bear testimony to the fact that suggestions of errors from unequal heat were carefully considered. Only in one or two cases did the captains of iron vessels ascribe their compass errors to this cause. Investigation showed that there was no support for the supposition; and subsequent experiment has proved that nothing appreciably affecting the navigation of a ship is possible from the effects of unequal heat on the compasses of iron vessels.

"Let us, however, for a moment examine the three examples brought forward by the Nautical Magazine in support of the hypothesis. No. 1. In an iron steamship going up the Red Sea, the master noted that the deviation in the evening was different from the deviation in the morning.' Very probably the difference' would have been the same in a wooden vessel. I remember that the same thing was observed on shore in Liverpool by a gentleman who wished to ascertain the exact variation of the compass. He found that the morning and evening observations did not correspond, and rightly attributed the difference to want of adjustment in his instrument. The compass in the iron ship in question, if taken ashore, would possibly have shown the same difference which it did on board. But we are not told that this was tried; or that any other compass on board showed the same peculiarity; or that it has been observed in any other steamer in the Red Sea; or whether it was large or small in amount-nothing, in fact, which one would reasonably expect in the shape of evidence to support the premises.

"Case No. 2 may be briefly stated thus :-In a steamer bound from Liverpool to New York, while running along the shoals between Georges and Nantucket, in twenty-five to thirty fathoms of water, an alteration' of ten degrees was observed in the compass in one hour. How this was ascertained is not stated-whether by celestial observation, or by comparison with another compass supposed to be correct, or supposed to be free from the influence which so seriously affected the one in question. So extraordinary a change might be supposed to demand immediate investigation on the part of a careful navigator, and a record of the direction of ship's head by each compass on board at the time. The record, too, would not have been complete without some notice as to whether the alteration' was permament, or belonged only to the special hour named, or whether the sun was shining' on one side of the vessel, as in the case No. 3-a vessel in which a deviation also of ten degrees was observed. In the cases No. 1 and No. 2 we can form some idea of the direction of the vessel; but in No. 3 it is not said whether she was in dock, or swinging at anchor, or whether she was lying alongside another iron vessel; whether the sun shone on her port or her starboard side; or if the deviation was easterly or westerly. The statements are, meagre and vague, and, although quite true, are entirely

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936 THE EFFECTS OF UNEQUAL HEAT ON THE COMPASSES OF IRON SHIPS.

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unfit to support the writer's position; but he appears unconscious of their baldness and imperfections, and boldly asks-how are these errors to be accounted for ? An error very similar to cases Nos. 2 and 3 was once very satisfactorily accounted for in this way: The door of the binnacle was opened, and, on looking inside, a magnet, originally placed horizontally, was found to have slipped from its supports, and was hanging loose at an angle of about forty-five degrees from the perpendicular. In other cases a blunt pivot, or a cracked agate, or some equally-simple mechanical cause, has accounted for the error supposed to have been due to the magnetism of the ship. But these simple causes will evidently not suit the writer of the article, for he has to support the proposition which he commences with, namely, that the compass on board iron vessels is subject to 'frequent,' to 'unknown,' to 'unsuspected," errors of a temporary' and 'ever-varying' character! Evidently something sensational is wanted-something to excuse getting a ship ashore-something to mystify the stipendiary magistrate and puzzle the kind-hearted seamen who assist him on a Board of Trade inquirysomething to raise the sympathy of the public and to divert attention from the true cause of the disaster.

“It is, I think, plain that the writer, besides offering imperfect evidence, carries his theory too far, for, after reading his paper, the real puzzle appears to be, not to account for one steamer getting ashore, but to account for the 700 steam voyages which are yearly performed between Liverpool and New York without accident, and without, so far as I am advised, ever noticing any change of deviation in the compass when passing into or out of those bands of cold and warm water which are always met with on this voyage.

"There are other parts of the article in question which are equally obnoxious to comment. Perhaps you will favour me with space for some further remarks upon them in another letter. Yours, &c., W. W. RUNDELL, Liverpool, October 3rd, 1873."

After reading the above communications, our readers will perhaps expect us to own that we are ashamed of ourselves; but our readers will be disappointed if they expect anything of the sort. We stand our ground, and say now what we said before; viz., that the subject is worth inquiry. We may be wrong, but, whilst we admit that the world owes more than it can repay to the two gentlemen who are so tremendously shocked by our heterodoxy, we do not admit that these two gentlemen, great and undoubted as are their powers and services, have yet unveiled the whole truth; nor do we admit, as their letters imply, that October, 1873, marks the end of all acquisition of knowledge of compass deviation. Finally, we say that the deviation of the compasses of iron steamers in the American trade is well worthy of early attention and investigation;

and that there is no difficulty, if there should be any desire, to obtain the data from the logs of large companies in the trade. Let Captain Evans and Mr. Rundell put their heads together, and get returns from the "Cunard" and "Inman" and "White Star" lines for six months; from the White Star line, especially, because of the iron decks and iron houses of their ships. We are not asking too much in the interests of the truth they so dearly love,—and we say that if those learned gentlemen can afterwards show that no alteration of deviation other than what might be expected from a change of magnetic latitude, is recorded in these logs when the learned gentlemen have done this, and not before, we will admit that our suggestion is of no moment. Whether they do it or not, it will, we trust, be done, and when it is done, it will, we believe, prove that the anonymous writer in the old Nautical is right, and knowledge will have been acquired by others.

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RULES OF THUMB.

In our July number is a paper on "Rules of Thumb for Steamship Speed, Power, and Coal in the Merchant Service." We are glad to find that a very great amount of interest is attached to that paper, not only by our readers, but also in quarters beyond the ordinary range of our circulation. Our contemporary, Naval Science, has been a good deal puzzled by the rules, and has subjected them to analysis: and our learned, but far more practical contemporary, Engineering, having discovered and pointed out some errors in the conclusions arrived at by Naval Science, has taken up the gauntlet on our behalf.

Engineering speaks of our "Rules of Thumb" as follows :-" Their publication in the Nautical Magazine entitles them to the careful consideration we propose now to bestow upon them, and justifies the very honest, very intelligent, very accurate, and calmly philosophic treatment they have received in the pages of Naval Science for this month. These two periodicals the one (the Nautical) a monthly of nautical sense, the other a quarterly of naval science; the one a light blue, and the other a dark blue in complexion, seem to be to some extent imbued with the distinguishing characters we are accustomed to associate with these rival colours; the light blue Nautical, in its utilitarian spirit, goes in for practical though only approximate rules as useful generalizations; the dark blue Naval, on the other hand, is quite shocked at the proposal even to approximate to speed, power, or displacement without, in the most orthodox fashion, taking into account, as far as possible, every considera-" tion affecting the result, and thereby, of course, adopting a method of

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calculation, that, except to technical men, would be quite impracticable.

"The Nautical considers, as we do, that convenience, and not possibility, should regulate how far every consideration affecting the result may be included in a "rule of thumb." Nautical sense would prefer a practical rule without the ornamentation of radical signs, the absence of which is objected to by Naval Science. Which is right? We cannot decide, not having yet exhaustively examined these rules; but, taking the evidence given by Naval Science, they do seem to possess a higher degree of general approximation than is attributed to them in the article from the Nautical Magazine, or than we were disposed to attach to them."

In criticising our Rule V., Naval Science had taken the case of the Elbe, and had stated that "We (Naval Science) shall be quite within the mark if we take the consumption as practically about 24 lb. per horsepower per hour; this, for a total horse-power of 3,182, corresponds to an actual daily consumption of 76 tons, or upwards of 50 per cent. more than the result given by the rule. We might cite other examples, but these, selected at random, are sufficient for the present purpose."

Thereupon, Engineering remarks as follows, pointing out that the conclusions of Naval Science are 250 per cent. wrong-viz. :—

"So much for what naval science says about the Elbe. Turn we now to Mr. Bramwell's paper on the Economy of Fuel in Steam Navigation,' read before the Institution of Mechanical Engineers at Liverpool last year; there we find a return for the Elbe, the same steamer; it is stated that the average consumption of coal per indicated horse-power per hour by steamships with compound engines in long sea voyages' was, in the case of the Elbe, on the voyage from Southampton to St. Thomas, in February, 1870, 2-18 lb., that the indicated horse-power was 1,452 horses, and that the coal consumption per 24 hours was 33.8 tons. It may not be fair to criticise the statements of naval science, as they are not arrived at by Rule of Thumb, but we merely notice in passing that the result arrived at in this case by detailed calculation is only 76 33.8

2.25, say, about one hundred and twenty-five per cent. too much!" In the pages of the Nautical we have no room to enter into controversy, and we are, therefore, only too glad that our daily, weekly, and quarterly contemporaries, knowing this, are thoughtful enough to reprint our articles, and to open their pages for the discussion of subjects started by us. Engineering, we are pleased to see, has expressed an intention to return to the subject shortly; and, judging from the talent in the staff of that paper, we feel that nothing but good can come of discussion in its pages, and we feel ourselves safe in its hands.

PASS the end of a chain down the Rudder Trunk, and take it up over the stern; lash the end link to a piece of spar (a, Fig. 1) fitted with a blade (b); put a mouse (c) on the chain, leaving two or three links drift between it and the spar for play; sling a pig of ballast, or other weight, at the lower part of the blade (d); fit a block (e) for a tackle to trice it up clear of the screw in case of making sternway; fit guys (f); lower the whole over the stern, and heave the chain tight until the mouse (c) is jammed tight in the lower part of the trunk (g); secure the chain (h, Fig. 2), reeve the guys (f) through blocks at the end of a spar across the stern (i), and take them to the barrel of the wheel. When the main piece of the Rudder cannot be unshipped, lash a large shackle to the end of the spar, and reeve the chain (j) through the shackle, drop the bight of the chain over the stern, and draw it tight under the counter, securing both ends in board at (k).

F. W. WITHERS, Examiner in Seamanship, London.

GUNS AND ARMOUR.-The first heavy guns were 12 tons weight; then 18, 25, and 35 tons. A gun is now to be made of 60 tons, firing a charge of 200lbs. of powder. The weight of the shot is to be more than half a ton. The Woolwich " infant" fires a shot of 700lbs. with a charge of 110lbs., can pierce the 14-inch turret of the Devastation at 500 yards; a 16-inch plate would render the turret proof against this gun. The 60-ton gun is intended to pierce a plate of 20 inches. The Devastation is safe beyond 500 yards from the most powerful gun existing, but will not be safe from the new 60-ton gun.