Part 9 (2/2)
The experiments were made in the usual manner by firing bullets into a ballistic pendulum, and recording the swing of the pendulum. Thompson suggested a modification of the ballistic pendulum, attaching the gun-barrel to the pendulum, and observing the recoil, and making allowance for the recoil due to the discharge from the gun of the products of combustion of the powder, the excess enabled the velocity of the bullet to be calculated. Afterwards he made experiments on the maximum pressure produced by the explosion of powder, and pointed out that the value of powder in ordnance does not depend simply on the whole amount of gas produced, but also on the rapidity of combustion.
While superintending the a.r.s.enal at Munich, Rumford exploded small charges of powder in a specially constructed receiver, which was closed by a plug of well-greased leather, and on this was placed a hemisphere of steel pressed down by a 24-pounder bra.s.s cannon weighing 8081 pounds. He found that the weight of the gun was lifted by the explosion of quant.i.ties of powder varying from twelve to fifteen grains, and hence concluded that, if the products of combustion of the powder were confined to the s.p.a.ce actually occupied by the solid powder, the initial pressure would exceed twenty thousand atmospheres.
Rumford's calculation of the pressure, based upon the bursting of a barrel, which he had previously constructed, is not satisfactory, inasmuch as he takes no account of the fact that the inner portions of the metal would give way long before the outer layers exerted anything like their maximum tension. When a hollow vessel with thick walls, such as a gun-barrel or sh.e.l.l, is burst by gaseous pressure from within, the inner layers of material are stretched to their breaking tension before they receive much support from the outer layers; a rift is thus made in the interior, into which the gas enters, and the surface on which the gas presses being thus increased, the rift deepens till the fracture is complete. In order to gain the full strength due to the material employed, every portion of that material should be stretched simultaneously to the extent of its maximum safe load. This principle was first practically adopted by Sir W. G.
Armstrong, who, by building up the breech of the gun with cylinders shrunk on, and so arranged that the tension increased towards the exterior, availed himself of nearly the whole strength of the metal employed to resist the explosion. Had Rumford's barrel been constructed on this principle, he would have obtained a much more satisfactory result.
These investigations were followed by a very interesting series of experiments on the conducting power of fluids for heat, and, although he pushed his conclusions further than his experiments warranted, he showed conclusively that convection currents are the princ.i.p.al means by which heat is transferred through the substance of fluids, and described how, when a vessel of water is heated, there is generally an ascending current in the centre, and a descending current all round the periphery. Hence it is only when a liquid expands by increase of temperature that a large ma.s.s can be readily heated from below. Water below 39 Fahr. contracts when heated. Rumford, in his paper, enlarges on the bearing of this fact on the economy of the universe, and the following extracts afford a good specimen of his style, and justify some of the statements made by Cuvier in his eulogy:--
I feel the danger to which a mortal exposes himself who has the temerity to undertake to explain the designs of Infinite Wisdom.
The enterprise is adventurous, but it cannot surely be improper.
The wonderful simplicity of the means employed by the Creator of the world to produce the changes of the seasons, with all the innumerable advantages to the inhabitants of the earth which flow from them, cannot fail to make a very deep and lasting impression on every human being whose mind is not degraded and quite callous to every ingenuous and n.o.ble sentiment; but the further we pursue our inquiries respecting the const.i.tution of the universe, and the more attentively we examine the effects produced by the various modifications of the active powers which we perceive, the more we shall be disposed to admire, adore, and love that great First Cause which brought all things into existence.
Though winter and summer, spring and autumn, and all the variety of the seasons are produced in a manner at the same time the most simple and the most stupendous (by the inclination of the axis of the earth to the plane of the ecliptic), yet this mechanical contrivance alone would not have been sufficient (as I shall endeavour to show) to produce that gradual change of temperature in the various climates which we find to exist, and which doubtless is indispensably necessary to the preservation of animal and vegetable life....
But in very cold countries the ground is frozen and covered with snow, and all the lakes and rivers are frozen over in the very beginning of winter. The cold then first begins to be extreme, and there appears to be no source of heat left which is sufficient to moderate it in any sensible degree.
Let us see what must have happened if things had been left to what might be called their natural course--if the condensation of water, on being deprived of its heat, had followed the law which we find obtains in other fluids, and even in water itself in some cases, namely, when it is mixed with certain bodies.
Had not Providence interfered on this occasion in a manner which may well be considered _miraculous_, all the fresh water within the polar circle must inevitably have been frozen to a very great depth in one winter, and every plant and tree destroyed; and it is more than probable that the region of eternal frost would have spread on every side from the poles, and, advancing towards the equator, would have extended its dreary and solitary reign over a great part of what are now the most fertile and most inhabited climates of the world!...
Let us with becoming diffidence and awe endeavour to see what the means are which have been employed by an almighty and benevolent G.o.d to protect His fair creation.
He then goes on to explain how large bodies of water are prevented from freezing at great depths on account of the expansion which takes place on cooling below 39 Fahr., and the further expansion which occurs on freezing, and mentions that in the Lake of Geneva, at a depth of a thousand feet, M. Pictet found the temperature to be 40 Fahr.
”We cannot sufficiently admire the simplicity of the contrivance by which all this heat is saved. It well deserves to be compared with that by which the seasons are produced; and I must think that every candid inquirer who will begin by divesting himself of all unreasonable prejudice will agree with me in attributing them both TO THE SAME AUTHOR....
”But I must take care not to tire my reader by pursuing these speculations too far. If I have persisted in them, if I have dwelt on them with peculiar satisfaction and complacency, it is because I think them uncommonly interesting, and also because I conceived that they might be of value in this age of _refinement_ and _scepticism_.
”If, among barbarous nations, the _fear of a G.o.d_, and the practice of religious duties, tend to soften savage dispositions, and to prepare the mind for all those sweet enjoyments which result from peace, order, industry, and friendly intercourse; a _belief in the existence of a Supreme Intelligence_, who rules and governs the universe with wisdom and goodness, is not less essential to the happiness of those who, by cultivating their mental powers, HAVE LEARNED TO KNOW HOW LITTLE CAN BE KNOWN.”
Rumford, in connection with his experiments on the conducting power of liquids, tried the effect of increasing the viscosity of water by the addition of starch, and of impeding its movements by the introduction of eider-down, on the rate of diffusion of heat through it. Hence he explained the inequalities of temperature which may obtain in a ma.s.s of thick soup--inequalities which had once caused him to burn his mouth--and, applying the same principles to air, he at once turned his conclusions to practical account in the matter of warm clothing.
After an attempt to determine, if possible, the weight of a definite quant.i.ty of heat--an attempt in which very great precautions were taken to exclude disturbing causes, while the balance employed was capable of indicating one-millionth part of the weight of the body weighed--Rumford, finding no sensible effect on the balance, concluded that ”if the weight of gold is neither augmented nor lessened by _one-millionth part_, upon being heated from the point of _freezing water_ to that of a _bright red heat_, I think we may very safely conclude that ALL ATTEMPTS TO DISCOVER ANY EFFECT OF HEAT UPON THE APPARENT WEIGHTS OF BODIES WILL BE FRUITLESS.” The theoretical investigations of Princ.i.p.al Hicks, based on the vortex theory of matter and the dynamical theory of heat, have recently led him to the conclusion that the attraction of gravitation may depend to some extent on temperature.
A series of very valuable experiments on the radiating powers of different surfaces showed how that power varied with the nature of the surface, and the effect of a coating of lamp-black in increasing the radiating power of a body. In order to determine the effect of radiation in the cooling of bodies, Rumford employed the thermoscope referred to by Cuvier. The following pa.s.sage is worthy of attention, as the truth it expounds in the last thirteen words appears to have been but very imperfectly recognized many years after it was written:--
”All the heat which a hot body loses when it is exposed in the air to cool is not given off to the air which comes into contact with it, but ... a large proportion of it escapes in rays, which do not heat the transparent air through which they pa.s.s, but, like light, generate heat only when and where they are stopped and absorbed.”
Rumford then investigated the absorption of heat by different surfaces, and established the law that good radiators are good absorbers; and recommended that vessels in which water is to be heated should be blackened on the outside. In speculating on the use of the colouring matter in the skin of the negro, he shows his fondness for experiment:--
”All I will venture to say on the subject is that, were I called to inhabit a very hot country, nothing should prevent me from making the experiment of blackening my skin, or at least, of wearing a black s.h.i.+rt, in the shade, and especially at night, in order to find out if, by those means, I could contrive to make myself more comfortable.”
In his experiments on the conduction of heat, Rumford employed a cylinder with one end immersed in boiling water and the other in melting ice, and determined the temperature at different points in the length of the cylinder. He found the difficulty which has recently been forcibly pointed out by Sir Wm. Thomson, in the article ”Heat,”
in the ”Encyclopaedia Britannica,” viz. that the circulation of the water was not sufficiently rapid to keep the temperature of the layer in contact with the metal the same as that of the rest of the water; and he also called attention to the arbitrary character of thermometer-scales, and recommended that more attention should be given to the scale of the air thermometer. It was in his visit to Edinburgh, in 1800, that, in company with some of the university professors, the count conducted some experiments in the university laboratory on the apparent radiation of cold. Rumford's views respecting _frigorific rays_ have not been generally accepted, and Prevost's theory of exchanges completely explains the apparent radiation of cold without supposing that cold is anything else than the mere absence of heat.
We must pa.s.s over Rumford's papers on the use of steam as a vehicle of heat, on new boilers and stoves for the purpose of economizing fuel, and all the papers bearing on the nutritive value of different foods.
The calorimeter with which he determined the amount of heat generated by the combustion, and the latent heat of evaporation, of various bodies has been already alluded to. Of the four volumes of Rumford's works published by the American Academy of Arts and Sciences, the third is taken up entirely with descriptions of fireplaces and of cooking utensils.
Before deciding on the best way to light the military workhouse at Munich, Rumford made a series of experiments on the relative economy of different methods, and for this purpose designed his well-known shadow-photometer. In the final form of this instrument the shadows were thrown on a plate of ground gla.s.s covered with paper, forming the back of a small box, from which all extraneous light was excluded. Two rods were placed in front of this screen, and the lights to be compared were so situated that the shadow of one rod thrown by the first light might be just in contact with that of the other rod thrown by the second light. By introducing coloured gla.s.ses in front of the lights, Rumford compared the illuminating powers of different sources with respect to light of a particular colour. The complementary tints exhibited by the shadows caused him to devise his theory of the harmony of complementary colours. One result is worthy of mention: it is a conclusion to which public attention has since been called in connection with ”duplex” burners. Rumford found that with wax tapers the amount of light emitted per grain of wax consumed diminished with the diminution of the consumption, so that a small taper gave out only one-sixteenth as much light as an ordinary candle for the same consumption of wax. He says:--
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