DECEMIiKK 2S, ISS;^.] 



SCIENCE. 



821 



oxygen. This, in relation to tlie eartli's mass, is 

 .0OOU(XISS25. 



5. Unoxidized carbon. — Tliis occurs not only in 

 coal-beds, but in pyroschists and petroleum. We find 

 that tbe oxidation of a layer of carbon 0.712.3 metre 

 in thickness would use up all the oxygen in the at- 

 mosphere. A layer 2.2.52 metres thick, and having a 

 specific gravity of 1.2.5, if converted into carbon di- 

 oxide, would exert a pressure of one atmosphere. 

 This would amount to 2,207,000 tons of 2,240 pounds 

 each on a square mile. Jlr. J. li. Mott calculates 

 that the amount of unoxidized carbon per square mile 

 cannot be less, and is probably n).any times greater, 

 than 3,000,000 tons. If we .idopt this determination, 

 it will imply a depth of 0.'.t82 metre, and the propor- 

 tion of the earth's mass will be .00000030:) 18. This 

 is the amount of carbon dioxide which must be de- 

 composed to yield a layer of carbon over the earth 

 only a trifle over three feet in tbiclcncss, while it is 

 probable that the carbonaceous deposiLs of tbe earth's 

 crust exceed this. Now. it will hardly be maintained 

 that the uncouibineS carbon of the earth's crust was 

 derived from any other source than the atmosphere, 

 and mostly through the agency of vegetation. The 

 earth's atmosphere must therefore have contained all 

 this amount of carbon dioxide. With the fixation of 

 the carbon, the freed oxygen, it may be said, might 

 have been employed, as far as it would go, in the 

 formation of ferric oxide, whose demands upon the 

 atmosphere have just been computed; but, as it would 

 only satisfy n.'.ij of those demands, it is hardly neces- 

 sary to consider the question. 



6. Meteoric contribulions. — If, as commonly as- 

 sumed, 400,000,000 meteors enter our atmosphere 

 daily, an average weight of ten grains each would 

 amount to a yearly addition of ",13,170 tons. This, 

 in 100,000,000 years, would amount to .000000001542 

 of the earth's mass, and would form a film .292, or 

 nearly ♦, of .an inch thick, having a density of 2.5.' 



Gathering together these various contributions to 

 the earth's mass during 100,000,000 years, we have 

 the following: — 



1. COj reprosentfdby tbfcarbunntos . . . .0003806 



2. CO, fixed In kaolinization of felspars . . .0000175826 



3. COj fixed in decay of hornblendlc and au- 



gltic rocks 000040320!! 



4. O fixed in conveision of ferrous oxide . . .OOOOOOSSJo 

 .5. CO3 repreBcnted by uncombined carbon . .00U0U0;'.6->1.S 

 i>. Meteoric contribulions 000000U0I.J42 



Aggregate 0004.39750722 



This is an addition of — Vi,; to the earth's mass; 

 and, in the present state of knowledge, it does not 

 appear on what grounds assent can be withheld from 

 the result, or some result of similar purport. It 

 must be left with the astronomer to determine what 

 relation this increase may sustain to the moon's ac- 

 celeration in its orbit and to other phenomena. It 

 may be noted, however, that the remote secular reces- 

 sion and retardation of the moon, which (J. H. Darwin 

 has recently brought to view, would have been delayed 

 by the cause here considereil, and the time required 

 for the attainment of the moon's present relations 

 would have been prolonged, but to what extent re- 

 mains to be determined. 



The evidences disclosed by these recent researches, 

 of the ^low accession of gaseous and solid matters to 

 the earth, posse.ss a profound interest. It would al- 

 most seem that the earth's atmosphere is only so much 

 of the intercosmical mixture of gases and vapors as 

 the earth's mass is capable of condensing around it, 



' The value given for tliia lilm In a note, p. 14, in my ' World- 

 life,' should be multiplied by 385,1. 



and that the proportions of these gases are determined 

 separately, each by its own weight and elasticity and 

 by its relative abundance in .space ; so that, as any one 

 becomes diminished by fixation in the planetary crust, 

 new supplies arrive to keep the ratio constant. As 

 under this view it is apparent that an atmosphere 

 should be accumulated around the moon, even after 

 the saturation of the pores of its rocks, it may be 

 said that the moon's mass and volume are such that 

 her atmosi)bere would possess only Vr;, or, according 

 to Neison, jVi, the density of tbe earth's atmosphere; 

 and this degree of tenuity might reduce the lunar 

 atmospheric refraction to the small value actually 

 observed. Alexander Wixcheli,. 



Regulation of electromotive force. 



In one of the articles — the first, I think — recently 

 published in Science (ii. 042) upon the subject of the 

 electric light on the U. S. fish-commission steamer 

 Albatross, the writer tells us that the brilliancy of 

 the Edison incandescent lamps is kept constant, 

 \\'hen other lamps upon the circuit are lighted or 

 extinguished, by placing an adjustable resistance in 

 the circuit of the field-magnets of the dynamo-electric 

 machine, ' whereby the internal and external resist- 

 ances are balanced.' 



The importance of the subject scarcely seems to 

 warrant any more space being devoted to it than 

 already has been. But the point that I bring up is not 

 an immaterial one, such as whether the engine is on 

 the port or the starboard side of the vessel : it is a 

 question which involves interesting and important 

 physical principles. 



The reason an adjustable resistance is required 

 in the field-circuit of an Edison dynamo, in order 

 to maintain a steady incandescence of the lamps, 

 results from the fact that the armature has some 

 resistance. This resistance is quite small, to be sure, 

 but it has a considerable e£fe;l, nevertheless. 



In order that a multiple arc system should be per- 

 fect, so that the dynamo or generator would require 

 no adjustment or regulation when lamps were turned 

 on or off the circuit, it would be necessary that this 

 generator should have absolutely no resistance: for, 

 if it were possible to reduce the internal resistance to 

 zero, then there wniild be no fall of potential within 

 the machine itself; that is, the fall of potential would 

 all be in the external circuit, and the difference of 

 potential between the poles of the generator would 

 be equ.al to the total electromotive force of the cir- 

 cuit. In that case, all that is necessary is to keep 

 the electromotive force constant; and then it follows, 

 that any number of the l.amjis in the system may be 

 lighted or put out without producing any fluctuation 

 whatever in the light of the other lamps, because the 

 incandescence of a given lamp depends only upon 

 the electromotive force with which it is supplied. 

 Now, we know that the electromotive force gener- 

 ated by a dynamo is constant, provided that tlie 

 speed of rotation of its armature, and the intensity of 

 the ficld-magnetisra, are kept constant. ' The arma- 

 ture is maintained at a constant speed because it is 

 driven by a steam-engine furnished with a governor, 

 the function of which is to secure a constant speed;' 

 and the field-magnets have a constant strength be- 

 cause the current which excites them is constant, 

 since this current, like the current in the lamps, is 

 produced by an electromotive force, which, by hy- 

 pothesis, is constant. 



Let us now consiiler the case where the resistance 

 of the armature is not zero (to which, of course, it 

 ' The speed would remain constant, but tbe powt^r required 

 would lacrease wltb the number of lamps in circuit. 



