Part 33 (1/2)
My contrivance consisted of an arrangement by means of which the metal bar or other solid substance, whose total expansion under a given number of degrees of heat had to be measured, was in a manner itself converted into a thermometer. Absolutely equal bulks of each solid were placed inside a metal tube or vessel, and surrounded with an exact equal quant.i.ty of water at one and the same normal temperature.
A cap or cover, having a suitable length of thermometer tube attached to it, was then screwed down, and the water of the index tube was adjusted to the zero point of the scale attached to it, the whole being at say 50deg of heat, as the normal temperature in each case. The apparatus was then heated up to say 200deg by immersion in water at that temperature. The expansion of the enclosed bar of metal or other solid substance under experiment caused the water to rise above the zero, and it was accordingly so indicated on the scale attached to the cap tube. In this way we had a thermometer whose bulb was for the time being filled with the solid under investigation,--the water surrounding it imply acting as the means by which the expansion of each solid under trial was rendered visible, and its amount capable of being ascertained and recorded with the utmost exactness, as the expansion of the water was in every case the same, and also that of the instrument itself which was ”a constant quant.i.ty.”
In this way we obtained the correct relative amount of expansion in bulk of all the solid substances experimented upon. That each bar of metal or other solid substance was of absolutely equal bulk, was readily ascertained by finding that each, when weighed in water, lost the exact same weight.
[Image] James Nasmyth's Expansometer, 1826.
My friend, Sir David Brewster, was so much pleased with the instrument that he published a drawing and description of it in the Edinburgh Philosophical Journal, of which he was then editor.
1827. A Method of increasing the Effectiveness of Steam by super-heating it on its Pa.s.sage from the Boiler to the Engine.
One or the earliest mechanical contrivances which I made was for preventing water, in a liquid form, from pa.s.sing along with the steam from the boiler to the cylinder of the steam-engine.
The first steam-engine I made was employed in grinding oil colours for my father's use in his paintings. When I set this engine to work for the first time I was annoyed by slight jerks which now and then disturbed the otherwise smooth and regular action of the machine.
After careful examination I found that these jerks were caused by the small quant.i.ties of water that were occasionally carried along with the current of the steam, and deposited in the cylinder, where it acc.u.mulated above and below the piston, and thus produced the jerks.
In order to remove the cause of these irregularities, I placed a considerable portion of the length of the pipe which conveyed the steam from the boiler to the engine within the highly heated side flue of the boiler, so that any portion of water in the liquid form which might chance to pa.s.s along with the steam, might, ere it reached the cylinder, traverse this highly-heated steam pipe, and, in doing so, be converted into perfectly dry steam, and in that condition enter the cylinder. On carrying this simple arrangement into practice, I found the result to be in every way satisfactory. The active little steam-engine thence-forward performed its work in the most smooth and regular manner.
So far as I am aware, this early effort of mine at mechanical contrivance was the first introduction of what has since been termed ”super-heated steam”--a system now extensively employed, and yielding important results, especially in the case of marine steam-engines.
Without such means of supplying dry steam to the engines, the latter are specially liable to ”break-downs,” resulting from water, in the liquid form, pa.s.sing into the cylinders along with the steam.
1828. A Method of ”chucking” delicate Metal-work, in order that it may be turned with perfect truth
In fixing portions of work in the turning-lathe, one of the most important points to attend to is, that while they are held with sufficient firmness in order to be turned to the required form, they should be free from any strain which might in any way distort them.
In strong and ponderous objects this can be easily accomplished by due care on the part of an intelligent workman. It is in operating by the lathe on delicate and flexible objects that the utmost care is requisite in the process of chucking, as they are easily strained out of shape by fastening them by screws and bolts, or suchlike ordinary means. This is especially the case with disc-like objects. As I had on several occasions to operate in the lathe with this cla.s.s of work I contrived a method of chucking or holding them firm while receiving the required turning process, which has in all cases proved most handy and satisfactory.
This method consisted of tinning three, or, if need be, more parts of the work, and laying them down on a tinned face-plate or chuck, which had been heated so as just to cause the solder to flow. As soon as the solder is cooled and set, the chuck with its attached work may then be put in the lathe, and the work proceeded with until it is completed. By again heating the chuck, by laying upon it a piece of red-hot iron, the work, however delicate, can be simply lifted off, and will be found perfectly free from all distortion.
I have been the more particular in naming the use of three points of attachment to the chuck or face-plate, as that number is naturally free from any risk of distortion. I have on so many occasions found the great value of this simple yet most secure mode of fixing delicate work in the lathe, that I feel sure that any one able to appreciate its practical value will be highly pleased with the results of its employment.
The same means can, in many cases, be employed in fixing delicate work in the planing-machine. All that is requisite is to have a clean-planed wrought-iron or bra.s.s fixing-plate, to which the work in hand can be attached at a few suitable parts with soft solder, as in the case of the turning lathe above described.
1828. A Method of casting Specula for Reflecting Telescopes, so as to ensure perfect Freeness from Defects, at the same time enhancing the Brilliancy of the Alloy.
My father possessed a very excellent achromatic spy-gla.s.s of 2 inches diameter. The object-gla.s.s was made by the celebrated Ramsden.
When I was about fifteen I used it to gaze at the moon, planets, and sun-spots. Although this instrument revealed to me the general characteristic details of these grand objects, my father gave me a wonderful account of what he had seen of the moon's surface by means of a powerful reflecting telescope of 12 inches diameter, made by Short-- that justly celebrated pioneer of telescope making. It had been erected in a temporary observatory on the Calton Hill, Edinburgh. These descriptions of my father's so fired me with the desire to obtain a sight of the glorious objects in the heavens through a more powerful instrument than the spy-gla.s.s, that I determined to try and make a reflecting telescope which I hoped might in some degree satisfy my ardent desires.
I accordingly searched for the requisite practical instruction in the pages of the Encyclopedia Britannica, and in other books that professed to give the necessary technical information on the subject. I found, however, that the information given in books--at least in the books to which I had access was meagre and unsatisfactory. Nevertheless I set to work with all earnestness, and began by compounding the requisite alloy for casting a speculum of 8 inches diameter. This alloy consisted of 32 parts of copper, 15 parts of grain tin, and 1 part of white a.r.s.enic.
These ingredients, when melted together, yielded a compound metal which possessed a high degree of brilliancy. Having made a wooden pattern for my intended 8-inch diameter speculum, and moulded it in sand, I cast this my first reflecting telescope speculum according to the best book instructions. I allowed my casting to cool in the mould in the slowest possible manner; for such is the excessive brittleness of this alloy (though composed of two of the toughest of metals) that in any sudden change of temperature, or want of due delicacy in handling it, it is very apt to give way, and a fracture more or less serious is sure to result. Even gla.s.s, brittle though it be, is strong in comparison with speculum metal of the above proportions, though, as I have said, it yields the most brilliant composition.
Notwithstanding the observance of all due care in respect of the annealing of the casting by slow cooling, and the utmost care and delicate handling of it in the process of grinding the surface into the requisite curve and smoothness suitable to receive the final polish,-- I was on more than one occasion inexpressibly mortified by the sudden disruption and breaking up of my speculum. Thus many hours of anxious care and labour proved of no avail. I had to begin again and proceed da capo. I observed, however, that the surplus alloy that was left in the crucible, after I had cast my speculum, when again melted and poured out into a metal ingot mould, yielded a cake that, brittle though it might be, was yet strong in comparison with that of the speculum cast in the sand mould; and that it was also, judging from the fragments chipped from it, possessed of even a higher degree of brilliancy.
The happy thought occurred to me of subst.i.tuting an open metal mould for the closed sand one. I soon had the metal mould ready for casting.
It consisted of a base plate of cast iron, on the surface of which I placed a ring or hoop of iron turned to fully the diameter of the intended speculum, so as to antic.i.p.ate the contraction of the alloy.
The result of the very first trial of this simple metal mould was most satisfactory. It yielded me a very perfect casting: and it pa.s.sed successively through the ordeal of the first rough grinding, and eventually through the processes of polis.h.i.+ng, until in the end it exhibited a brilliancy that far exceeded that of the sand mould castings.
The only remaining difficulty that I had to surmount was the risk of defects in the surface of the speculum. These sometimes result from the first splash of the melted metal as it is poured into the ring mould.
The globules sometimes got oxidised before they became incorporated with the main body of the inflowing molten alloy: and dingy spots in the otherwise brilliant alloy were thus produced. I soon mastered this, the only remaining source of defect, by a very simple arrangement.