SCIENCE. 



[Vol. XV. No. 366 



the positive pole to tbe negative, and the latter is built up at 

 the expense of the former. I do not know that the velocity of 

 these particles has even been estimated, but it must be exceed- 

 ingly small as compared with that of electricty (186,000 miles 

 per second) . Suppose we have a positive and a negative 

 electric field, or dual condition, in a dusty atmosphere: may we 

 Bot say that the dust in the positive field, if sufficiently elec- 

 trified, will have a tendency to pass toward the negative field? 

 Or. if we consider that moisture particles take the place of 

 dust, why may not these, positively electrified, have a tendency 

 toward the negative field? We have an illustration on a large 

 scale in the case of thunder-clouds which have been repeatedly 

 seen to approach each other. Mr. Dolbear writes me that he has 

 himself noted a most remarkable and sudden clearing of clouds 

 after a thunder-storm. I have myself observed a line of black- 

 ness gradually advance in a clear sky, the line stretching from 

 the south-east to the north-west. The demarcation between 

 the clear sky and the black cloud was almost geometrical in its 

 sharpness. No rain was felt till the edge of the cloud reached 

 the zenith; and then rain fell in torrents, though there was 

 blue sky almost directly overhead. 



But there is a still more important consideration. The 

 difficulty of changing the moisture contents of tbe air is 

 universally recognized. The number of grains per cubic foot 

 will remain absolutely constant for days at a time, no matter 

 what maybe the heat conditions of the earth, its winds, clouds, 

 or any other changes in the meteorological elements. A six- 

 teen-hours' steady rain has not been sufficient to saturate the 

 air. Noth withstanding these facts, we now know that accom- 

 panying a storm, and independent of the sun's heat, there are 

 most extraordinary fluctuations in the moisture contents of the 

 air. Frequently, over an area of 160.000 square miles, this 

 moisture may be doubled, and immediately following the storm 

 it may be diminished three-quarters of this ; and this, too, 

 absolutely independent of the wind, pressure, or temperature. 

 I will give but one illustration. On Dec. 22, 1889, at 3.11 

 P.M., I observed 4.09 grains of moisture per cubic foot in the 

 air, which was calm at the time. At 5.2 P.M., or 111 minutes 

 later, there were only 1.04 grains per cubic foot. This was 

 certainly the greatest diminution I ever observed, but several 

 times 1 have observed it almost as great. Without going into 

 the questions, which this discussion must raise, it seems to me 

 that such extraordinary changes can be abundantly accounted 

 for on the principles enunciated in this journal, and cannot 

 be accounted for in any other way. What we need most of all 

 are experimental determinations showing the possibility of such 

 transfer in electric fields. Have we any help from the difficulty 

 of running a Holtz machine in a damp room, from the gather- 

 ing of dust and lint on electrified glass rods? Is it possible to 

 electrify a mass of air so as to test any of these questions ? Thus 

 far I have hoped only to interest others more familiar with the 

 subject than myself. I do not expect that I have added any 

 thing to our knowledge; but as Professor Holden has said 

 recently, regarding photographic magnitudes of stars, "any 

 discussion of the question at this stage can but be advanta- 

 geous, " so it seems to me in this field of research we may well 

 consider that any consideration of the questions involved must 

 tend to bring out the best thoughts of many minds ; and ' 'in 

 the multitude of counsellors there is wisdom." 



H. A. Hazen. 

 -Northfleld, Minn,, Jan. 28. 



In my communication on physical fields published in Science 

 of Dec. 27, what I was most desirous of pointing out was the 

 character of the physical re-action of a field of a given sort 

 upon a body in it. The explanation of the various steps was 

 unessential, entirely so; and if my explanations were not the 

 true explanations, the conclusions reached in the main thesis 

 "would not be vitiated. 



Mr. N. W. Perry takes some exceptions to my terminology, 

 which are proper enough if I have not used appropriate terms. 

 I most heartily agree that in all departments of science the 

 terms used should be explicit, definite, and not misleading; 



but it is unfortimate indeed that all through physics, to say 

 nothing of other sciences, there is no general agreement as to the 

 proper use of terms. Take, for instance, the term "heat." 

 Some say "heat is vibratory atomic or molecular motion," 

 others just as competent say "heat is a form of energy." 

 Now, both cannot be right, unless a mode of motion is a form 

 of energy. Again, note the long controversy lately had in 

 England over the proper use of the words "mass" and 

 "weight.' ' 



The significance of it is this: that, until there js a well- 

 settled use of a word in a technical sense, one cannot be 

 altogether blamed if he uses the word in a sense different from 

 some other one. Now, Mr. Perry is certain that I do not use 

 the word ' 'stress' ' properly ; that it ' 'is not proper to say that a 

 stress travels;" that Maxwell and others do not believe that 

 electrification involves motion in any way; that potential con- 

 ditions or energy are static, and that I have made a funda- 

 mental mistake in not discriminating between static and kinetic 

 energy. 



To all this I have to reply, 



1st, Suppose an electrometer to be, say, one metre from a • 

 glass rod which I electrify with a piece of silk. If the electrom- 

 eter gives any indication of electrification, the condition that 

 incites it has travelled with a finite velocity. Whether it be 

 called a stress, a strain, or any thing else, is immaterial; 

 whether it is a condition of the ether or action at a distance in 

 the sense the older philosophers thought, does not matter so much 

 if it takes time to go from the glass rod to the electrometer. 

 One may call it potential or kinetic energy if he chooses : a static 

 condition will presently be reached, but not instantly. And 

 the same is true of the effect produced by magnetizing a piece 

 of iron. 



Mr. Perry seems to say, that, if there was but one body in 

 the universe, it could not have an electric field, even if it could 

 be electrified. If that be his meaning, I must say that his con- 

 ception of electrical re-actions is totally different from mine. 

 As Tait has it, "every action between two bodies is a stress." 

 The body and the ether about it are two bodies; and. if they 

 can act at all upon each other, there will then be a field. 

 Perhaps, however, Mr. Perry calls the ether matter, which has 

 not been my habit, and against which I was not on my guard 

 when I wrote the statement to which he objects. Until we 

 have some evidence that ether is subject to the law of gravi- 

 tation, it seems to me to be improper to speak of it as matter. 

 If "every particle of matter attracts every other particle of 

 matter, ' ' and if there is no evidence that ether is so attracted, 

 it is not conducive to good terminology to call it matter. 



2d, This term "stress" has not been long in use at all, and 

 the adoption of it into electrical science I suppose to be due 

 chiefly to Maxwell. I have therefore looked to see how he 

 employed it, and I find the following in his treatise on 

 "Electricity and Magnetism," Art. 866: — • 



' 'Now, we are unable to conceive of propagation in time, 

 except either as the flight of a material substance through 

 space, or as the propagation of a condition of motion or stress in 

 a medium already existing in space." The Italics are mine, 

 as I interpret them to mean precisely what I meant. Evidently 

 Maxwell did conceive that stress could travel. 



Again, in Art. 863 he says, "The emitted potential ^zos to 

 the body ; ' ' and once more, ' 'The potential as received by the 

 attracted body is identical with, or equal to, the potential 

 that arrives at it;" and once more, "The velocity of trans- 

 mission of the potential is not like that of light, constant 

 relative to the ether or to space, but rather like that of a 

 projectile, constant relative to the velocity of the emitting par- 

 ticle at the instant of emission." 



These quotations seem to me to justify me in the use of the 

 word ' 'stress' ' as a condition capable of translation from one 

 point to another. It is not unlikely, though, that within the 

 past few years, and since Maxwell's death, the term has become 

 more precise; and that, if true, would justify calling attention 

 to a departure from such use. A. E. Dolbeae. 



College HiU, Mass , Feb. 3. 



