April 20, 1883.] 



♦ KNOWLEDGE ♦ 



239 



current flows, and, vhcn allowed to rise, the circuit is 

 immediately disconnected. 



So much for the Wheatstone Bridge. It is a set of 

 apparatus the value of wliich is incalculable. IJy its 

 means the practical electrician is able to detect almost 

 erery possible fault in a line, whether it be a disconnection, 

 a partial oxidation of the conductor, an earth fault or leak, 

 or, what amounts almost to the same thing, a metallic con- 

 tact Notwithstanding its paramount utility, there are 

 engaged in the newer applications of electricity several 

 so-called electricians who have no more knowledge of this 

 method of testing than the proverbial lunar inhabitant. It 

 will doubtless surprise many to learn that a man who was 

 in charge of one of the most important experimental instal- 

 lations of the electric light once asked us to recommend a 

 work on the rudiments of electricity; while on another 

 occasion an " electrician " in the same company's employ, 

 having to trace three or four wires, declined the offer of a 

 battery and simple galvanometer, and preferred to cut a 

 notch in a piece of wood and putting each wire in it in 

 succession, followed the wires throughout their length 

 over ceilings and under roofs, and through no end 

 of rubbish to the various lamps. To the ama- 

 teur and the experimenter in general the apparatus 

 is equally a boon, and its use was well brought out 

 at a meeting of the Society of Telegraph Engineers, 

 on the 12th inst., when Mr. Shelford Bid well showed how 

 by its aid he had gauged the resistances of microphonic 

 contacts under a multitude of circumstances. 



The subject of electrical measurement, fraught as it now 

 is with the highest importance, must be suspended for a 

 time in these columns, to make room for a concise de- 

 scription of the various forms of batteries, their manu- 

 facture, principles, efficiencj-, and applications. 



OX THE FORMATION OF COMETS' 

 TAILS. 



By a. S. Davis. 



IN his letter to Knowledge, for March IG, 1883, Mr. 

 Eanyard has piiuted out that the mode in which he 

 conceives the force of recoil from evaporation to act in 

 producing a comet's tail differs materially from that which 

 has been proposed by me. 



According to Mr. Ranyard's supposition, a mist, consist- 

 ing of small particles which do not themselves undergo 

 evaporation, envelopes the nucleus of a comet. Upon the 

 particles of this mist matter of a more volatile kind is 

 being continually precipitated on the cool side, and evapo- 

 rated on the warmer sunny side. The matter thus pre- 

 cipitated and e\aporated is, in the first instance, evapo- 

 rated from the nucleus, and in streaming away into space 

 is caught by the particles of mist Mr. Ranyard thinks 

 that each particle of the mist, before its passage into the 

 tail, may condense and evaporate a quantity of matter 

 many times its own mass, and may, in this way, acquire 

 the velocity necessary to account for the formation of a 

 tail, a velocity, namely, which, in many cases, must be 

 many times greater than the relative velocity with which 

 the molecules escape from the particle of mist upon 

 evaporation. 



Xow against this view there appears to me to be an 

 insuperable objection. Mr. Eanyard has, in fact, left out 

 of account one of the forces acting upon the supposed par- 

 ticles of mist In addition to the foiir forces he enume- 

 rates, — viz., the gravitation towards the nucleus and the 

 heat repulsion from the nucleus, the gravitation towards 



the sun and the heat repulsion from the sun, — there will be 

 the force arising from the resistance to motion through the 

 supposed atmosphere. 



Now, by the time that the particle has acquired a 

 velocity away from the sun equal to the average velocity 

 with which the molecules escape on evaporation, this 

 resistance will have become equal to the force due to recoil 

 from evaporation ; for by the hypothesis as much matter is 

 met with as is afterwards evaporated, and since the relative 

 velocity with which the molecules meet the particle will 

 then be at least equal to the relative velocity with which 

 they leave the particle, any further increase of velocity 

 away from the sun will be impossible. Thus, on Mr. 

 Ranyard's hypothesis, the velocity of the particles tailwards 

 could never be as great as the average velocity of transla- 

 tion of the molecules of gaseous matter due to thermal 

 energy, however great might be the quantity of matter 

 precipitated on them and afterwards evaporated. 



Mr. Ranyard states that he has been dri%-en to make 

 this hypothesis of a mist of small particles on which matter 

 is being precipitated and afterwards evaporated, from the 

 difficulty of conceiving of the precipitation and subsequent 

 evaporation of the same substance in the neighbourhood of 

 a comet's nucleus. But, I would ask, where does this 

 difficulty arise? Let us suppose that a comet on its 

 approach to the sun consists of an agglomeration of blocks 

 of matter more or less loosely compacted about a nucleus, 

 that these blocks are of very various sizes, and that some 

 of them consist of matter almost wholly volatilisable by 

 the sun's rays, if continued long enough. From the ex- 

 treme smallness of the mass of a comet, it would be im- 

 possible for it to retain any permanent atmosphere, and 

 any matter volatilised by the sun's heat would at once 

 fly oft" into space. On its approach to the sun, such a 

 current of gaseous matter would be set up by the sun's 

 heat. At and about the nucleus, where the cometary 

 matter is most thickly clustered, this current would 

 be sufficiently powerful to carry along with it some of the 

 smaller masses of meteoric matter. It may be noted that 

 a very small force would suffice for this, seeing that the 

 gravitating force of the comet must be almost nil. The 

 matter thus carried away, and for the most part towards 

 the sun, since the evaporation would be on the sunny side 

 of the nucleus, would, however, itself be undergoing 

 evaporation, and when the matter had been carried so far 

 from the nucleus as to be almost entirely free from the 

 current of gas setting outwards, then the force of recoil 

 due to the evaporation of its own substance would begin 

 to tell upon its motion. 



My illustration of a block o ice, equal in mass to a 

 cubic metre of water, enclosing a gramme of sand, was 

 chiefly taken as one admitting of very ready calculation. 

 It would appear more probable that the volatile substance 

 in many comets is some hydro-carbon, either frozen solid 

 or in the liquid state. Some of the meteoric masses would 

 probably be almost entirely made up of this hydro-carbon, 

 either broken off, or splashed off, from larger blocks by 

 their collision, and containing, perhaps, the merest trace of 

 non-volatile material enclosed or dissolved in them. Such 

 masses on volatili-sation would ultimately attain an enor- 

 mous velocity. As regards the continued emission of large 

 tails by a comet, having the constitution I have supposed, 

 even after several returns to perihelion, this would be quite 

 possible if the comet contained a nucleus having a core, 

 even a few miles in diameter, in which the masses were so 

 thickly clustered as to make it impenetrable by the sun's 

 rays. 



Mr. Ranyard observes that the velocity I obtain in my 

 example is only 29.3,000 mUes per day, whereas velocities 



