Dec, 27, 1877] 



NATURE 



161 



steady and without the slightest shake. It is this system 

 of centrin:.^ that constitutes the patent by which Messrs. 

 Batchelor's drawings are protected. The pivots are made 

 entirely of card and paper cut in a most ingenious manner, 

 by which both freedom and steadiness are insured. 

 Nor are the centres the only parts of these drawings 

 interesting for their ingenuity; the contrivance for holding 

 down the sliding parts is equally good. It consists of a 

 band of thin paper passing over the sliding part, and 

 printed exactly like the part it covers, so that it is invisible 

 except on close examination. 



The first of these drawings which is before us is a 

 diagram in illustration of the action of the "trunk engine," 

 the characteristic feature of which consists in making the 

 piston-rod hollow and of sufficient internal diameter to 

 allow the connecting-rod to be attached at one end direct 

 to the piston, and to oscillate within the trunk, the other 

 end embracing the crank-pin. By this means the crank 

 shaft can be brought nearer to the cylinder, considerable 

 space thereby being saved, and the alternative system by 

 which direct connection between the piston and crank is 

 effected, viz., the oscillating cylinder, is avoided, with its 

 more complicated valve gear and expensive construction. 



The double trunk system represented in the drawing was 

 the invention of the late Mr. John Matthew, who for many 

 years was a partner in the eminent firm of Messrs. John 

 Penn and Sons, and it is the system upon which, almost 

 without exception, the large screw engines of Messrs. 

 Penn are constructed, with which so many of the ships in 

 her Majesty's navy are fitted. 



There is nothing to be desired in the execution of the 

 diagram before us, of which the name of Messrs. Maclure 

 and Macdonald is a sufficient guarantee. It is litho- 

 graphed in white upon a blue ground, and all the parts 

 come out with singular distinctness. We could have 

 vsfished that, in the choice of an example for illustration, 

 a more modern design of engine had been selected. 

 The eccentric rod, with its lattice bracing, is that em- 

 ployed in the old beam engines, and a trunk engine made 

 to the drawing before us could hardly work, for the crank 

 pin is evidently inserted into one of the spokes of the 

 fly-wheel, and unless projecting to an impossible extent, 

 the trunk could not clear the wheel ; this could very 

 easily have been remedied by shovving the '' throw " of a 

 crank behind the connecting-rod, which would have 

 aided rather than detracted from the clearness of the 

 diagram. 



While thus criticising the particular design of engine 

 selected for representation, we can only express admira- 

 tion of this most ingenious system of illustrating mecha- 

 nical motions and the action of machines. For educational 

 purposes it will be of the highest value, and there are 

 many of the examples in Reuleaux's masterly work upon 

 the " Kinematics of Machinery," ^ so ably translated by 

 Prof. Kennedy, to which it might with great advantage be 

 applied. 



We feel sure that Messrs. Batchelor's drawings will be 

 a great boon to inventors for explaining their inventions 

 to others ; and as supplementary to scientific evidence in 

 dispu-ied patent cases and other litigation, they will be 

 found of value. 



C. W. C. 



' "Thoretisthe Ki emit'k." 



LETTERS TO THE EDITOR 



\The Editor does not hold himself responsible for opinions expressed 

 by his correspottdents. Neither can he undertake to return, 

 or to correspo7id with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. 



The Editor urgently requests correspotidents to keep their letters as 

 short as possible. The pressure on his space is so great that it 

 is impossible otherwise to ensure the app arance. evf.n of com- 

 municatiens containing interesting and novel facts.\ 



Oxygen in the Sun 



From the time of the discovery by Prof. Draper of the 

 presence of oxygen in the sua down to the present moment I 

 have devote 1 m;)st of iny leisure time to the consideration of the 

 question as to why the oxygen lines should appear brii^ht while 

 the mefallic lines should appear dark in the solar spectrum. I 

 was led into this inquiry under the firm belief that the new fact 

 made known by Dr. Draper might learl to a modification of 

 existing views of the sun's atmosphere, and it was consequently 

 with th<i greatest pleasure that I read in last week's Nature a 

 communication from Ur. Schuster on this subject. 



The views which I have arrived at being in my opinion hardly 

 matured enough for publication, I reserve further statement at 

 present, but will so far anticipate as to say that the explanation 

 which I am disposed to maintain necessitates the assumption that 

 oxygen possesses two different spectra — a low temperature hand- 

 spectrum and the well-known line-spectrum of high temperatures. 

 This assumption I thought warranted by the behaviour of other 

 njn-metals as made known by the researches of many spectro- 

 scopists, but more particularly by those of Salet and Lockyer, 

 and endorsed by the low temperature absorption spectra of the 

 metals discovered byRoscoeand Schuster, I.ockyer and Roberts. 

 The recent research of Dr. Schuster, however, has now placed 

 this assumption in the posidoa of a fact, and all who have fol- 

 lowed recent spectroscopic advancement will recojjnise the value 

 and importance o; this last discovery. 



As Dr. Schuster's explanation of the brightness of the oxygen 

 lines differs fundamentally from that which I am inclined to hold, 

 and as he considers ihis view wat ranted by the result of his 

 investigation, I will beg permission to make a few brief remarks 

 upon ttie chief points of difference between us, being convinced 

 that their discussion cannot fail to elicit opinions ot interest to 

 all concerned in the progress of solar physics. This ventilation 

 of op.nion is the more necessary as views very similar to, if not 

 identical with, those of Dr. Schuster's had occurred to me and 

 had been abandoned for reasons which I will now explain. 



According to Dr. Schuster, "the temperature of the sun, 

 at some point intermediate between the photosphere and 

 the reversing layer " is the same as that at which the 

 spectrum of oxygen changeii ; that is to say, if I rightly 

 interpret these words, above the photosphere the temperature Is 

 such that we get the line spectrum of oxygen and above the 

 reversing layer the temperature is such that we get the band 

 ("compound line") spectrum. This state of affairs would 

 doubtless account for the reversal of the " compound line " spec- 

 trum which Dr. Schuster has now shown to be present in the 

 solar spectrum, but I fail to see at present how it is to be 

 reconciled with the bright line oxygen spectrum. Let us 

 consider the conditions more closely. All observers agree in 

 placing the reversing layer at the base of the chromosphere — the 

 present hypothesis necessitates a space between the photosphere 

 and the leversing layer — i.e., a space sufficiently extended to 

 contain the incandescent oxygen giving the line spectrum. 

 Neglecting for the present the an'agonism between these views, 

 let us assume that such a space exists, and for the sake of 

 simplicity let us also neglect the other elements which may be 

 present. Now it cannot be assumed that the supposed zone is 

 higher in temperature than the photosphere — it might be of the 

 same temperature, but, being auove the photosphere it would 

 more probably be at a loi.uer temperature. Let us, however, 

 make the assumption most favourable to Dr. Schuster's view, 

 viz., that the hypothetical zone is of the same temperature as the 

 photosphere. Then we have a zone of oxygen exterior to the 

 photosphere and of the same temperature as this last region, and 

 above the oxygen the cooler reversing layer. Thus the light of 

 the photosphere passes unchanged through the oxygen zone, and 

 we should see no dark lines corresponding to the line-spectrum 

 of this gas. — As a matter of fact, however, the oxygen lines are 

 bright — hence it must be at a higher temperature than the photo- 

 sphere, or we must be looking through an enormous stratum of 



