I02 



NATURE 



{yttne 2, 1887 



the white house, whilst the tree was still almost bare of leaves. 

 Not wishing for the noise and dirt so near the windows, I 

 removed it, and they began another ; again it was removed, and 

 this time, though apparently little more than a flat beginning, it 

 had eggs' upon it. They tried again, and on removing it the 

 third time 1 found that the birds were overlaying it on all sides 

 with the flowers of some sweet Alyssum that was growing below ; 

 the intention being, evidently, to render it more like the back- 

 ground of white wall, and therefore less conspicuous. 



Sidmouth. J- M. H. 



Earthquakes and the Suspended Magnet. 



During the afternoon of May 3 at Lyons, N.Y., a peculiar 

 quivering motion of the suspended magnet was noted, especially 

 at about i o'clock p.m., and a strong westward deflection con- 

 tinued during the afternoon. Similar phenomena have been 

 noted repeatedly when earthquakes were in progress, in this 

 case the shock being quite severe, and occurring at 3.8 p.m. at 

 El Paso, Texas. M. A. Veeder. 



Lyons, N.Y., May 4 



Units of Weight, Mass, and Force. 



The letter of Prof. Greenhill (Nature, vol. xxxv. p. 486) is 

 both timely and suggestive. Herbert Spencer's chapter on 

 space, time, matter, motion, and force, supplemented by his 

 chapter on the persistence of force, in "First Principles of 

 Philosophy," gives all that can be desired by the student for a 

 complete comprehension of the subject. One who assimilates 

 the basic truths there so clearly given need never be perplexed 

 by any statement found in the mechanical and mathematical 

 text-books. It is simply impossible to use language in regard to 

 these matters without employing expressions that are true only 

 in a certain sense. We say that "the sun rises " and " the sun 

 sets," and that "the heavens revolve." If these words are used 

 to indicate the cause of the progressive shadows on a sun-dial, 

 or the time of day, they serve a practical need as well as if they 

 were true. But a student who should infer the constitution of 

 the solar system from such phrases would go far astray. 



When the significance of Spencer's explanation of motion is 

 grasped, a great part of the ambiguity will have vanished. We 

 constantly think of motion as an entity, which is a pure delusion. 

 We also say of force that it is the cause of motion. Nothing 

 can be more untrue. Force is the cause of change of motion 

 only. There is not a conceivable difference between rest and 

 motion otherwise than as the expression of a relation. Whether 

 a body be at rest or in motion depends wholly upon the body to 

 which it is related. 



When the student sees that motion is no entity, and is familiar 

 with the process by which the conceptions of matter, force, 

 space, and time, are built up from sensations, he will be in no 

 danger of mistaking the sense in which certain text-book state- 

 ments are to be taken, much less will he be captured by those in 

 which the errors are unpardonable. I. Lancaster. 



Chicago, 111., April 28. 



With regard to Mr. Geoghegan's letter in your issue of 

 April 7 (vol. xxxv. p. 534), my experience in teaching physics 

 long ago led me to the same conclusions. For three years I 

 have used in my classes in this the oldest existing University in 

 Ontario, and with the greatest advantage, the terms iach, gram- 

 tach, prcin, and dyntach for the units of velocity, momentuai, 

 pressure-intensity, and rate of working respectively, in the 

 C.G.S. system of units. These may be found in my "Intro- 

 duction to Dynamics," which was printed last year for my junior 

 class. Prem was chosen after failure to get a euphonious mono- 

 syllable from the Greek. A name for the unit of acceleration I 

 have not found to be necessary. Vel seems to me to be a good 

 word for the unit of velocity in the F.P.S. system of units, but, 

 for fear of hanging on a sour apple-tree, I would shudder to 

 mention poundvcl and poundal-vcl. The term squeeze would be 

 suitable in several respects for a p.ttndal per sqture foot, but in 

 mixed classes, such as we have here, it mdght lead to disorder. 

 , D. H. Marshall. 



Queen s University, Kingston, Ontario, Canada, April 27. 



Remarkable Phenomenon seen on April 26, 1687. 



A phenomenon was seen here this evening quite distinct 

 from anything I have before observed. It was an exact copy of 

 streams of aurora borealis rising from a low arch, but instead 

 of being in the northern heavens it was near the south horizon. 

 The sky was cloudless, except a long thundercloud which extended 

 from near south-south-west to almost south-south-east, the upper 

 portion of this cloud being about 12° above the horizon. From this 

 cloud issued from one to three streams of conspicuous white light, 

 the north-easterly stream being the largest and brightest, and this 

 continued visible from 9.40 until 10.5 (the others were only 

 seen for five minutes). The streams were at an angle of 

 about 53°, and moved slowly easterly (the cloud moving in the 

 same direction). The longest stream reached an altitude of 25°, 

 and at 10 o'clock exactly (G.M.T.) the base was immediately 

 over the Avonmouth Lighthouse. The light of the streams was 

 more persistent and less flickering than is usually the case w ith 

 aurora borealis. 



There was also a confused luminosity behind the cloud, which 

 varied considerably in brightness ; this made the outline of the 

 cloud at one time distinctly visible, and at another scarcely 

 discernible ; this alsj gave the clouds a black appearance. After 

 10 p.m. other clouds rose above the cunuL -stratus, and the 

 streams became hid. Three hours afterwards there was a snow- 

 storm, and the ground was white till 7 a.m. Reports from 

 Somerset, Dorset, and Devon would be valuable. 



Shirenewton Hall, near Chepstow. E. J. Lowe. 



Pear-shaped Hailstones. 



Are pear-shaped hailstones as uncommon as some of your 

 correspondents suppose ? 



We have had here to-day a succession of heavy showers of 

 rain and hail together, the hailstones being small, but many of 

 them pear-shaped, and the rest of shapes which might easily 

 have been derived from that form by attrition or partial melting. 



About half past six this evening we had a storm of hail only, 

 heavier than any that preceded it, in which nearly or quite all 

 the stones were pear-shaped, from a fifth to a third of an inch in 

 diameter. B. WoouD SMITH. 



Penmaenmawr, N. Wales, May 20. 



P.S. — May 21. At g.30 this morning we had another shower 

 of hail and rain, in which the stones showed no sign of any 

 pear-shape, but were of irregular rounded forms. 



" A Junior Course of Practical Zoology." 



In the review of Messrs. Marshall and Hurst's book refeiTcd 

 to in my friend Prof. Bourne's letter, I sought to compare that 

 work with others devoted to the familiar type-system, to which 

 alone the words "all other books current " were meant by me 

 to refer, to the exclusion of general text-books such as those from 

 which he quotes. I admit that I might have made my meaning 

 soAiewhat plainer than I did, and would beg to be allowed to 

 state that I had it in my mind, at the time of writing, to refer 

 the reader to the impartial statements made on the subject rn 

 question by Prof. Rolleston in his " Forms of Animal Life " ; 

 the first of the series of what we are now pleased to term ' ' type " 

 or "junior course " books. 



Wiih respect to my critic's second objection, I would ask the 

 readers of Nature to judge for themselves how far the quota- 

 tions which he so skilfully weaves into his letter do justice to my 

 contention. His view is, like my own, but an expression of 

 opinion, and time alone can show which of the two will come 

 nearest the truth. G. B. H, 



South Kensington. 



Bishop's Ring. 



The letter by Prof. G. H. Stone in Nature, vol. xxxv. p. 

 581, is interesting, as showing the disappearance of "Bishop's 

 ring" in Colorado. It has not wholly disapp-ared here, being 

 still plainly visible about sunset. In the middle of the day, 

 however, I have rarely seen any trace of its red colour since May 

 last year ; but up to that time, although growing much fainter,, 

 it was still frequently plain here, and I also saw it in the south 

 of England, both in May and June 1886, but only feebly. Since 

 then, when there has been a slight tinge of red, it has usually 



