250 



SCIENCE 



LVol. IV., No. 84. 



called attention to the fact that one solid cannot 

 slide over another without tearing. 



Professor Reynolds also presented an interesting 

 paper on a method of illustrating the second law of 

 thermo-dyn amies by means of kinetic elasticity. If 

 a long flexible cord or chain be suspended with a 

 weight at the lower end, the weight may be lifted a 

 considerable distance by communicating a vibratory 

 motion in a horizontal plane to the upper end of the 

 chain. It then represents an absolutely reversible 

 engine. If the weight, when at an elevated point, 

 be removed from the chain, to straighten the chain 

 out will require as great an expenditure of energy, 

 not counting dissipation through friction, etc., as was 

 consumed in raising the weight. It was shown that 

 in this model the mean square of ,the velocity of the 

 chain, multiplied by the weight per unit of length, 

 corresponds to the heat in Carnot's engine. Another 

 form of the device consisted of two vertical cords to 

 which a number of horizontal bars of wood were at- 

 tached at equal distances. In discussing the paper, 

 Professor Fitzgerald described a very pretty illus- 

 tration of the same principle by means of a 'balanced 

 governor,' with a chain and weight attached in such 

 a way as to be in equilibrium in all positions, the 

 details of which are difficult to describe without 

 the aid of a diagram. 



The subject of the relative vapor tensions of a 

 body in the liquid and solid state at the same tem- 

 perature was discussed in a paper by Professor Ram- 

 say and Mr. Sydney Young. 



Professor James Thomson long ago pointed out 

 that there must be a sudden change in the curve of 

 vapor density of water at the point of solidification; 

 and showed that this change was really to be detected 

 in Regnault's results, but that Regnault himself 

 had not thought such a break to occur, and had 

 ' smoothed ' his curve at this point ; believing errors 

 of observation to be sufficient to cover the discrep- 

 ancies. 



Messrs. Ramsay and Young, by means of ingenious 

 devices, had overcome some of the difficulties of the 

 experimental investigation, and had experimented 

 upon camphor, benzine, water, and several other sub- 

 stances. The results were in accordance with the 

 previously accepted views, and in the case of water 

 were found to agree with those based upon Professor 

 James Thomson's formula. 



Radiation was the subject of two or three papers. 

 Professor Dewar offered the methods and results of 

 an investigation of the law of total radiation at high 

 temperatures. The plan and arrangement of the ap- 

 paratus for the research were ingenious and effective ; 

 and Professor Dewar stated that he had just learned 

 from Professor Newcomb, that he had some time be- 

 fore devised and described an arrangement for the 

 same purpose, identical in principle with that made 

 use of in his own work. 



For relatively low temperatures, Professor Dewar 

 made use of an iron vessel containing mercury, into 

 which a thermometer-bulb was pushed. The radia- 

 tion measured was that from one side of the vessel, 

 which was made of exceedingly thin iron; and the 



heat was received upon the face of a thermopile 

 enclosed in a case properly screened, and arranged 

 so that a steady current of water would be used to 

 maintain constancy of temperature at one face. For 

 these lower temperatures, the equation expressing the 

 amount of radiation was of the ordinary parabolic 

 form, the radiation being nearly proportional to the 

 square of the temperature. The difficulty in dealing 

 with high temperature is, that most substances un- 

 dergo an alteration in the character of the surface 

 when the temperature is very much raised. The 

 arrangement finally adopted consisted essentially of a 

 platinum air thermometer, the bulb of which was en- 

 closed in a small furnace with a small opening through 

 which the radiation took place. The walls of the 

 platinum bulb were very thick, nearly a quarter of an 

 inch in the actual experiment, and the bulb was con- 

 nected with a mercury manometer for determining 

 the pressure. Experiments were also made to deter- 

 mine the radiation when the thermopile was protected 

 by an iodine screen. The results were as follows, 

 the numbers being in arbitrary units: — 



Radiation at 600° . . 15.5, screen used, 8. 



" 700° . . 19.5, " " 12.2 



" 800° . . 29.0, " " 19.5 



" 900° . . 42.5, " " 29.9 



''1000°. .60.5, " " 44.0 



" "1100°. .84.5, " " 66.0 



The assumption was made, that the radiation was 

 represented by some power of the temperature ; and 

 this power was found in the first case to be 3.4, and 

 in the second 3.3, thus showing a tendency to ap- 

 proach the fourth power ; and attention was called to 

 the fact that many of the results of Dulong and Petit 

 were well represented by the equation R=a£ 4 . 



Mr. J. T. Bottomly offered a paper on the loss of 

 heat by radiation and convection as affected by the 

 dimensions of the cooling body, and on cooling in a 

 vacuum ; which, on account of the absence of the au- 

 thor, was read by Sir William Thomson. The paper 

 was based on an extensive series of resistance meas- 

 urements of copper wires under various conditions, 

 accepting the well-known coefficient of increase of 

 resistance of copper for increase of temperature. The 

 conclusion was reached, that the emission power was 

 greater for small wires than for large ones, and that it 

 diminished with the pressure. 



It would be almost impossible to give a perfectly 

 clear idea of Sir William Thomson's paper on a 

 gyrostatic working-model of the magnetic compass, 

 without quoting the paper entire. What he proposes 

 to accomplish may, however, be readily understood. 

 At the last meeting of the association, at Southport, 

 he had explained several methods for overcoming the 

 difficulties which seemed to have defeated all previous 

 attempts to realize Foucault's "beautiful idea of dis- 

 covering with perfect definiteness the earth's rota- 

 tional motion by means of the gyroscope." He had 

 there shown that a gyrostat supported, without fric- 

 tion, on a fixed vertical axis, with the axis of the 

 fly-wheel approximately horizontal, will behave ex- 

 actly as does a ' magnetic compass,' only with ref- 



