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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[October, 



was a remarkable fact that the water washing to the north of Ireland was a 

 foot higher than on the south coast. There were several other remarkable 

 facts whicli the Astronomer-Royal detailed, to which he gave philosophical 

 explanations, and his observations were of that highly interesting character 

 as to demand for them the applause of the section. 



Jlr. Scott Russell made his final report " On the Tides of the East Coast 

 of Scotland." He said he had little more to add to the report which he had 

 submitted to tlie last meeting. He recommended that observations of this 

 sort should be made at the highest and lowest point of the tide, and should 

 be kept perfectly continuous, which might be done by employing two sets of 

 observers, one for the night time, and the other for the day. He mentioned 

 a self-registering tide gauge which had been invented by a gentlemen at Port 

 Glasgow, and which gave the heights in the most simple and accurate 

 manner. 



Mr. Russell also made some remarks upon " The Nature of Sound Waves," 

 which were principally details of experiments in which he has been engaged. 



A paper was then produced, written by the Rev. ■ — ■ O'Brien, '■ On the pro- 

 pagation of Waves in a resisted medium, with a new explanation of the dis- 

 persion and absorption of light, and other optical phenomena." The com- 

 munication was read, and its contents verbally described by J. J. Sylvester, 

 Esq. 



On Specific Heat. — By J. P. Joule. — After examining the law of Duloug 

 and Petit, that the specitic heat of simple bodies is inversely proportional to 

 their atomic weights, the author proceeded to detail the attempts made by 

 Haycraft, De la Rive, and Mercet, to discover the specific heats of gases and 

 liqiiids. The observarions of Newraann and Regnault on the specific heats of 

 simple and compound bodies were next examined. Mr. Joule then exhibited 

 to the Section a table, in which the theoretical specific heats of a variety of 

 bodies impartially selected were calculated, on the hypothesis, that the capa- 

 city for heat of a simple atom remains the same in whatever chemical com. 

 bination it enters. On the whole, the coincidence between the theoretical 

 and experimental results was such as woidd induce a belief that the law of 

 Dulong and Petit, with regard to simple atoms, is capable of a greater degree 

 of generalization than chemists have hitherto been inclined to admit. 



On the Alteration that takes place in Iron by being exposed to long-con- 

 tinued Vibration. By Mr. W. Lucas. — At Cork, this subject was 

 brought forward, and certain specimens of iron exhibited, in order to show 

 the effects produced upon the iron by being exposed to a certain degree of 

 concussion or vibration during the process of swaging, and again restored to 

 its original state by being annealed, in accordance with the results detailed 

 by Mr. Nasmyth, at Manchester, in 1842 ; in addition to these, also were ex- 

 hibited specimens of portions of the same iron that had been exposed to the 

 concussion of a large till hammer, working at the rate of about 350 strokes 

 per minute, which occasioned the bars of iron to break short off at the point 

 of bearing in the course of twenty-fonr hours; there was also shown a por- 

 tion of one of the hammer shafts, the texture of which had evidently been 

 altered, probably by the long-continued and repeated concussions to which 

 it had been exposed, for instead of breaking with the peculiar splintery frac- 

 ture common to wood, it broke with a peculiar short fracture, and this, I am 

 informed, is a common occurrence. In continuance of these experiments 

 upon the effects of concussion or vibration, Mr. Lucas laid before the section 

 the results of some further experiments. 



EARL OF RO-SSE'S REFLECTING TELKSCOPE. 

 The Earl of Rosse commenced by stating, that the Council having inti- 

 mated tlipir opinion that some account of the experiments in which he had 

 been engaged on the Reflecting I'ele.s'cope would not be altogether devoid of 

 interest, he would endeavour to describe, as briefly as possible, the manner 

 in whtch he had attempted to accomplish the object in view, and the princi- 

 pal results (ibtained. Two objects required to be kept in view : first, to give 

 the telescope sufficient aperture to secure a sufficiency of light; secondly, to 

 increase to a sufficient extent the magnifying power. On tliese depended 

 what might be called the optical power of the instrument, but particularly 

 upon the former. For instance, the large telescope, of which a model stood 

 before them, to be used effectually, must have a magnifying power of 300 

 times. Now, another instrument, very inferior in size, might have a much 

 higher power, but. from the vast quantity of light which it collected into the 

 image, objects in it became distinct w hich could not be at all seen by those of 

 inferior aperture. The next question he had to determine was, whether he 

 should attempt refractors or reflectors. Just at that time very large and very 

 fine discs of the proper glass had been produced upon the Continent, and a 

 strong hope was entertained of bringing the refracting telescope to a degree 

 of perfection which had been hitherto rather hoped for than attained. But, 

 upon a calm balancing of all thedifficulties which oppused their construction, 

 he determined to attempt the improvement of the Newtonian reflector, and 

 that notwithstanding it was well known that an error of form of the reflector 

 produced an error in the image more than five times as great as the same 



error in the refractor would produce. It was to the steps by which he at- 

 tained this object that he was now about to direct the attention of the Sec- 

 tion. 



" Having concluded that upon the whole there was a better prospect of ob- 

 taining by reflection, rather than by refraction, the power which would be 

 required for making any effectual progress in the re-examination of the ne- 

 bul;e, the first experiments were undertaken, in the hope of obviating the 

 difficulties which had previously prevented the application of the brilliant 

 alloy which may be formed of tin and cojiper in proper proportions to the 

 construction of large instruments. The manner in which the difficulty had 

 been met, was, by adding an excessive proportion of copper to the alloy, but 

 tlie mirror was no longer susceptible of a durable polish, and, wiien used, its 

 powers declined rapidly. It appeared to me, therefore, to be an object soim- 

 portant to obtain a reflecting surface which would reflect the greatest quan- 

 tity of light, and retain that property little diminished for a length of time, 

 that numerous experiments were undertaken and perseveringly carried on. 

 After a number of failures the difficulties appeared to be so great that I con- 

 structed three specula, where the basis of the mirror was an alloy of zinc 

 and copper in the proportion of 1 zinc to 2"74 copper, which expands with 

 changes of temperature in the same proportion as speculum metal. This was 

 subsequently plated with speculum metal, in pieces of such size as we were 

 enabled to cast sound. These specula were very light and stiff, and their 

 performance upon the whole satisfactory ; l^ut they were afl!ected by diflfrac- 

 tion at the joinings of the plates; and althougli very brilliant and durable, 

 defining all objects well under high powers, except very large stars, still as 

 the effect of diffraction was then perceptible they could not be considered as 

 perfect instruments. In the course of the experiments carried on while these 

 three specula were in progress, it was ascertained that the difficulty of cast- 

 ing large discs of brilliant speculum metal arose from the unequal contraction 

 of the material, which in the first instance, produced imperfections in the 

 castings, and often, subsequently, their total destruction ; and it appeared 

 evident, that, if the fluid mass could be cooled throughout with perfect regu- 

 larity, so that at every instant every portion should be of the same tempera- 

 ture, there would be no unequal contraction in the progress towards solidifi- 

 cation, nor, subsequently, in the transition from a red heat to the tempera- 

 ture of the atmospliere. Although it was obvious that the process could not 

 be managed so that the exact condition required [should be fulfilled, still, by 

 abstracting heat uniformly from one surface (the lower one), the temperature 

 of the mass would be kept uniform in one direction, that is, horizontally; 

 while in the vertical direction, it would vary in some degree as the distance 

 from the cooling surface. These conditions being satisfied, we should like- 

 wise have a mass which would be free from flaws, and, when cool, would be 

 free from sensible strain , nothing could be easier than to accomplish this, ap- 

 proximatelj-, in practice ; it would be only necessary to make one surface of 

 the mould (the lower one) of iron of a good conducting material, while the 

 remainder was of dry sand. On trial, this plan was perfectly successful ; 

 there was, however, a new, though not a very serious defect, which was im- 

 mediately apparent— the speculum metal was cooled so rapidly that air-bub- 

 bles remained entangled between it and the iron surface; but the remedy 

 immediately suggested itself, by making the iron surface porous, so as to 

 suffer the air to escape; in fact, by forming it of plates of iron placed verti- 

 cally side by side, the defect was altogether removed. It only then remained 

 to secure the speculum from cooling unequally, and for that purpose it was 

 sufficient to place it in an oven raised to a very low red heat, and there to 

 leave it till cold, from one to three or four weeks, or perhaps longer, accord- 

 ing to its size. 



" The alloy which I consider the best, differs but little from that employed 

 by Mr. Edwards : I omit the brass and arsenic, employing merely tin and 

 copper in the atomic proportions, namely, one atom of tin to four atoms of 

 copper, or. by weight. !)S9 to 126'4. As it was obviously impossible to cast 

 large specula in earthen crucibles, the reverberatory furnace was tried ; but 

 the tin oxidized so rapidly, that the proportions in the alloy were uncertain ; 

 and after some abortive trials with cast-iron crucibles, it was found, that 

 when the crucible is cast with the mouth up it is free from the minute pores 

 through which the speculum metal would otherwise exude ; and therefore 

 such crucibles fully answered the purpose. It was very obvious that the 

 published processes for grinding and polishing specuU, being in a great mea- 

 sure dependent on manual dexterity, were uncertain, and not well suited to 

 large specula ; accordingly, at an early period of these experiments, in 1827 ; 

 a machine was contrived for the purpose, which has subsequently been im- 

 proved, and by means of it a close approximation to the parabolic figure can 

 be obtained with certainty ; as it has been described in the Philosophical 

 Transactions for 1840, it is unnecessary to do more than to point out the 

 principle on wfiicli it acts. The speculum is made to revolve very slowly, 

 while the polishing tool is drawn backwards and forwards by one eccentric or 

 crank, and from side to side, slowly, by another. The polishing tool is con- 

 nected with the eccentrics by a ring, which fits it loosely, so as to permit it 

 to revolve, deriving its rotatory motion Irom the speculum, but revolving 

 much more slowly. It is counterpoised, so that it may be made sufficiently 



