Feb. 2 2, 1872] 



NATURE 



321 



Composition of Vibrations 



While holding one of Konig's large polished tuning-forks in 

 my hand, I happened to give it a swaying movement on the plane 

 on which its vibrations were being performed, and immediately 

 noticed that the space through which the fork swmig was occu- 

 pied by a series of bright straight lines arranged in a fan-like 

 form. The lines spread out, or drew together, as the rate of 

 movement impressed on the fork increased or diminished. The 

 case was clearly one of composition of vibrations, the bright 

 lines being merely the edges of the prongs seen in positions of 

 instantaneous rest, where the proper motion of a prong was equal 

 and opposite to that communicated to it by the hand. 



By taking forks of different pitch, and causing them to swing 

 with equal velocities, the dependence of pitch on the number of 

 vibrations performed in a given time was easily exhibited. 



In case this simple observation has not yet been made or de- 

 scribed I ask its insertion in Nature. 



Trinity College,. Cambridge Sedley Taylor 



Eclipse Photography 



Mr. J. BoEStNGER, in the last number of Nature, expresses 

 his surprise at the ignorance of the photographers attached to the 

 late expeditions, and favours them with hints, observations, and 

 instructions still more surprising. Because he cannot see their 

 reasons for employing equatorial stands, plates in separate frames, 

 and long exposures, he concludes these were unnecessary ; and 

 affirms " there must have been a great want of balance in their 

 chemicals." No doubt there is a want of balance somewhere, 

 and I diffidently submit the probability tliat Mr. Boesinger has 

 lost his. 



I would briefly state to those few of your readers who may 

 have been misled by this correspondent, that equatorial stands 

 driven by clock-work are absolutely necessary in the production of 

 the best results, either by short or long exposure of photographic 

 plates; a picture "not perfectly sliarp but valuable as a 

 memorial," was what Mr. Boesinger aimed at (and I sincerely 

 hope he obtained it), but the expeditions had higher aims and 

 greater expectations. Single large plates were exposed separately, 

 that should a corona extending many degrees be actinically pre- 

 sent, it might find ample room to put in an appearance ; in such 

 a case had *' repeating backs " been used to give many pictures 

 on one plate, there would have been great danger from the corona 

 of one picture over-lapping that of another, to the ruin of all. 

 Comparatively long exposures were found necessary to secure 

 impression from the faint extremities of the rays. 



Henry Davis 



Tidal Friction according to Thomson and Tait 



I .VM so afraid that this letter will convict me of hopeless 

 stupidity that I conceal my name. For I am going to confess 

 that I do not understand, and even feel inclined to dispute, the 

 reasoning of Thomson and Tait, on pp. 191-194 in their great 

 work, respecting the effect of tidal friction on the motion of the 

 earth and moon. It will be a convenience to your readers if I 

 quote the passage at full length : — 



" Let us suppose the moon to be a uniform spherical body. The 

 mutual action and reaction of gravitation between her mass and 

 the earth's will be equivalent to a single force in some line 

 through her centre, and must be such as to impede the earth's 

 rotation as long as this is performed in a shorter period than the 

 _ moon's motion round the earth. It 



must therefore lie in some such direc- 

 tion as the line MQ in the diagram, 

 which represents, necessarily with enor- 

 mous exaggeration, its deviation, OQ, 

 from the earth's centre. Now, the 

 actual force on the moon in the line 

 MQ, may be regarded as consisting of 

 a force in the line MO towards the 

 earth's centre, sensibly equal in amount 

 to the whole force, and a compara- 

 tively very small force in the line MT 

 This latter is very nearly tangential 

 to the moon's path, and is in the direction -u^ith her motion. 

 Such a ibrce, if suddenly commencing to act, would, in the first 

 place, increase the moon's velocity ; but after a certain time she 

 would have moved so much farther from the earth, in virtue of 

 this acceleration, as to have lost, by moving against the earth's 



perpendicular to MO. 



attraction as much velocity as she had gained by the tangential 

 accelerating force." 



The consequences are then shown to be that the moon's 

 distance would be increased in the ratio i : i . 46, and her 

 periodic time increased, and the earth's period of rotation 

 lengthened. 



This reasoning perplexes me ; for if the effect of a certain 

 amount of fluid friction is to throw the line of action of the force 

 from MO to MQ, a fluid friction is conceivable which should 

 throw it outside the earth altogether. Moreover, the line of 

 attraction of the earth on the moon would be in a line not 

 passing through the earth's centre, a result I cannot understand, 

 especially if the fluid friction were increased as just suggested. 

 Nor can I see that a force in MQ, the centre of the earth being 

 free, would tend to stop the rotation of the earth. 



As I view the matter, fluid friction generates a couple tending 

 to stop the rotation of the earth, and it is impossible to combine 

 this couple with the force in MO, and represent the resultant by 

 a single force. The energy lost in the form of momentum of 

 rotation of the earth is gained in the heat devolved by the fluid 

 friction, which is ultimately dissipated. And the final result 

 would be that the orbit of the moon would not be appreciably 

 altered, while the period of rotation of the earth is gradually 

 lengthened. 



Am I wrong, for the thousandth time in my life ? and if so 

 will some one try and enlighten me. Perhaps Prof. Tait will 

 spare a few minutes to an old friend. M, A. 



Circumpolar Lands 



In Nature (Feb. 8) Mr. Murphy seems to admit the sound- 

 ness o' the reasoning by which I endeavoured to .show (Jan. 25) 

 that the earth's form is probably undergoing a slow progressive 

 change, but he thinks that the statements in the first and last 

 parts of my letter are contradictory. 



If Mr. Murphy will be good enough to read again the para- 

 graph immediately foUovving the one which he quotes, I think 

 he will find that there is no contradiction. " Transmission of 

 pressure towards the poles " must tend to elevate the land in those 

 regions. How that pressure is produced and transmitted I have 

 endeavoured to show in the same paragraph. 



However, the main proposition which I sought to establish in 

 my paper of 1S57, before alluded to, is that any spheroid of 

 equilibrium, whether earth, sun, or any other, in motion .about 

 an axis, in cooling from a fluid stale, undergoes a change of form, 

 and with this proposition Mr. Murphy seems to agree. 



Mr. Murphy has inadvertently omitted part of a sentence in 

 making his quotation from my letter, thus representing me as 

 speaking of a ratio with one quantity only. 



Queen's Coll., Liverpool, Feb. 16 George Hamilton 



The Spheroidal State of Water 



I HAD the pleasure a few days ago of visiting Messrs. Johnson's 

 celebrated iron wire manufactory in Manchester. There may be 

 seen a series of furnaces and rolling mills which in twenty-four 

 hours can convert a truck load of the best Swedish iron into the 

 bright and polished galvanised wire which is now being so ex- 

 tensively employed to complete our very perfect system of Post 

 Office telegraphs. Every stage of the process passes beneath the 

 eye of the observer; the melting of the pigs, the formation of 

 tire billets, the puddling of the bloom, the shingling of the balls, 

 the rolling of the bars, and their subsequent extension by further 

 rolling, and drawing into telegraph wire. 



The bars are cut off into loft. lengths, and are placed in a 

 Siemen's regenerative furnace, where they are raised to a brilliant 

 white heat. They are then drawn out of the mouth of the glow- 

 ing furnace, and pass through a series of consecutive rollers of 

 varying dimensions, and rotating with varying speed, ultimately 

 flowing out in a continuous stream of iron wire. In fact, the 

 metal is at such a high temperature and so plastic that the curves 

 it takes in falling convey the idea of a thin, fine unbroken jet of 

 liquid matter. 



The rollers are kept cool by the constant play upon them of 

 jets of water. The first pair of rollers is fixed close to the mouth 

 of the furnace, which is partially closed by a moveable screw that 

 is only raised when the attend.int sprite requires to direct another 

 bar to the attenuating process of the continuous rollers. The 

 jet of water that cools the first pair of rollers in one furnace fell 

 in a broken shower upon the foot-plate of the mouth of the 

 furnace, which, from its proximity to the fire,, was raised to a 



