Jan. 6, 1876] 



NATURE 



187 



What I most have meant to say is, that the sodium sulphide 

 appears like a precipitate, i.e. as a powder, in the partly 

 dissolved mass. The reason of its appearing to be black on 

 aluminium, and not, as it ought orthodoxically to be, brown, I 

 cannot tell, as the metal is not attacked. Perhaps my reviewer 

 can ? I only say that it is so. 



May I take this opportunity of soliciting you to afford, if 

 possible, a little more space in your valuable journal, to the 

 admittedly neglected subject (in England) of blowpipe analysis ? 

 It is, I can assure any of our chemists who have not much em- 

 ployed it, a most fascinating study, which will amply repay any 

 leisure time expended upon it. W. A. Ross 



Meteor in the Daytime 



Dec. 22, about 2 p. m., as our servants were sitting at dinner 

 by the kitchen window, two of them were startled by the sudden 

 appearance of a b riUiant meteor descending in the E. with a 

 litde inclination to the N. It was not as large as the moon, but 

 much larger than Mars or Saturn, white, and like lightning, with 

 a very quick course, leaving a train as broad as itself, and pre- 

 serving its full size till it was lost behind the top of an oak tree 

 at a Utde distance, whose branches, though leafless, seem to have 

 concealed it from view. The next day I found by means of a 

 compass and a joined ruler, that its azimuth was E. by N., its 

 inclination towards N. about 10% the upper window frame, 

 where it probably came into sight, 48°, and the top of the tree 

 21° above the horizon. I have not, as yet, heard of any other 

 observation of this remarkable meteor. The position of Hard- 

 wick Vicarage, where it was seen, according to the Ordnance 

 Map, is Long. W. 3° 4' 23", Lat. N, 52° 5' 20'. 



T. \V. Webb 



Protective Resemblance in the;Sloths 



As "mimicry" and " protective resemblance " have chiefly 

 been noticed among insects and the lowest of vertebrated animals, 

 the following observation regarding the three-toed sloth, made 

 at the beginning of this centurj', and therefore much in advance 

 of the period at which attention had been directed to this subject, 

 is, in these days, not without interest. It is taken from a work 

 not frequently met with, namely. Baron Albert von Sack's 

 "Narrative of a Voyage to Surinam" (London, 1810). In 

 chap. xvi. at p. 170, he says : — " The colour and even the shape 

 of the hair are much in appearance like withered moss, and serve 

 to hide the animal in the trees, but particularly when it gets that 

 orange-coloured spot between the shoulders, and lies close to the 

 tree ; it looks then exactly like a piece of branch where the rest 

 has been broken off, by which the hunters are often deceived. " 

 The colour of the hair of the body is thus distributed in Arcto- 

 ^E pithecus casta neiceps, A. griseus, and A. Jlaccidus (" Notes on the 

 ^m Species of Bradypodidje in the British Museum," by the late 

 ^ Dr. J. E. Gray. Proa ZooL Soc., 1871, p. 428, Plates xxx v. - 

 xxxviL). 



Brants, in his " Dissertatio Zoologica Inauguralis de Tardi- 

 gradis " (Lugdun. Batav., 182S), p. 28, says of the sloths :—" At 

 provida natura, cum animanti negaverit arma et tela, velleri cum 

 colorem tribuit, quo subducatur oculis ferarum et adverjarioram 

 fere eadem ratione, ac Pallas retulitde Ptcromye zolantc." The 

 passage to which reference is made is in the "Nova: species quadru- 

 pedum e glirium ordine," p. 357 :— " Dum vero in Betuletis prse- 

 sertim vitam agunt, sapienter a natura perspectum est, ut omni 

 tempore anni exalbido canescentem colorem velleris servent, quo 

 cortici betularum ita fiunt similes, ut scandentes vix, imo sub 

 diluculum, quo tempore prjesertim excurrunt, plane non conspici 

 eminus possint, coque ab avibus rapacibus noctm-nis securiores 

 sunt" Reference is also made to Prince Maximilian of Nieu- 

 wied's " Beitrage zur Natnrgeschichte von Brasilien," tome ii. 

 «• 480. J. C. Galton 



Dec. 29, 1875 



Coffee in Dominica 

 In- Nature (vol. xui. p. 38, and under the head of " Coffee 

 in Dominica"), it is stated that the " falling off in the cultivation of 

 the coffee plant, in a soil and climate which experience showed 

 was eminendy suited to it in every respect, was due to the exten- 

 sive destruction of the plants by what was there known as the 

 coffee blight" The foregoing statement requires this qualifi- 

 cation, that after the appearance of the coflfee blight, and 

 when the coffee crop was gradually decreasing in quantity, the 



old coffee planters made no attempts to check the ravages of the 

 destroying insect, but, in many instances, cut down the valuable 

 trees, planted the sugar-cane, and converted their coffee-works 

 into sugar- works. I could mention the names of several estates 

 where what I have described was done. I think it right also to 

 add that in some portions of Dominica, where the coffee-trees 

 were simply abandoned, they now stand,' and, considering their 

 age and the neglect to which they have been exposed, they bear 

 fairly well. During the last two years, and since the disastrous 

 fall in the price of cane sugar, I have been endeavouring to re- 

 introduce here the coffee culdvation, and, on the Tabery esUte, 

 12,000 young trees of my own planting are doing well. You 

 will confer a great and lasting benefit upon this beautiful but 

 neglected and almost unknown island by calling attention to its 

 capabilities as a coffee-producing country. 



Edmuxd Watt 

 South Chiltem, Dominica, Dec. 11, 1875 



The Law of Storms 



I HAVE to thank you for publishing, in Nature of Dea 2, 

 1875, my letter in reply to M. Fayes theory of cyclones, and I 

 have now to submit some remarks on his theory of waterspouts. 



I understand him to maintain that the dark part of the water- 

 spout, which we see, contains a core of transparent air, which is 

 descending at the centre of a vortex, and that the dark visible 

 external part is a cloud formed by an ascending coimter-current 



All this is unproved, and I think baseless. No dynamical 

 reason can be assigned why there should be a downward current 

 at the centre of the vortex. If the waterspout is formed in a 

 vortex, which I think probable, though I am not certain of it, 

 the vortical modon will produce not a downward but an up- 

 ward current at its centre, in consequence of the diminution of 

 barometric pressure, due to the air being thrown to the circum- 

 ference by the centrifugal force. We see such upward currents 

 formed in the little dust-whirlwinds that form themselves over 

 streets and roads in windy weather. 



Further, if M. Faye's theory were true, and if the waterspont 

 were transparent at the centre, it could not be so well defined 

 and solid as it usually is, nor could it be formed so rapidly. 



The true theory of waterspouts is expounded in Espy's " Philo- 

 sophy of Storms," a work which, notwithstanding its great error 

 of deaying the rotation of cyclones, made an era in meteorol<^y, 

 and, so for as I am aware, is not yet superseded. 



When vapour is condensed into water, forming cloud, the 

 latent heat of the vapour is liberated and expands the air. A 

 simple calculation shows that, after deducting the destroyed 

 volume of the condensed vapour, the increased volume of the air 

 due to this expansion is between four and five times as great as 

 the volume of the vapour before condensation. I^ then, the air 

 is neariy saturated with moisture, and the temperature in a state 

 of convective equilibrium for dry air (that is to say, when the 

 difference between the temperatures of any two strata is that due 

 to the difference of their pressures), and condensation b^ins iu 

 any column of air, the effect of liberating this heat will be to 

 make the air of that colunm warmer and lighter than the air at 

 corresponding heights in the surrounding columns. What fol- 

 lows is from Espy's work, page 44 : — 



"It begins, by its diminished specific gravity, to rise, and 

 then, if all circumstances are favourable, the cloud will increase 

 as it ascends, and finally become of so great perpendicular 

 depth, that by its less specific gravity the air below it, in conse- 

 quence of diminished pressure, will so expand and cool by ex- 

 pansion, as to condense the vapour in it ; and this process may 

 go on so rapidly that the visible coae may appear to descend 

 to the surface of the sea or earth from the place where it 

 first appears, in about one or two seconds. The terms here em- 

 ployed must not be understood to mean that the cloud actually 

 descends ; it appears to the spectator f o descend, but this is an 

 optical deception, arising firom new jKJruons of invisible vapour 

 constantly becoming condensed, while all the time the individual 

 particles are in rapid motion upwards." 



To this I wiil add as very probable, if not quite certain, that 

 the rarefaction thus caused at the waterspout wdl produce an 

 inflow of air from ad sides, and this will produce a vortex at the 

 centre ; this again, by its centnfiigal force, will increase the 

 rarefaction, and thus will intensify the effect. But the com- 

 mencement of the waterspout is in the way described by Espy 

 in the above extract. Joseph John Murphy 



Old Foi^e, Dunmorry, Co. Antrim, Dec. 12, 1875 



