Marvels of Scientific Invention Part 17

It can also be hollowed out, ballasted with weights inside, and so made to sink to any desired level, thereby representing the vessel when fully loaded, half loaded and so on. All sorts of unequal loading can be produced if needed, indeed every condition of the real ship can be imitated in the model.

It can then be towed to and fro in the tank by the travelling carriage described above. The speed of towing can be varied by changing the speed of the motors which drive it. The force needed to pull the model through the water is measured by means of a dynamometer which registers the pull on the towing apparatus.

A matter very often needing investigation is the shape and size of the wave thrown up by the bow of the vessel, and of that left behind her, known as the "bow wave" and the "stern wave" respectively. These waves represent wasted energy, for they are no use and are produced actually by the power of the engines of the ship as they drive her along. The ideal ship would cause no waves, but since that is a degree of perfection impossible even to hope for, the shipbuilder has to content himself by so designing his ships that these waves shall be as small as possible.

The waves are recorded photographically, in some cases by the kinematograph.

Some of the large shipbuilders have their own tanks, and so have the naval authorities of the great naval Powers. The one at Teddington was established through the munificence of a famous British shipbuilder, Mr Yarrow, who not only defrayed the cost of construction, but gave an endowment to a.s.sist in its upkeep. It is intended to serve the needs of the smaller builders who have not tanks of their own, and also for the investigation of matters of general interest to shipbuilders, and for such tests as that relating to the _Hawke_ and _Olympic_. In this last-named case, of course, two models were made, one to represent each ship, and they were towed along in such a way as to imitate very closely the movements of the ships at the time when they collided. It was as the result of these tests that the _Olympic_ was ordered to pay damages to the Admiralty, it being held that she was the cause of the accident.

A very interesting investigation of this kind was recently carried out in the tank at the United States Navy Yard. The port of New York consists very largely of jetties projecting out from the banks of the river. With the growth of the Atlantic liner the old jetties had become too short, and questions arose as to the elongation of them. If it were done, how would it effect the current in the river, and the handling of shipping generally? If, on the other hand, it were not done, what would be the effect of the ships lying with their ends projecting out into the stream unprotected by a jetty.

To determine these points the experimental tank was converted into a model of the New York Harbour, or at all events of that part in connection with which these questions arose.

A false floor was put in, so as to make the depth exactly right in proportion to the width. Little model jetties were arranged to represent exactly the real ones, while against them were moored model vessels, so that the effect upon them could be observed as the model of the large vessel was towed past.

In addition to this, special appliances were arranged for finding out what the disturbance might be which the movement of a giant liner produces under the surface as well as above it. For this purpose buoyant b.a.l.l.s were employed, moored at various distances below the surface, from which thin rods projected upwards, the movement of which rendered visible the movements of the submerged b.a.l.l.s and therefore the effects of the under-water currents.

All these things had to be observed at one and the same time--the moving model itself, the models alongside the jetties, the commotion on the surface, the swayings to and fro of the rods attached to the submerged floats--all, or most of which, at all events, it was impossible to make self-recording. Yet, seeing that it was of the utmost importance that the relations between all these things should be observed, and recorded from time to time as the model was towed along, it is evident that something must be done, and a cunning use of the kinematograph solved the problem quite easily. At various points commanding a good view of the model harbour and its shipping these machines were placed, and so several series of photographs were obtained, by the study of which all the different movements could be seen and compared. A large dial too was rigged up upon the travelling carriage by which the model was towed, a finger on which denoted the distance which the carriage had travelled at any moment. This large dial came into each photograph, of course, and so each picture bore upon itself a clear record of that particular moment in the voyage of the model to which it referred.

Thus we see an instance of how the very latest and most up-to-date methods of amus.e.m.e.nt are sometimes applied to serve very practical purposes.

Akin to the experiments upon ships are aerial experiments to determine matters connected with the navigation of the air. At Barrow-in-Furness the great firm of Vickers, shipbuilders and armament manufacturers, and latterly builders of aerial craft for the British Admiralty, have erected a machine for testing the efficiency of aerial propellers and other things of a kindred nature. Upon the top of a tall tower there is pivoted a long arm of light iron framework. To the end of this a propeller can be fixed, so that as the arm revolves there is produced almost exactly the same conditions as those which prevail when a propeller drives an aeroplane or steerable balloon.

By means of suitable mechanism the propeller can be turned at any desired speed, with the result that it drives the arm round and round upon its pivot on the top of the tower. The force which the propeller thus exerts can easily be measured, and so can be determined such questions as the most efficient speed for each type of propeller, the power which any particular one can develop, the best form for each particular need, and so on.

Materials, too, require the most careful testing, in order that they may be put to the best possible use in modern machinery and structures. For example, anyone can measure the strength of a spring, but what do we know as to its lasting power? Springs often have to form part of a machine in which they are stretched and compressed millions of times, and the question arises as to what is the best shape and material for the purpose. It may be that the spring which works best a few times will be the first to become "weary," for with repeated strain such things as steel get tired, just as the human frame does. Now that is a matter which will yield to no calculation, the only way to determine it is actual test. So a mechanism has to be employed which will extend and compress the spring over and over again, just as it will be in actual use, with a counter of the nature of a cyclometer to count how many times it has been subjected to this distortion. Then the apparatus is set going and left to itself for hours, or even for days, during which time it may work the spring millions of times. This may go on until it breaks, or else it may be done a prearranged number of times, and then the spring taken out and tested by other means to see how its strength has been affected.

Metal bars are often subjected to sudden blows, light in themselves but oft repeated. The point to be determined then is how many times the blow may fall before permanent injury is done to the bar. To investigate such matters we have the "repeated-impact" machine. The bar is held in a suitable holder, under a hammer which gives it a blow, the force of which can be easily regulated, at regular intervals, the number of blows being counted by a suitable recording mechanism. Ultimately the bar breaks, under a blow the like of which it can endure singly without any apparent strain at all. The machine, by the way, can be caused to turn the bar round to some degree after each blow, so that it is struck from all directions in succession.

The microscope, too, has established its place in the testing laboratory. It is a very valuable adjunct to chemical and mechanical tests.

Suppose, for example, that a bar of steel is being investigated; it can be put into a machine and pulled until it breaks in two. The machine registers the amount of the pull which was applied. Or a small piece can be put under a press and compressed to any desired degree. It can also be tested by impact or even pulled apart by a sudden blow, as described in _Mechanical Inventions of To-day_. The bar can be supported by its ends and loaded or pulled down in the centre, so that its power of resisting bending can be determined. It can be judged, too, from its chemical composition. Steel, in particular, depends for its properties very largely upon its chemical composition. The difference between cast-iron, wrought-iron and steel, also the differences between the innumerable varieties of steel, are due almost entirely to the admixture of a certain percentage of carbon with the metal. This can be ascertained by chemical a.n.a.lysis. This form of inquiry has the advantage over the more purely mechanical methods in that the latter, for the most part, have to be applied to the bar as a whole, whereas the quality may vary in different parts, the surface in particular being liable to differ from the interior. In such cases, one a.n.a.lysis can be made of a piece cut from the surface and another of a piece from the centre.

And it is here, too, that microscopical a.n.a.lysis comes in. For this purpose a piece is sawn off the bar, and the end ground perfectly smooth. This is then washed in a suitable chemical, such as a mild acid, which acts differently upon the different materials of which the "metal"

is built up, thereby rendering them visible one from another. A photograph taken through a microscope then shows the structure of the metal; how the different const.i.tuents are built together.

This is known as metallographic testing, and its advantage as compared with chemical a.n.a.lysis is that the latter shows, as we might say, what are the bricks of which the thing is built, while the former shows how the bricks are arranged. Indeed it is hardly correct to speak of the advantage or superiority of one over the other, since each is the complement of the other, supplying the information which the other fails to give.

And there are other mechanical tests which have not yet been mentioned.

There are machines which twist a bar so as to discover its power to resist torsion, there are others which apply a downward pressure on one part of the bar and an upward one on an adjacent part, so as to show its capabilities in withstanding shearing strain.

Moreover, many of these tests are nowadays, in a well-equipped testing-house, carried out in conjunction with the use of heat. It stands to reason that a part of a machine which will have to work under considerable heat may have to be of different material from a part which works under a normal temperature. In some cases the bar is surrounded by a spiral wire through which electric current is pa.s.sing, and by the regulation of this current any desired temperature can be set up in the bar. Or it may be placed in a bath of hot oil in such a way that the bar shall be raised to any temperature required, without interfering with the machinery which exerts the tension or pressure, or whatever it be.

Years ago such elaborate tests as these were never thought of. There are certain well-known figures, to be found in all engineering text-books, which give what stresses different materials ought to be able to stand, and these were, and are still, to a large extent, relied upon, it being taken for granted that the material used will be up to the average standard. In large and important works, however, the testing has been developed upon scientific lines, so that it is known from actual experiment what each particular thing is capable of. This not only means security but economy, for it is sometimes found that a substance is stronger than it is thought to be, and so things made of it can be designed to give the requisite strength lighter and cheaper than they would have been otherwise.

Some of the machines employed are of enormous strength, capable of exerting a pull or a compression of, it may be, 100 tons or more. They are often made, too, with self-recording appliances, whereby the course of the test is set down automatically upon a chart. For example, when a bar is being tested for tension, it is desirable to know not only the actual pull under which it came in two, but the behaviour of the test piece during the period before that. It begins to stretch as soon as the tension is applied, theoretically at all events, and if the metal were perfectly ductile it would stretch continuously as the load increases, until at last the breaking stress is reached. But in actual practice it probably stretches somewhat by fits and starts, and a record of that fact will be of great value in estimating the strength of the material in actual work. For such, an automatically made record, which can be studied at leisure, is of the utmost importance.

But perhaps the finest instance of scientific methods in manufacture is to be found in the methods by which standard parts of machines are measured, so as to ensure that they shall be interchangeable.

It may surprise the casual reader to be told that an absolutely exact measurement is an impossibility. It is safe to say that out of a million similar articles--articles made with the intention that they shall be exactly alike--there are no two which are, in fact, absolutely similar. They may be made with the same machines and the same tools, handled by the same man, but machines and tools wear or get out of adjustment, while man's liability to err is proverbial. Astronomers are the greatest experts in the art of measurement, and they recognise the possibility, nay, the probability, of error so frankly as to make every measurement several times over; if it be an important one they make it, if possible, a great many times over, and then take the average of the results. By this means they eliminate, to a certain extent at any rate, the error which cannot be avoided. That process is to allow for errors on the part of their instruments, for the most part. To deal with personal errors another method is used as well, for it is known that some observers have a natural tendency to err on one side more or less, while others tend to make mistakes in some degree on the other side.

This tendency to err is known as the "personal equation" of the observer, and there are machines and tests by which the personal equation of each man can be determined, or perhaps it would be more correct to say estimated, so that in all observations made by him the proper allowance can be added or deducted.

But of course it would be extremely difficult to apply such methods in a workshop. It would never do to have to measure everything several times over, hoping that the average would come out in such manner as to indicate that the thing being measured was the size required. Instead, therefore, of wasting time seeking an accuracy which is known to be unattainable, the manufacturing engineer adopts a scientific system of measurement wherein a certain amount of inaccuracy is determined upon as permissible, and then simple appliances are used to see that it does, in fact, fall within those limits. For instance, a round bar is to be made, say, an inch in diameter. Now we know from what has just been said that, when made, we have no means of telling whether the bar is really and truly an inch in diameter or not. We consider, then, what it is for, and decide, say, that it will be near enough so long as we are sure that it is not larger than one inch plus one thousandth, nor less than one inch minus one thousandth. So long as it does not exceed or fall short of its reputed size by more than one thousandth of an inch, then we know that it will answer its purpose.

Now, having come to that decision, we can build up a system upon which any intelligent workman can proceed, with the result that all the inch bars which he makes will be the same size within the limits of 1/1000 over or under, so that the greatest possible difference between any two will be 1/500.

This system involves the use of two gauges for every size. The man employed upon making one-inch bars has a plate with a hole in it 1-1/1000 inches in diameter and another hole 999/1000 of an inch in diameter. One of these is the "go in" gauge; the other is the "not go in." So that all he has to do, in order to be quite sure that his work is right, is to see that it can be poked through one of these holes, but not through the other. No trouble at all, it will be observed, adjusting fine measuring appliances, simply a plate with two holes in it, and the workman can be sure that he is turning out articles every one of which is practically correct, with no variation beyond a slight inequality too small to matter.

And probably at some other part of the factory there is a man making articles each of which has a hole in it, into which this bar must fit.

How does he manage? He is provided with a gauge somewhat the shape of a dumb-bell, one end of which is slightly larger than the other. One is the "go in" end, the other the "not go in" end. If the hole which he makes will permit the former to enter, but will refuse admittance to the latter, then he knows that that hole is sufficiently near its reputed size to answer its purpose.

[Ill.u.s.tration: _By permission of The Mining Engineering Co., Sheffield_

A MINERS' RESCUE TEAM

These men are equipped with breathing apparatus which enables them to pa.s.s safely through the deadly fumes after an explosion, to rescue their unfortunate comrades]

In the instances mentioned, a thousandth of an inch either way has been mentioned as the limit of inaccuracy, or the "tolerance," as it is sometimes termed, but often the limits are much narrower than that. The gauges themselves are a case in point, for they must be true within, say, a ten-thousandth, or even less. And they too are checked by master gauges of a finer degree of accuracy still, being made by the most laborious methods, and checked over and over again, so as to reach the utmost limits in the way of correctness.

So this methodical "scientific" system of "limit gauges" is based upon the principle of having one gauge limiting the error one way and another defining it in the other. Anything simpler or more effective it would be impossible to conceive. It is due very largely to this system that many manufactured articles are now so much cheaper than they used to be. For it enables each individual part to be made wholesale on a large scale, by machines specially adapted to the work, operated by men specially trained to work them, with the practical certainty that these parts when a.s.sembled together will fit each other.

In conclusion, there is another very interesting instrument which was first made for a purely utilitarian use--namely, the investigation of the methods of making coloured gla.s.s--but which has since been applied to some interesting problems in pure science. It is called the "ultra-microscope."

It must first be pointed out that there is a limit to the power of the ordinary microscope, beyond which the skill of the optician cannot go.

He is baffled at that point not because of any lack of ability on his own part, but because of the nature of light itself. An opaque object, unless it be self-luminous, which few things are, can only be seen by reflected light. Generally speaking, we see things because they reflect in some degree the light which falls upon them. But light consists of waves, and when we reach an object so minute that its diameter is about half the wave-length of light, then we cannot see it because it is unable to reflect the light on account of its smallness. We can see this any day by the seaside, or by a river or large pond. There it is evident that the waves and ripples are reflected by such things as large stones, wood posts or anything of any size which come in their way; but when a wave encounters an object much smaller than itself it simply swallows it up, as it were, flows all over it or around it, without being in any way reflected by it. And it is just the same with the waves of light; they are unaffected by obstacles below a certain size, and so are not reflected by them. For this reason things smaller than about a seven-thousandth of a millimetre cannot possibly be seen by a microscope in the ordinary way.

But if an object can be made self-luminous, then it can be seen, whatever its size, if the magnifying power of the microscope be great enough. So this ultra-microscope, as it is called, is really an ordinary microscope of the highest power possible, with an added apparatus for making the tiny particles which are being sought for self-luminous. This is done by directing upon them a pencil of light of exceeding intensity.

Generated by powerful arc lamps, the light is concentrated by a system of lenses until it is of an almost incredible brightness, after which it falls upon the object.

Now at first sight this seems to be no different from the usual procedure with a microscope, and there appears to be no reason why it should be more successful, but the explanation is this: light is a form of energy, and the waves of this very intense beam, falling upon the object, throw it into a state of violent agitation, by virtue of which it shines, not with reflected light, but with light of its own. It is not that the waves are reflected, but that they so shake up the particle that it gives off light waves itself. And thus it comes within the range of human vision.

In this way, not only have the very small particles of colouring matter in gla.s.s been seen individually, but it is thought that the actual molecules of matter have been seen, or if not the molecules individually, little groups of molecules, dancing and capering about, just as scientific people for years have believed them to be doing, although they could not see them. So here we have an instance in which manufacture has aided science--an inversion of the usual order of things.

CHAPTER XVI

COLOUR PHOTOGRAPHY

Photography has introduced many of the general public to a branch of practical science which otherwise they would never have cared much about. The action of light upon certain chemicals, the subsequent action upon the same of other chemicals, such as developers, toning solutions and so on, form a very well-known region of the domain of science. And this is, too, a branch of chemistry in which the practical inventor has been very busy. The efforts, therefore, which have been made to invent ways of producing photographic pictures which shall give to the objects their natural colours, will probably be of special interest in a book like this.

Of these there are two very well-known systems, and to them we will mainly confine our attention.

It should first be pointed out, however, that what we are discussing is quite different from the simple "orthochromatic" plates which are used by many photographers. These latter are coated somewhat differently from other plates, with a view to their giving a more realistic picture, but the result is still in one colour. They are, in fact, a little more sensitive to differences in colour than ordinary plates, so that colours which appear, when the latter are used, very much the same, appear, when orthochromatic plates are employed, a little different. But the difference in colour in the object photographed is only, even then, represented by a difference in shade in the picture. The object is, it may be, in many colours, in all the colours, very likely, but the picture is only in one.

And the step from that to a coloured picture is a very long one. True, the solution of the problem is very simple in principle, yet the practical difficulties are so great that even now they have not been entirely overcome.