To the Editor of the Mechanics' Magazine.
Sir,— A paper from a correspondent signing himself "H.Y.P.", published in Number 1743 of your Magazine, has directed my attention to Mr. Bland's brochure on the "Forms of Ships and Boats." I was anxious to make myself acquainted with the experiments and consequent deductions by reason of which, according to "H.Y.P.", "Mr. Bland deserves the thanks of his country." The perusal of the book, however, has not left upon my mind impressions in harmony with the tenor of the paper to which I allude. I therefore request that you will extend to me your usual courtesy; and permit me, by means of your columns, to place my views also before your readers.
I should be as ready, I think, as most, to accord thanks or any other reward to an inventor who had established a just claims to them; but I cannot consent to join your correspondent in treating as important that which is really insignificant, and as dignified that which is undoubtedly puerile. With no other knowledge of the matter, one would finish the perusal of "H.Y.P's" letter with the impression that Mr. Bland had performed a series of experiments which had authorised him to come to some serious conclusions relating to the speed and forms of ships, either at variance with, or in advance of, those principles which are ordinarily received; and yet in the book itself is found only one proposal pretending to be an improvement; and this is quoted in the letter, and relates to an increase of breadth, and diminution of draft of water, in line of battle ships; and in the statement of this, your correspondent seriously contradicts himself, and renders himself unintelligible. He says:
"The draught amidships is fixed at such a measurement, as under a bracteted form of bottom **** would amount to only two fifths of the beam. The reader perceives that in the latter case the draught of present three deckers would be reduced from 25 feet to about 12 feet!"
He may well close such a sentence with a note of admiration. First, we are informed that two-fifths of the beam shoudl be the draught of water, and are afterwards told that by this arrangement a three decker would draw 12 feet; now, 12 ffet are two-fifths of 30 feet; hence, it follows that a three-decker should be 30 feet broad, and draw 12 feet of water! This certainly is a noteworthy improvement; but I must be excused if I hesitate, at least for the present, to acquiesce in the propriety of the adoption of such dimensions for the first rates of our Navy.
Another false impression which your readers may possibly have derived from the same source is, that Mr. Bland has endeavoured to solve by experiments, accurately and carefully conducted, some of the problems which have hitherto baffled the attempts of the most eminent mathematicians and naval architects. Mr. Bland, however, has condined his attention to testing the relative merits of bluff and sharp bows, as to speed, and of flat and sharp bottoms, as to stability — subjects on which we have knowledge far more extensive, and data far more trustworthy, than any to be obtained from experiments made with models which are certainly small enough, but too rude for toys for the nursery. It would not, of course, be my place to criticise any gentleman who chooses to spend his time in so innocent an amusement as these experiments are doubtless capable of affording; but I object very strongly to the advancement of such absurd pretensions in their regard, and I regret that the ingenuity observable in them was not more productively employed.
In order to justify my strictures, perhaps, Sir, you will allow me to transcribe a specimen or two of the experiments on the character of which I si seriously differ from your correspondent. There is nothing of more importance in a set of scientific experiments than that the first principles which form the substructure of the whole should be free from serious error, because an error in these is often repeated and muliplied to such an extent as utterly to mar the result. To enable your readers to judge the present case from this point of view, I here quote from the first chapter of Mr. Bland's book a passage relating to the resistance experienced by rectangular bodies of various dimensions in their motion through the water in which they float. To investigate this subject Mr. Bland says:
"Four pieces of deal were selected, of the same uniform density and thickness, and each 12 inches long, but varying in width.
| No. 1 | model | 2 inches wide and 12 inches long. |
| No. 2 | " | 4 |
| No. 3 | " | 6 |
| No. 4 | " | 8 |
These were two at a time attached to the two ends of a balance rod, of the length of 20˝ inches; a third string, acting the part of a fulcrum whilst suspending the rod, was so put on the rod as to admit of being readily slipped along it at the will of the experimenter; the other end being fastened to the small extremity of a long pole, for the purpose of reaching far enough over a pond of water to tow the models upon the surface, clear of all obstacles.
The two models selected for the experiment were then drawn on the water, and whichever of them preponderated, by meeting with greater resistance than the others, had the suspending string shifted along the balance rod until both the floating bodies attained an equilbrium of resistance, when the measure of their respective resistance was denoted by the inverse length of arm or lever to which they were fastened. The shorter arm was made, in each experiment, to balance correctly the longer arm, by means of a moveable weight applied to the shorter arm.
| Models | Width. | Length. | Weight. |
|---|---|---|---|
| No. 1 | 2 ins. | 12 ins. | 10 ozs. |
| No. 2 | 4 ins. | 12 ins. | 10 ozs. |
| Difference. | Weight. | ||
| 1 1/8 inch of lever, or 2 ozs. | |||
| Models | Width. | Length. | Weight. |
|---|---|---|---|
| No. 2 | 4 ins. | 12 ins. | 12˝ ozs. |
| No. 3 | 6 ins. | 12 ins. | 12˝ ozs. |
| Difference. | Weight. | ||
| 1 1/8 inch of lever, or 2 ozs. | |||
| Models | Width. | Length. | Weight. |
|---|---|---|---|
| No. 3 | 6 ins. | 12 ins. | 19 ozs. |
| No. 4 | 8 ins. | 12 ins. | 19 ozs. |
| Difference. | Weight. | ||
| 1 1/8 inch of lever, or 2 ozs. | |||
"In these experiments the dimensions of the models were to each other as 1, 2, 3, and 4; and the head resistance, compared two at a time and of equal wieght, gave the smae results; consequently the law of the head resistance is, that it increases directly with the increase of the square surface opposed: and therefore, in this instance of equal additions, assumes the arithmetic ratio."
Perhaps, Sir, I ought to apologise for requesting you to occupy your valuable space with such unintelligible stuff as this. My excuse is that is seems to me the only way by which your readers can be enabled to judge of the real merits of a work for which so much has been claimed by "H.Y.P." Such experiments as these three make up the volume. The whole is as meaningless and as worthless as this sample.
All your readers will, no doubt, perceive the extremely misty nature of the above extract, yet it may not be useless to point out a few of its peculiar incongruities. In the first place, the method of conducting the business is quite unadapted for the observatión of those minute differences which would be of great importance in such small models. The forces by which the motion is communicated and kept up are not applied in the direction* of that motion; for the strings to which the models are attached cannot be held in a horizontal directions, because it is necessary that the rod should be kept at a distance above the water's surface sufficient for the accompanying weight to move clear of the fluid. And thus applied, they plainly tend to raise the fore parts of the models out of the water, and so to upset their trim and alter the whole character of the experiment. Another difficulty is to understand what is meant by the number of ounces placed in various positions in the tablular experiments. In experiments No. 1 is the force to move the first model through the water at a certain speed 10 ozs., and that for the second model at that speed also 10 osz.? And yet, in spite of this, is there a difference between these two forces amounting to 2 ozs.? Further, when No. 2 is tried with No. 1, it has a force or weight of 10 ozs.; when tried with No. 3 it has a force or weight of 12¾ osz. associated with it: wy is this? Is the model heavier or more sluggish in the second experiment than in the first? Again, is the difference between the lengths of the two arms of the bar 1 1/8th inches in all cases? If so the forces which balance on this constant lever must have a constant ratio to one another, and so the series of resistances would then be geometrical, and not arithmetrical, as inferred by Mr. Bland.
With regard to the stability of floating bodies, this volume furnishes a few vary strange propositions, which however are as false as they are novel. Take this:— There are two floating bodies of similar and equal rectangular sections with the length of one double that of the other; Mr. Bland asserts that when inclined from like positions of equilibrium through equal angles, the stability of the longer is treble that of the shorter. The simplest hydrostatical considerations assure us of the error of such a statement. Indeed it does not require even a knowledge of the first principles of hydrostatics to enable a person endowed with a little common sense to see the falsehood of this principle. It can be seen at first sight that if two floating bodies, of precisely the same dimensions, be placed together, end to end, they would thus form one double the length, and possessing exactly double the stability of either.
From this we may form a just estimate of the value of this collection of experiments. The may be divided into two parts, one relating to questions wich have received thorough and satisfactory investigations, and with regard to which truth and falsehood have been accurately defined; the other part relating to questions as yet imperfectly solved, and to which therfore such authors as Mr. Bland consider themselves at liberty to attach such answers as accord with their peculiar tastes, Those of the former division are proved erroneous by the simplest tests, and we are thus justified in condemning the whole as unworthy of reliance.
Another mistake is made in the method given for finding the centre of gravity of a ship. I will make the following quotation on this subject my last;
"The place of the centre of gravity between the heaf and stern is ascertained pretty correctly by the surface of the water coinciding with the loadwater line obtained and laid down from a correct model. But the axis of its height is extremely difficult of detection; and the readiest mode which presents itself would be, the placing of three or more cups or open vessels, filled with water, upon separate yet moveable shelves, a few inches or more perpendicularly above each other, at the centre of the ship's width and centre of gravity, taken lengthwise. This being done, and a lateral rolling motion communicated to the ship artificially, or the taking advantage of a light wind upon smooth water, and observing particularly the surface of the water in the cups, — then if the water in any one of them be seen to rise up first on one side, afterwards on the other, but in the remaining cups if the motion of the water be more rapid, even to overflowing, that first cup, wherever situated, cannot be far from the axis of lateral motion. Should any doubt on this question arise, just shift the said cup a trifle higher or lower, until the due quietude of its water surface be obtained."
That the centre of gravity of the ship and that of the water section are in the same vertical line is quite untrue; and the adoption of such a principle as a basis of calculation would be a source of serious error. Of course the centre of buoyancy can always be assigned without appreciable error, by means of the draught of the vessel; and this point is in the same vertical as the centre of gravity of the ship. The method given above for dertermining by experiment the height of the centre of gravity when the ship is afloat, is rather amusing than instructive. The philosopher who would set about this business in such a way would no doubt obtain, as he would certainly derserve, the laughter of all beholders. It will perhaps surprise Mr. Bland to be told that there is a ready method in which this experiments may be conducted to accurate succes. This method he will find explained in many works of naval architecture, some of which works he would do well to study before he again appears before the world in the capacity of author in this branch of knowledge.
I think, Sir, you will now agree with me in the opinion that such a production as this affords no claim to the national thanks which "H.Y.P." wishes to award to Mr. Bland.
I am, Sir, yours, &c.,
A Mechanic.
"A Mechanic": On the Form of Ships.
The Mechanic's Magazine, 1857. p 129-131, ill.
Transcribed by Lars Bruzelius.
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