6r4 



NA TURE 



[October 29, 1891 



number of Ahnenplasmas varies from species to species ; (2) 

 tiiat the number in the combination and not the character of the 

 Ahnenplasmas determines the species. And as there is not a 

 particle of evidence for the latter postulate, we may say that 

 on hypothesis B the theory breaks down by its non-conformity 

 vith the facts. 



We have then the dilemma, from which I see no escape, that 

 the theory is inconsistent, on A with itself, on B with the facts. 

 When once worked out and fairly put into words, which was not 

 so easy as it may appear, this argument seemed so obvious that 

 I felt sure it must have been long since urged, confuted, and dis- 

 missed. But not having found any reference to it, I now state 

 it fully, in the hope that the question raised may be thoroughly 

 discussed. Marcus Hartog. 



Dublin, October 12. 



Rain-making Experiments. 



Your last number contains an article by Prof. Curtis on the 

 "rain-making" experiments in Texas, in which no reference is 

 made to the report published in the October number of the North 

 American Kevie%v by General Dyrenforth, who directed the 

 operations. I wish to call attention to the remarkable differences 

 which exist between the statements of Prof. Curtis, the meteoro- 

 logist of the expedition, and General Dyrenforth, its director. 

 On August 10, Prof. Curtis, who had not yet arrived at the 

 scene of the experiments, believes that only sharp showers or 

 "good grass-rain" fell; General Dyrenforth says the amount 

 was nearly 2 inches. On August 18, Prof. Curtis says that only 

 002 inch of rain fell ; General Dyrenforth says that "drenching 

 rain fell in torrents for two and a half hours," and that driving 

 from the encampment to Midland, a distance of 25 miles, the 

 road traversed was covered for 6 or 8 miles under 4 to 40 inches 

 of water. It is impossible, under these circumstances, for those 

 interested to come to any conclusion at present with regard to 

 the actual results of the experiments. May I draw your 

 attention further to an article which appeared in the Manchester 

 Guardian of the 13th inst., in which a suggestion was made 

 precisely similar to that put forward by Prof. Giglioli in your 

 last number. If, as setms probable, the experiments of Mr. 

 Aitken amply suffice to explain any positive results obtained, it 

 is evident that the explosions of hydrogen and oxygen, on which 

 General Dyrenforth relies so much, are useless, and that the 

 smoke-producing rackarock does all the work. In an ex- 

 tremely sceptical and very justly critical article, which follows 

 that of General Dyrenforth in the North American Review, 

 Prof. Simon Newcomb, while scouting the "concussion" 

 theories of General Dyrenforth, says, indeed, that smoke 

 particles may possibly seive as nuclei for the condensation of 

 water vapour ; but he is evidently unacquainted with the re- 

 markable work of Mr. Aitken, which throws so much light on 

 the matter. H. 



Manchester, October 24. 



A Rare Phenomenon. 



Having just returned from Norway, it may be of interest to 

 record that the band of light which was observed by many of 

 your correspondents on September il, was remarkably brilliant 

 in N. lat. 62°, extending from the horizon to the zenith, but 

 not beyond. It was nearly, but not quite, equal in width 

 throughout the 90°, and therefore must either have been much 

 wider at the base than at the apex, or else at an immense alti- 

 tude. Some clue to the estimation of this altitude would be 

 afforded by an accurate record of the zenith distance as observed 

 in England. 



I may add that the aurora borealis was distinctly visible in the 

 north and north-west at the same time, but this band rose from 

 the north-east, which led me to conjecture that it might belong 

 to a comet ; however, on the following night it did not recur, 

 and I then thought it might have been caused by some sun-lit 

 cirri at a great elevation, but it is now obvious that this was not 

 the case. The remarkable feature was its concurrence with, 

 and yet apparent difference from, the ordinary aurora. 



Richmond, Surrey, October 24. W. Duppa-Crotch. 



The phenomenon observed by Dr. Copeland (Nature, 

 September 24, p. 494) at 11. 18 p.m. on September 10 at 

 Dunecht, by Mr. W. E. Wilson at 9 p.m. on September 11 

 in Co. Westmeath, and by other observers on the lith in 



NO. I 148, VOL. 44] 



several parts of England, was observed by a party of three 

 including myself, at 9.30 p.m. on September 25 at Ballater 

 Aberdeenshire. 



It appeared as an intense white beam of light stretching 

 from east to west and directly overhead, of uniform width and 

 perfectly steady. It seemed quite low down, almost as if it 

 might light up the summit of the church spire were it move, 

 a little further towards the south. At 11.30 the light ha' 

 become diffuse, and it appeared at a much greater eleva 

 tion, though maintaining its general direction from east K 

 west. W. N. Hartley. 



October 23. 



Earthquake at Bournemouth, 

 We had a sharp momentary shock of earthquake here at foi' 

 o'clock this afternoon. I happened to have my eyes fixed on ; 

 plant with long variegated leaves on my dining-room table. 

 Suddenly there was a heavy sound as of some subterranean fall, 

 and simultaneously the leaves of this plant were violently agi- 

 tated — waved up and down — for some seconds. It was as if it 

 had risen vertically and then fallen. It was wholly unmoved 

 by so much as a tremor the rest of the afternoon. I tried to 

 reproduce anything like the same disturbance by hand, but with- 

 out success. Henry Cecil. 

 Bregner, Bournemouth, October 25. 



W = yi.g. 



I had read Mr. Slate's letter (Nature, vol. xliv. p. 445), 

 and admired it ; moreover, I found mjself in agreement with 

 him. But it seems to me strange that Prof. Greenhill should 

 approve of it. For Mr. Slate takes as his gravitational unit of 

 force "the weight of one pound under circumstances specified 

 . . . {locality, vacutim)." Surely this implies that he agrees 

 with the theorists (Prof. Greenhill's foesj when they say that 

 " the weight of a given body depends on the local value of g." 

 Prof. Greenhill, on the contrary, speaking of goods, says that 

 " the weight cannot be said to vary with the local valut of g " 

 (Nature, vol. xliv. p. 493), I would ask him, then — 



(i) What name does he give to the earth's pull on a given 

 body ? Or, what is it that a spring balance measures when the 

 said body is hung from it ? He cannot say " its weight" ; for 

 the pull referred to varies with g, while Prof. Greenhill's 

 "weight " does not. I conclude that he has no special name 

 for it. The theorists have ; and they thereby gain in brevity 

 without losing by ambiguity, since they do not employ the word 

 " weight " in any other sense in their text-books. 



I would also repeat the still unanswered question — (2) 

 How does Prof. Greenhill give the expression for hydrostatic 

 pressure at a given depth in any locality, if he banishes " g"f 

 (Nature, vol. xliv. p. 341). And does he conclude that Mr. 

 Slate does not use "^" in hydrostatics? 



Again ... (3) Does Prof. Greenhill, in common with Mr. 

 Slate and the theorists, use the word mass in speaking of the 

 fundamental units ; and, if so, in what sense? 



In the science of dynamics we recognize two properties of 

 matter: . . . (\.) \is inertia ; . . . {\\.) the attraction between 

 it and other matter. The theorists use the word mass when 

 they refer to quantity of matter as measured by its inertia ; and 

 they use the word weight when they refer to the attraction of a 

 given body to the earth. For commercial purposes it is con- 

 venient to measure quantity of matter by balancing its weight 

 against that of the standard lump of platinum, its multiples, 

 and sub-multiples. Hence the every-day, slightly ambiguous, use 

 of the word " weight " in matters in which we are not concerned 

 with inertia. But in the science of dynamics, of which Newton's 

 laws are the foundation, we are concerned primarily with 

 inertia. The theorists, therefore, in their text-books, regard 

 the well-known lump of platinum as the standard pound, the 

 British tinit of mass. They thus have the word ' ' weight " free, 

 and say [e.g. ) that the weight of the stc: ndard pound is measured by 

 the resultant pressure that it exerts (in vacuo) on the bottom of the 

 box in which it lies. It requires more than general expressions 

 of condemnation to show that any other system of nomenclature 

 is clearer or less free from ambiguity, or that the equation 

 W — M^ has not as much meaning as any other dynamical 

 equation. (I may refer back to my letter, Nature, vol. xliv. 

 p. 493), W. Larden. 



Devonport, September 26. 



