew Observatory, where special apparatus has been erected for that purpose. But it is a far cry from China to Kew, and if a sextant gets out of order on a far distant station, or meets with an accident, an occurrence which is not uncommon, the ship herself is very likely to meet with another accident on her way home, for she has to be brought back by a faulty sextant and thereby runs considerable risks.It have, however, been fortunate enough to hit upon a method for finding the total error of a sextant which is simplicity itself, and which I published in the Nautical Magazinefor September, 1901. By taking the angular distance between two stars, when they are in a line, or nearly in a line with the Zenith—that is, in other words, on the same great circle of altitude—and then applying the corrections due to refraction, a true observed distance is obtained. The difference between this and the calculated distance is the total error for that particular angle. If the real or calculated angle is already known, the 224 error can in this manner be found in less than a couple of minutes. I am desirous of seeing a table of these calculated distances published in the Nautical Almanack. I am aware that all the executive officer of the Royal Navy, and many of those belonging to first-class lines of mail steamers, are capable of making this calculation for themselves. These is, however, no reason why their time should be wasted in calculating and recalculating the fixed distance of fixed stars when they have other duties to attend to. These calculations would be much better done once for all by a single computer on dry land. As, however, some stars have an appreciable movement, this work would require revision about once in five years. But by far the larger portion of the tonnage of Great Britain is in the charge of men who have not much knowledge of nautical astronomy, and who have possibly never made a logarithmic computation to seconds of are in the course of their existence. The sextants they use are often cheap and secondhand. But though their sextants are cheap, the lives of these men are of value, and so also are the lives of the men under their command; and it is principally in their interests that I am asking the noble Earl at the head of the Admiralty to make this addition to the Nautical Almanack. It would be a boon to mariners to publish it, and it would be one of those boons that do not consist of robbing Peter to pay Paul. With the permission of the House, I will quote an extract from an article recently written by Mr. H. B. Goodwin, who was at one time Naval Instructor at the Royal Naval College at Greenwich. Mr. Goodwin wrote—
To give an idea of the magnitude of the errors to which the sextant is liable. we may mention the result of thirty years' experience at the Kew Observatory, taken from a report made to the British Association at its meeting at Aberdeen in 1885. For errors in centring and defects in graduation, 20 per cent. of the instruments gave half a minute of error, 30 per cent. gave one minute, 25 per cent. gave two minutes, and 7 per cent. reached three minutes.Now, a minute of angle is one sea mile at the surface of the globe. In Staff-Commander Martin's work on "Navigation," a sextant is referred to which had a centring error which varied from nothing to ten minutes. Many years 225 ago the late Admiral Sir Charles Shadwell, under whom I had at one time the honour of serving, and from whom I learnt much, published a little book that contained a table which gave the angular distances of twenty-seven pairs of stars. The last edition came out in 1870, and is therefore more than thirty years old. These pairs of stars were, however, not selected with the view of correcting sextants, but for the purpose of shortening the calculations for obtaining the latitude by a method that has now become obsolete. The principle on which these pairs of stars should be chosen, is that the seaman should have the opportunity of correcting his sextant at about every 15 degrees of are. Different pairs of stars should be selected for different zones of latitude, and the seaman should be able to correct his sextant for all angles during the first and middle watches of two consecutive starlight nights, at any time of the year and at any place on the globe, except, of course, in high latitudes in summer time when stars cannot be seen. I would suggest that this table be published separately at first, and afterwards as part of the Nautical Almanack.To sum up, I suggest that the seaman should take the angular distance of a pair of stars in line with the zenith, apply the error due to refraction, and then take the difference between the observed angle and the one in the Admiralty table. That will be that total error of his sextant, which is what he wants. I wish to enable every navigator to find the error of his own sextant while on board his own ship, instead of running the risk of being drowned while on his way to Kew to find it out. I do not wish it to be understood that I am saying anything disrespectful to the Kew Observatory. I look upon it as one of the most valuable institutions that the country possesses, and whether this table is published or not, I think that a man is unwise who purchases a sextant that does not possess a Kew certificate. I now beg to ask the First Lord of the Admiralty the question of which I have given notice.
§ THE FIRST LORD OF THE ADMIRALTY (The Earl of SELBORNE)My Lords, I am afraid I am at this great disadvantage, that I have none of the 226 personal knowledge of this very scientific question which the noble Lord himself possesses. All I can do is to give him and the House the opinion of the expert advisers of the Admiralty and the Board of Trade on the point that he has brought to your Lordships' notice. I am advised that nearly all the errors of the sextant can be readily ascertained by an ordinary observer without any difficulty. The only errors that cannot be so ascertained are due to false centring or graduation. These are usually supplied with a good sextant after it has been tested at Kew Observatory, but they are usually so small that it is unnecessary to consider them in ordinary navigation. When very accurate positions are required, these can be obtained by observing celestial objects on each side of the zenith at, or about, the same altitude. This not only eliminates all sextant errors, but also those due to refraction of the horizen, a much more important matter. It is not, therefore, considered neccessary to calculate angular distances between stars, or to publish tables, which would never be absolutely correct owing to the differences in the refractoin at different altitudes. I am afraid that the answer does not correspond to the noble Lord's views, but the opinions it contains are those of the experts, not only of the Admiralty but of the Board of Trade.