Oct. ii, 1888] 



NA TURE 



573 



fluid were quiescent, is easily explained ; and the explanation is 

 illustrated by the diagram of curves, § 7 below, with the time- 

 values given for sugar and common salt. Look at curve No. I, 

 and think of the corresponding curve with vertical ordinates 

 diminished in the ratio of 1 to 40. The corresponding diffusion 

 would take place for sugar in 1 1 seconds, and for salt in 3^ 

 seconds. The case so represented would quite correspond to a 

 streaky distribution of brine and water or of syrup and water, in 

 which portions of greatest and least salinity or saccharinity are 

 within half a millimetre of one another. This is just the condi- 

 tion which we see, in virtue of the difference of optic refractivity 

 produced by difference of salinity or of saccharinity, when we 

 stir a tumbler of water with a quantity of undissolved sugar or 

 salt on its bottom. If water be poured very gently on a quantity 

 of sugar or salt in the bottom of a tumbler with violent stirring 

 up guarded against by a spoon — the now almost extinct Scottish 

 species called " toddy ladle " being the best form, or, better 

 still, a little wooden disk which will float up with the water ; 

 and if the tumbler be left to itself undisturbed for two or three 

 weeks, the condition at the end of 17 x io 5 seconds (twenty days) 

 for the case of sugar, or 54.x io 5 seconds (six days) for salt, 

 will be that represented by No. 10 curve in the diagram. 



6. If the subject be electricity in a submarine cable, the 

 "quality" is electric potential at any point of the insulated 

 conductor. It is only if the cable were a straight line that x 

 would be (as defined above) distance from a fixed plane : but 

 the cable need not be laid along a straight line ; and the proper 

 definition of x for the application of Fourier's formula to a sub- 

 marine cable is the distance along the cable from any point of 

 reference (one end of the cable, for example) to any point of the 

 cable. For this case the diffusivity is equal to the conductivity 

 of its conductor, reckoned in electrostatic units, divided by the 

 electrostatic capacity of the conductor per unit length insulated 

 as it is in gutta-percha, with its outer surface wet with sea-water, 

 which, in the circumstances, is to be regarded as a perfect con- 

 ductor. For demonstration of this proposition see vol. ii. 

 Art. lxxiii. (1855) of my collected papers. 



7. Explanation of Diagram showing Progress of Laminar 

 Diffusion. — In each curve— 



NP = 



v* - . 



run 



where x denotes the number of centimetres in ON, and i the 

 "curve-number." The curves are drawn directly from the 

 values of the integral given in Table III., appended to De 

 Morgan's article "On the Theory of Probabilities," "Ency- 

 clopaedia Metropolitana," vol. ii. pp. 483-84. 



NP denotes the "quality " 

 (defined below) 



at distance = ON from initia 

 surface or interface, 



and at time equal in seconds to 

 [" curve-number "]- divided 

 by sixteen times the diffus- 

 ivity in square centimetres 

 per second. 



Subject of Diffusion. 



Motion of a viscous fluid 



Closed electric currents within 

 a homogeneous conductor 



Heat 



Substance in solution 



Electric potential in the con- 

 ductor of a submarine 

 cable 



' Quality" ( represented by ,',, NP). 



Ratio of the velocity at N to 

 the constant velocity at O 



Current-density 



Ratio of temperature minus 

 mean temperature to mean 

 temperature 



Ratio of density minus mean 

 density to mean density 



Ratio of potential at N to 

 constant potential at end O 



Prof. G. II. Darwin sent a paper On the Mechanical Con- 

 ditions of a Swarm of Meteorites and on Theories of Cosmogony. 

 — This is an abstract of a communication made to the Royal 

 Society, in which the author proposes to apply the principles of 

 the kinetic theory of gases to the case of a swarm of meteorites 

 in space. In the author's theory the individual meteorites are 

 considered to be analogous to the molecules of the gas ; and thus 

 a swarm of meteorites, in the course of conglomeration into a 

 star, possesses mechanical properties analogous to those of a gas. 

 Lockyer and others have expressed their conviction that the 

 present condition of the solar system is derived from an accretion 

 of meteorites, but the idea of fluid pressure seems necessary for 

 the applicability of any theory like the nebular hypothesis. 

 The author then proposes to reconcile the nebular and meteoric 

 theories by showing that the laws of fluid pressure apply to a 

 swarm of meteorites. The case of a globular swarm of equal- 

 sized meteorites is considered, and then the investigation is 

 extended to the case in which the meteorites are of various 

 sizes ; the latter extension does not affect the nature of the proof, 

 and only slightly modifies the result. In the case of a swarm of 

 meteorites condensing under the mutual attraction of its parts, 

 the author shows that the larger meteorites will tend to settle 

 towards the centre of condensation, and that consequently the 

 mean size of the meteorites will decrease from the centre towards 

 the outside of the swarm. 



NOTES. 

 We mentioned some time ago that the executors of the late 

 Sir William Siemens, desiring to have his biography authori- 

 tatively published, had placed its preparation in the hands of 

 Dr. William Pole, F.R.S., Honorary Secretary of the Institu- 

 tion of Civil Engineers, who had long been a personal friend of 

 Sir William and his family. The work is now finished, and will 

 be published immediately, in one volume, by Mr. Murray. It 

 will be followed by other volumes, containing reprints of Sir 

 William's most important scientific papers, lectures, and 

 addresses, edited by his secretary, Mr. E. F. Bamber. 



All who take an interest in questions relating to technical 

 education have reason to be grateful to the Goldsmiths' 

 Company for the way in which it has associated itself with the 

 movement for the establishment of technical and recreative 

 institutes in South London. By an act of splendid generosity it 

 has secured that there shall soon be a great centre of technical 

 instruction at New Cross. Subject to the sanction of Parlia- 

 ment, which will of course be readily granted, the following 

 proposal has been accepted. Out of the surplus funds of the 



