96 



NATURE 



\yu7iei, 1880 



It does not seem likely, however, that the invfestigation would 

 lead to interesting physical results, because this kind of motion 

 is essentially dynamically unstable. 



Towards the end of the same article there occurs the following 

 jiassage : — 



" The old idea that a ship (or more correctly a totally im- 

 mersed body, such as a fish) encountered a mysterious resistance 

 in addition to the mere friction of the molecules on its sides, is 

 now known to have been a pure delusion." 



This statement appears to me either erroneous or very mis- 

 leading. The resistances to the motion of a ship have been 

 classified under three heads, viz., wave-making resistance, eddy- 

 making resistance, and surface-friction. ^ For a totally-immersed 

 body the wave-making resistance is nonexistent, but Mr. Pres- 

 ton would appear only to take notice of the last of the three. 

 Now whilst for a body with "fair line-," such as a fish, the 

 eddy-making resistance may be small, yet if the lines are not 

 fair it may be very large. Thus a fish leaves scarcely any wake, 

 whilst an oar leaves a very great amount of disturbance. 



Helmholtz, Kirchhoff,' and Lord Rayleigh^ have made some 

 interesting hydrodynamical investigations on the resistance suf- 

 fered by a vane exp Dsed to a current, on the hypothesis that in 

 the wake of the vane there is dead water, separated from the 

 moving water by surfaces of finite slip. 



It has been already noticed that such a motion is dynamically 

 unstable, but there is in many respects a remarkable accordance 

 between the resistance as determined by this theory and that 

 found experimentally,'' so that it seems probable that the actual 

 stable motion of flow, with eddies in the wake, does not differ 

 very much from the theoretically unstable motion, with dead 

 water in the wake. It will be noticed that this theory of resist- 

 ance, which gives approximate results for bodies with very bad 

 lines, such as fiat vanes, actually entirely neglects surface-fric- 

 tion, to which Mr. Preston's statement would seem to refer the 

 whole resistance. G. H. Darwin 



Trinity College, Cambridge, May 27 



The Inevitable Test for Aurora 

 In reference to Prof. Piazzi Smyth's courteous criticism of our 

 communication to the Royal Society on the aurora borealis, we 

 regret that we are unable to say whether the critical' citron line, 

 to which he directs attention, was present or not in the spectra 

 of the electric discharges in atmospheric air from which we 

 deduced the probable heights of auroral displays. The experi- 

 ments quoted Avere made without reference to the aurora, and 

 this particular line was consequently not SDUght for, nor indeed 

 have many measurements been made of the spectra of discharges 

 in atmospheric air, on account of the time required and conse- 

 quent gi'eat consumption of the life of the battery which such 

 observations entail. Warren De la Rue 



73, Portland Place, W., May 29 Hugo ML/ller 



Cloud Classification 



There is a proverbial objection to "looking a gift-horse in 

 the mouth," and M. Poey's Cloud Book is such a valuable 

 addition to the scanty literature on the subject that it would 

 be highly ungracious to make captious objections to his views. 

 On the other hand, M. Poey, when he differs from others, puts 

 forward his views with such fairness and conrtesy that I believe 

 he would be the last man to deprecate full discussion. 



Allow me then to put in a plea for certain old pubhc servants, 

 that they should neither be cashiered altogether, nor transported 

 to strange regions, without full examination into their character 

 and their merits. 



First, then, for the slratiis. 



M. Poey— happy man !— has carried on his observations under 

 tropical .skies and in the clear atmosphere of Paris. Had his 

 lot been cast on the clays and gravels of the London basin I 

 venture to think that he would have regarded the "stratus" 

 with more respect, if with no increase of affection. He would 

 have had frequent opportunities of observing it — at times 

 resting entirely on the ground,^ at others rising with a clearly 



' Froude, Proc. of Roy. Inst., December, 1876. 



- " Math. Vorlesungen," zist and 22nd lectures. 



3 Pki'i. Maff.f December, 1S76. 



4 In particular Lord Rayleigh's investigation throws light on the theor>' 

 of the balanced rudder. 



5 Howard's Kssay says, "its inferior surface commonly'* (not "invari- 

 ably" or "necessarily") "rests on the earth ;or water." P. 7, Edit. 



defined lower and upper [ surface, a few feet (or even 

 inches) from the earth, cutting the taller trees in a horizontal 

 line, leaving their tops and bottoms free, and then being 

 gradually dissipated, to be absorbed in the warmer air or to form 

 cumuli at a higher elvation. He could hardly have failed to 

 recognise it as a clear and distinct variety of cloud, the lowest in 

 altitude of all the family, but none the less a member of it. If 

 every cloud which has contact with our baser earth is to be 

 casliiered on that account, what \vill become of M. Poey's own 

 cumulus on Plate XV. ? Every mountaineer knows to his cost that 

 if he happens to be on the mountain where such a cumulus is 

 resting, he will be enveloped in a fog undistinguishable from 

 what he finds on the Thames marshes. 



Whether, on the other hand, it is desirable to use the term 

 "stratus" for clouds in a totally different sky-region, -which 

 differ both in their origin and their nature from the true 

 "stratus," is a question too long to be fully discussed here. 



Next with regard to the ninUnts. 



M. Poey's view appears to be that Howard's term applies;to 

 an isolated sliDwer-cloud, and is unsuitable for a rain-cloud over- 

 spreading the sky. After careful reading of M. Poey's remarks 

 on the " pallium," and comparing them with Howard's descrip- 

 tion of the "nimbus," I entirely fail to see where lies sufficient 

 difference to consign the " nimbus " to oblivion ; and I can only 

 imagine that iM. Poey has taken his idea " of what Howard 

 meant almost entirely from the illustration, without noticing that 

 Howard first describes the forming and behaviour of the cloud 

 overhead in words curiously similar to those which Poey himself 

 uses for his " pallium," and then says, " But we see the nature of 

 this process more perfectly in viewing a distant shower in 

 profile." This clearly shows that the illustration was only 

 chosen as the easiest form in which the cloud, vel nubiiini 

 congeries, could be depicted, while the context guards completely 

 against the name being limited to an isolated shower-cloud. 



It would occupy to much space to place the descriptions of 

 the two savants side by side, but I think that any one who will 

 take the trouble to read the two together can hardly fail to see 

 that Howard's " nimbus '' fulfils all that Poey describes as the 

 rain-discharging cloud, including the upper " veil," = or pallium 

 of cirrus, the lower " sheet," ^ or " pallium " of cumulus, and 

 the "lower clouds arriving from the windward," which " move 

 under this sheet and are successively lost in it " (Howard, p. 

 II ; compare Poey, Plate XII.). In fact, to use an expression 

 frequently employed iir the discussion of patents, you can take 

 the description of the one inventor and "read it on to ", the 

 drawing of the other, or vice versa. 



M, Poey's term "pallium" is certainly expressive, and will 

 probably make itself a home in cloud terminology ; but it 

 appears after all only to mean that a certain modification over- 

 spreads the whole or a large part of the sky (compare Howard, 

 p. 11), and does not by any means cover that combination of 

 clouds which produces rain ("nimbus.") 



I must leave it to a future time or to other pens to discuss the 

 merits of the "cumulo-stratus," and pass on to examine shortly 

 JI. Poey's views about the "cumulus." The Rev. W. C. Ley, 

 in his review of M. Poey's work, in your pages, has already 

 pointed out the illogical nature of the author's repeated remark 

 that the " cumulus " only exists in the horizon, forgetting that 

 a cloud which is on the horizon of one place must be in the 

 zenith of another. Now I venture to suggest that this curiously- 

 distorted mental view aftects M. Poey's classification far more 

 than appears at first sight. If clouds are considered not ob- 

 jectively according to their whole form and structure, but 

 subjectively as they present themselves to an individual observer, 

 we naturally need new modifications as the clouds are viewed in 

 different positions. Are not many of the clouds which M. Poey 

 calls " fracto-cumulus " simply "cumuli" viewed from beneath? 

 Just as (to borrow a simile from Mr. Ley) an elm-tree seen from 

 beneath presents a spreading, ragged edge, and shows the blue 

 sky through its interstices, whereas on the horizon it appears 

 compact, rounded, and sharjily defined. 



May I add a practical suggestion as to the popular terms 

 proposed by M. Poey on p. 39 ? These terms are put forward as 

 an alternative to the scientific Latin names, for the use of iion- 

 scioitific observers, who may be of great service in collecting 

 information at out-stations where no trained meteorologist is at 

 hand. It is therefore all-important that they should be as short, 



^ See Poey, p. 33. 



= "At a greater .ahitude a thin light veil," &c., Howard, p. 11, and 

 again, "supeme cirrata," p. 4. 

 3 " The lower clouds .... form one uniform sheet," p. 11. 



