On February 12th, there was found on the shore at Basse-Point (Martinique) a sealed bottle, enclosing a visiting card, on which was found the following piece of writing:-" May 19, 1842, at half-past six in the evening, from the Nisus, before Saint-Lucia, one of the Cape Verd Islands. This bottle has just been emptied by the officers and passengers, in full conviviality." This note was signed "REY." ON FINDING TIME AND LATITUDE, SIR.-On perusing your useful and widely diffused Magazine, I observe many valuable rules for finding the latitude and time by sea observations. Permit me to contribute "a mite" to the list already published, as I fancy it will be found acceptable to many of your Nautical readers. During the Sulphur's "late Voyage round the World" I have had an opportunity of examining most of the Nautical Tables of the present day, and also in forming an estimate how far those tables are available for the Navigator, to keep pace with the new acquisitions which are daily being added to Astronomical Tables. In the course of the above examination, I have practically tested numerous rules for finding the time and also for latitude by altitudes off the meridian. Most of them for latitude are unavailable in practice, because they are dependent on having the apparent time, found by a previous observation. The rule by which the following example is computed, has been tested by observations in various parts of the globe, taken on shore by sextant and artificial horizon, with every possible degree of accuracy. The results compared, are found to agree so closely that the rule appears available for sea observations. Your most obedient servant, To the Editor, &c. LATITUDE. CHRISTOPHER GEORGE, A concis and easy method of finding the latitude by two altitudes, off the meridian. The Limits are the same as in all methods for finding the latitude by altitudes near noon; viz., the minutes from noon must not exceed the degrees of zenith dis tance. RULE. Take two altitudes and note the times; find the interval between the observations, and the corresponding change in altitude. Enter Table A* with the approximate latitude and declination, and call the corresponding log. A. Then add together Interval of time, in minutes and decimal parts log. a c Change of alts. in seconds + A a c Log. Add these three Logarithms together, and find the Nat. number, and with it enter Table B, in line with it will be found two quantities, one in time, which added to the middle time of observation, gives the approximate time from noon, the other quantity call T. To the Logarithm of T, add the Logarithm A, the Nat. number of their sum * The formation of this, and tables B, C, D, will be given hereafter. is the correction in seconds, to be added to the middle altitude, and gives the meridian altitude. METHOD 1.-FOR FINDING THE LATITUDE. Nicoya, 24th Jan., 1839.-Lat. 9° 56' N.-Sun's Declin. 19° 14' S. By a watch shewing 6h. 56m. 36s. The sun's alt. was 60o 25' 5" 66 60 32 55 Under the Latitude, place the Declination, and find the Meridian Zenith Distances corresponding thereto. Under the Meridian Zenith Distance place the Zenith Distance at the time of observation. Enter table C, with the Latitude and Declination, and the corresponding Logarithm call C. Enter table D with the Zenith Distances; and the Logarithm corresponding thereto, add to C. The sum of these Logarithms found in table D, is the apparent time. By the same Tables and Method the Azimuth may be found, by changing places with the Declination and Zenith Distance at the time of observation. METHOD NO, 2.-FOR FINDING THE TIME. The following is worked by tables found in most Navigation Books, only differently arranged. The above, requiring only two tables will be found very useful in computing the Latitude from the Time, as proposed by Capt. Owen. MODERN WORKS ON NAVIGATION (Notes and Mems. "for the use of Seamen."-Second Series.) LAX.* The Rev. Wm. Lax was many years, Professor of Astronomy in the University of Cambridge, in which capacity he distinguished himself, by the untiring zeal with which he discharged his important duties. It was well known that he had devoted much of the latter part of the time he was there to Nautical Astronomy, and calculating tables for the easier working of its different problems. In 1821 appeared the work now before us, in the preface of which he informs us, "that it is no hasty production; that, on the contrary, he had been employed upon it for some years; and that he had spared no pains to render it worthy of the approbation of the British Seamen." It is on these grounds that we notice it; for we believe, it was published by the Board of Longitude, and it was fully expected that it would entirely supersede all the other tables hitherto used with the Nautical Almanac. In spite however of all these favourable auspices it did not take, and has never been much used; indeed it is hardly known to the generality of Navigators. Yet, this work has many of the requisites, which fit it well for general use, and distinguish it from the Norie's and Hamilton Moore's of the day. It is an original work, no mere compilation, written and calculated by a Mathematician, and a finished scholar, well acquainted with all that had been done by those who had preceded him; he carefully avoids their mistakes, and did not, like Mendoza Rio, produce a ponderous quarto, in order to do little more than solve the problem of finding the longitude by the lunar observation. He makes no blunders from ignorance of his subject, his tables are so arranged that almost at night every part can be taken out with sufficient accuracy, and above all a similarity runs through the different methods, that he uses to solve the various problems. Yet, in spite of these requisites, Navigators were right in neglecting this book, it is not well fitted for general practice,-in short it is not the working man's book. Paradoxical as it may appear, although the rules used are easy, they are not simple, but unnecessarily complicated. • "Tables to be used with the Nautical Almanac, for finding the Latitude and Longitude at sea, with easy and accurate methods for performing the computations required.-By the Rev. W. Lax, A.M., F.R.S., Lowndes, Professor of Astronomy and Geometry, at the University of Cambridge.-London, Murray, 1821.” 2 U ENLARGED SERIES.-NO. 5.-VOL. FOR 1843. Thus, in order to render all the proportional parts for seconds additive, he is obliged to employ parts for no seconds, and thus the computer cannot, if he wishes, work to whole minutes. This artificial system we look upon as a complete mistake, since it directly opposes all our habits of acting and thinking, according to which we naturally take the greatest, or most important first, and come to minutiæ afterwards. Lax, though he works his variation, and every thing else to seconds, understood his subject thoroughly, no doubt; but failed for want of a practical view of it. Such words as "Lemina" and "Scholium," are not likely to become familiar among seamen-fortunately. Mem. The work contains no Navigation. The author uses versed sines, which have grown into a kind of fashion of late years, though they were used, in the work of Sir Jonas Moore, in 1681, to 7 places. Against these we have strong objections. The last figures are always put down,-as 7 places are used, the computer cannot work to any other number; if he was to try to work to 6 or 5 places he must count from the last or seventh place, unless ciphers are prefixed to make up the 7 places, as in Dr. Inman's Tables; the end of all which is that, the student is told to throw out the first figure; and thus these numbers stand in complete opposition to all other numbers natural and logarithmic, and as soon as a computer has recourse to them he at once runs counter to all his usual habits. For our part we profess ourselves of that school which begins at the beginning, and employ 3, 4, or more places of figures, according as the degree, minute, or further accuracy is wanted. Besides, we dislike increasing the number of Tables, especially the larger ones, for more time is often lost in turning the leaves over than is gained by the additional facility, for the direct end of all tables is merely the saving of time. The perfection of tablework would be to require only one table; but as this is impossible, the smallest number is the next step. M. CLEMENT'S NAUTICAL INVENTIONS. On the Sillométre, Sub-Marine Thermometer, Steam Thermometer, Derivométre and External and Internal Thermometer. SUCH are the names given to five instruments lately invented by M. Clement of Rochefort, and which have for some time been adopted in the French Royal Navy. As a trial has recently been made with some of them, on board H.M.S." Lightning," and as they have since been fitted to the "Blazer," at Woolwich, we purpose to lay before our brother seamen a brief description of these several instruments, followed by the report of Mr. LARGE, of Woolwich Dock Yard, on the trials in the "LightLing;" reserving for our next number a more detailed account of these ingenious inventions (which we hope to be enabled to illustrate with wood cuts,) together with Captain Washington's Report, of the recent trials of them on board H.M.S." Blazer." 1st. The Sillométre, as its name denotes, is an instrument to measure the rate of a ship's sailing, which is shewn by a dial on deck, and is intended as a substitute for the common log. 2nd. The Sub-Marine Thermometer is a very delicate thermometer, composed of platina and silver, placed at a depth of about ten feet below the surface of the water, and communicating with a dial on deck. 3rd. The Steam Thermometer, points out the temperature, and consequent pressure of the steam in the boilers, which is also shewn by a dial on deck. 4th. The Derivomêtre is an instrument somewhat on the principle of the sillomêtre, and intended to measure the drift of a ship. 5th. The External and Internal Thermometer, as its name indicates, is a highly sensible thermometer, so placed against the wall of an observatory, or house, as to shew the temperature of the air without and within, the two pointers which mark this are on the face of the same dial. We now proceed to the trial of the three first of these instruments, in October last, on board the "Lightning," Lieutenant Snell, R.N., on a passage from Woolwich to Portsmouth and back. MR. LARGE'S REPORT. I.-Trials with the Sillometre. (During the passage from Woolwich to Portsmouth.) Thursday, 13th October, 1842.-About one mile and a quarter below Gravesend, commenced a trial between Massey's patent log and M. Clément's sillomêtre. After a run of 24 hours, (being off Sheerness,) Distance given by Massey's log 15 miles. 66 Sillomêtre 15 Distance from Nore light to Deal. By Sillomêtre By tables reckoning from buoy to buoy 66 At 4h. 25m. P.M., altered the course four points, during which operation the sillomêtre showed a diminution of speed, from 8 miles per hour to 7 miles. At 8h. 50m. P.M., off South Foreland, commenced a trial between Massey's log and the sillomêtre. Friday morning, took in Massey's log. Distance from abreast the South Foreland to about 7 miles to the eastward of the Owers, By Massey's log By sillomêtre By tables, reckoning from buoy to buoy 84 miles Moved sundry weights aft, viz., boat, brass guns, anchor, oars, &c.; for an instant the speed, as shown by the sillomêtre diminished to 7.4 miles per hour, but it almost immediately increased to the former speed of 8 miles. Moved the same weight forward: no sensible difference in the speed of the vessel. The speed, as shown by the sillomêtre varied from 81 to 8.2 miles per hour. Tried the speed of the vessel by the common log which gave. 8 miles. The sillometre the same, viz. 81 At 10h. A.M., about two miles past the Nab light, tried Massey's log. By Massey's log By Sillomêtre 6 miles nearly. 6" exactly. |