5 2 4 



NA TURE 



[Sept. 25, 1 1 



To do away with the part of the emissivity which is due to 

 convection and conduction by the air, I have commenced experi- 

 ments on loss of heat by small wires in the nearly perfect vacuum 

 afforded by the modern mercurial air-pump. This part of the 

 subject was experimented on long ago by Dulong and Petit, and 

 within the last few years by Winkelman and Kundt and Warburg ; 

 lastly, and much more perfectly, by Mr. Crookes (Proc. Roy. 

 Soc, vol. xxxi. p. 239), though in no case, I believe, were the 

 emissivities in absolute measure determined. The conclusion 

 come to by all these experimenters is the same, namely, that 

 there is a decrease of emissivity due to lowering of the air- 

 pressure, this decrease being very small for a reduction down to 

 one-half or one-third of the ordinary atmospheric pressure, but 

 becoming very great as the vacuum approaches completeness. 



The very interesting experiments of Mr. Crookes seem to 

 show that, even with the high vacuum which he obtained, the 

 effect of the residual gas in carrying off heat from the cooling 

 body was far from being annulled. 



The following table shows the emissivity of a copper wire 

 with bright surface half a metre long, 0-40 mm. in diameter, and 

 sealed into a glass tube about 1 - 5 cm. in internal diameter : — 



The following table may also be found interesting. It shows 

 the emissivity in absolute measure of some materials commonly 

 used as insulating coverings for wires. 



On Some Phenomena Connected with Iron and other Metals in 

 the Solid and Molten Stales, with Notes of Experiments, by W. J. 

 Millar, C.E., Sec. Inst. Engineers and Shipbuilders in Scotland. 

 —I. Object of Paper. — Results of experiments by the author 

 with various metals, such as cast-iron, gun-metal, phosphor- 

 bronze, lead, copper, and type-metal. The object being to 

 determine the cause of the well-known phenomena of the flot- 

 ation of cold cast-iron on molten cast-iron, and as to whether 

 iny expansion took place upon solidification in the metals above 

 noted. 2. Notes of some of the experiments from which the 

 author concludes that the cause of flotation of the solid metal 

 on liquid metal of the same kind is buoyancy, due to expansion 

 suddenly set up in the invnersed pieces, and that this expansion 

 was found by careful measurement to be at least equal to the 

 shrinkage or total decrease in length of the piece from white hot 

 solid to finally cooled down solid. Further, that the expansion 

 observed is obtained within much lower limits of temperature 

 than the shrinkage; as the pieces, which were in all cases re- 

 moved from the molten metal, immediately on appearing floating 

 hardly showed redness, and when broken it was found that the 

 crystalline character of the metal remained. 3. Notes of experi- 

 ments made by gradually heating pieces of cast-iron — the results 

 of all these experiments leading the author to conclude that the 

 rate of expansion in cast-iron is at first much more rapid at low 

 1 Temperature probably much too low. The wire, sagging down, touched 

 the inside of the glass tub in several points. 



temperature than afterwards at high temperature. 4. From 

 experiments carried on with pieces of lead and copper and type- 

 metal, it was found that if any flotation occurred it was only 

 with small light pieces — heavy pieces sinking and remaining at 

 bottom of ladle. Gun-metal and phosphor-bronze behaved like 

 cast-iron. 5. Consideration of the peculiar appearance, or 

 "break," observed on the surface of molten cast-iron, the figures 

 presenting a geometrical pattern, like interlacing circles or stars. 

 The author believes that this appearance is due to cracks forming 

 upon the rapidly forming skin — these cracks taking more or less 

 a circular form from the convex forms into which the various 

 parts of the surface are thrown, due to the bubbling up of gas or 

 air. This appearance is limited to cast-iron, and experienced 

 observers can tell the quality of the iron from the form of pattern 

 or figures showing on the molten surface. 6. From observation 

 and experiments carried out from time to time, the author con- 

 cludes that no perceptible increase of volume of the metals noted 

 occurs at the moment of solidification; at least when free from 

 air or gas confined within the casting. 



On a Gyrostatic Working Model of the Magnetic Compass, by 

 Sir William Thomson. — In my communication to the British 

 Association at Southport, 1 I explained several methods for over- 

 coming the difficulties which had rendered nugatory, I believe, 

 all previous attempts to realise Foucault's beautiful idea of dis- 

 covering with perfect definiteness the earth's rotational motion 

 by means of the gyroscope. One of these, which I had actually 

 myself put in practice with partially satisfactory results, was a 



Gyrostatic Balance for Measuring the Vertical Component of the 

 Earth's Rotation. 

 It consisted of one of my gyrostats supported on knife-edge 

 attached to its containing case, with their line perpendicular to 

 the axis of the interior fly-wheel and above the centre of 

 gravity of the fly-wheel and framework by an exceedingly 

 small height, when the framework is held with the axis of 

 the fly-wheel and the line of knife-edges both horizontal, 

 and the knife-edges downwards in proper position for per- 

 forming their function. The apparatus, when supported on 

 its knife-edges with the fly-wheel not spinning, may be dealt 

 with as the beam of an ordinary balance. Let now the frame- 

 work bear two small knife-edges, or knife-edged holes, like those 

 of the beam of an ordinary balance, giving bearing-points for 

 weights in a line, cutting the line of the knife-edges as nearly as 

 possible, and of course (unless there is rea-on to the contrary in 

 the shape of the framework) approximately perpendicular to 

 this line, and, for convenience of putting on and off weights, 

 hang, as in an ordinary balance, 1 vo very light pans by hooks 

 on these edges in the usual way. Now, with the fly-wheel not 

 running, adjust by weights in the pans if necessary, so that the 

 framework rests in equilibrium in a certain marked position with 

 the axis of rotation inclined slightly to the horizontal, in order 

 that the axis of the fly-wheel, whether spinning or at rest, may 

 always slip down so as to press on one and not on the other of 

 the two end plates belonging to its two erhls. Now, unhook the 

 pans and take away the gyrostat and spin it ; replace' it on its 

 knife-edges, hang on the two pans, and find the weight required 

 to balance it in the marked position with the fly-wheel now 

 rotating rapidly. This weight, by an obvious formula which was 

 placed before the Section at Southport, gives an accurate measure 

 of the vertical component of the earth's rotation. 2 



Gyrostatic Model of the Dipping Needle 

 I also showed at Southport that the gyrostatic balance de- 

 scribed above, if modified by fixing the knife-edges, with their 

 line passing as accurately as possible through the centre of 

 gravity of the fly-wheel and framework, and with the faces of the 

 knives so placed that they shall perform their function properly 

 when the axis of the fly-wheel is parallel to the earth's axis of 

 rotation, and the rotation of the fly-wheel in the same direction 

 as the earth's, will act just as does an ordinary magnetic dipping 

 needle ; but showing latitude instead of dip, and dipping the 

 south end of the axis downwards instead of the end that is 



No report of this communication has, s 

 print. 

 : The formula is 



_§7e= - W/.--W7 ; 



know, hitherto appe; 



where ?(/ denotes the balancing weigh 

 the arm on which this force 

 radius of gyration ; « its angul. 

 / the latitude of the place. 



the force of gravity upon it ; a 

 W the weight of the fly-wheel; k its 

 city ; 7 the earth's angular velocity ; and 



