CONTENTS:-Finding the Way at Sea-Thought and Language-First Star Lessons-Philosophy of Clothing-Household Insects-Great Red Spot on Jupiter-Young Electrician-Isle of Wight Steam Ferry-Reviews-Geometrical Chats-Correspondence-Whist-Chess. KNOWLED AN ILLUSTRATED MAGAZINE OF SCIENCE CONDUCTED BY RICHARD A. PROCTOR. VOL. VIII.-No. 199.] "Put up a Picture in your room."-LEIGH HUNT. THE AUTOTYPE FINE - ART GALLERY, MEMORY & SUCCESS. HODDER & STOUGHTON'SLIST. 74, NEW OXFORD STREET, W.C. SPLENDID COPIES OF THE OLD MASTERS, From the most celebrated Galleries of Europe. REPRODUCTIONS OF WORKS OF MODERN ART, COPIES OF REYNOLDS, GAINSBOROUGH, LAWRENCE, And rare Works from the Print-room, British Museum, AN ILLUSTRATED PAMPHLET, "Autotype in relation to Household Art," with Press notices, free per post. Fine Art Catalogue, 166 pp., price 6d., post free. THAT IS IT THAT every one can obtain from NATIONAL GALLERY, LONDON. W PROFESSOR LOISETTE? A good memory-the Now Ready. PHOTOGRAPHY. 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I Correspondence: Meteoric Display -Invisible Suns-Life in the Moon Evolutionism What Lies outside the Mechanism of the Universe ?-Musical Temperament-Smell-clouds, &c....... 163 Our Whist Column 164 Our Chess Column FINDING THE WAY AT SEA. BY RICHARD A. PROCTOR. 166 PROPOSE to give a brief sketch of the methods in use for finding the way at sea, in order that the general principles on which safety depends may be recognised by the general reader. It is known, of course, to everyone, that a ship's course and rate of sailing are carefully noted throughout her voyage. Every change of her course is taken account of, as well as every change in her rate of advance, whether under sail or steam or both combined. If all this could be quite accurately managed, the position of the ship at any hour could be known, because it would be easy to mark down on a chart the successive stages of her journey, from the moment when she left port. But a variety of circumstances render this impossible. To begin with, the exact course of a ship cannot be known, because there is only the ship's compass to determine her course by, and a ship's compass is not an instrument affording perfectly accurate indications. Let anyone on a sea voyage observe the compass for a short time, being careful not to break the good old rule which forbids speech to the "man at the wheel," and he will presently become aware of the fact that the ship is not kept rigidly to one course even for a short time. The steersman keeps her as near as he can to a particular course, but she is continually deviating, now a little on one side now a little on the other of the intended direction; and even the general accuracy with which that course is followed is a matter of estimation, and depends on the skill of the individual steersman. Looking at the compass card in steady weather, a course may seem very closely followed; perhaps the needle's end may not be a hundredth part of an inch (on the average) from the position it should have. But a hundredth part of an inch on the circumference of the compass card would correspond to a considerable deviation in the course of a run of twenty or thirty knots; and there is nothing to prevent the errors so arising from accumulating in a long journey until a ship might be thirty or forty miles from her estimated place. To this may be added the circum stance that the direction of the needle is different in different parts of the earth. In some places it points to the east of the north, in others to the west. And although the actual "variation of the compass," as this peculiarity is called, is known in a general way for all parts of the earth, yet such knowledge has no claim to actual exactness. There is, also, an important danger, as recent instances have shown, in the possible change of the position of the ship's compass on account of iron in her cargo. But a far more important cause of error, in determinations merely depending on the log-book, is that arising from uncertainty as to the ship's rate of progress. The log-line gives only a rough idea of the ship's rate at the time when the log is cast; and, of course, a ship's rate does not remain constant, even when she is under steam alone. Then, again, currents carry the ship along sometimes with considerable rapidity; and the log-line affords no indication of their action: while no reliance can be placed on the estimated rates even of known currents. Thus the distance made on any course may differ considerably from the estimated distance; and when several days' sailing are dealt with, an error of large amount may readily accumulate. For these and other reasons, a ship's captain places little reliance on what is called "the day's work,' "that is, the change in the ship's position from noon to noon as estimated from the compass courses entered in the log-book, and the distances supposed to be run on these courses. It is absolutely essential that such estimates should be carefully made, because under unfavourable conditions of weather there may be no other means of guessing at the ship's position. But the only really reliable way of determining a ship's place is by astronomical observations. It is on this account that the almanac published by the Admiralty, in which the position and apparent motions of the celestial bodies are indicated four or five years in advance, is called, par excellence, the "Nautical Almanac." The astronomer in his fixed observatory finds this almanac essential to the prosecution of his observations; the student of theoretical astronomy has continual occasion to refer to it; but to the seacaptain the "Nautical Almanac" has a far more important The lives of sailors and passengers are dependent upon its accuracy. It is, again, chiefly for the sailor that our great nautical observatories have been erected and that our Astronomer-Royal and his officers are engaged. What other work they may do is subsidiary, and as it were incidental. Their chief work is to time this great clock, our earth, and to trace the motions of those celestial indices which afford our fundamental time-measures, in such a way as to ensure, so far as possible, the safety of our navy, national and mercantile.+ use. *The log is a flat piece of wood of quadrantal shape, so loaded at the rim as to float with the point (that is, the centre of the quadrant) uppermost. To this a line about three hundred yards long is fastened. The log is thrown overboard and comes almost immediately to rest on the surface of the sea, the line being suffered to run freely out. Marks on the log-line divide it into equal spaces, called knots, of known length; and by observing how many of these run out, while the sand in a half-minute hour-glass is running, the ship's rate of motion is inferred. The whole process is necessarily rough, since the line cannot even be tautened. + This consideration has been altogether lost sight of in certain propositions for extending Government aid to astronomical inquiries of another sort. It may be a most desirable thing that Government should find means for inquiring into the physical condition of sun and moon, planets and comets, stars and all the various orders of star-clusters. But if such matters are to be studied at Government expense, it should be understood that the inquiry is undertaken with the sole purpose of advancing our knowledge of these interesting subjects, and the work should not be brought into comparison |