Nov. 27, 1 8 73 J 



NATURE 



61 



course which both keeps down the damp and ven- 

 tilates the ground-floor. Proceeding to fire-proofing 

 methods, Mr. David Hartley's simple but little known 

 contrivance for protecting dwelling-houses from fire by- 

 interposing sheet-iron or copper between the floor boards 

 and the joists is mentioned. The plan described a little 

 farther on, p. 46-47, would probably not be so effective as 

 Hartley's. 



In pp. 48-58 fire-grates are mentioned, and with a de- 

 cided preference (perfectly justified in the experience of 

 the writer of these remarks) for Mr. John Taylor's smoke- 

 consuming grate ; but the author should hardly have left 

 Dr. Arnott's smoke-consuming contrivances unnoticed ; 

 and when at pp. 61-66 he speaks of ventilation, he should 

 have mentioned at greater length Dr. Arnott's ventilating 

 valve. Boyle's ingenious ventilators, however, quite de- 

 serve the praise given them in p. 63. 



It would be interesting to have had some references 

 given to sanction our author in claiming the authority of 

 the Duke of Wellington, together with that of Aaron and 

 the High Priests, his successors, for the practice of 

 placing their beds nearly north and south so as to be in 

 the line of the magnetic current. The theory no doubt 

 has its advocates, but can hardly be of universal appli- 

 cation, as there are many sound sleepers at all degrees 

 of orientation. 



Chapter iv. contains some good suggestions respecting 

 London street improvements and the Sanitary Recipes at 

 the end will be found deserving attention. 



OUR BOOK SHELF 



Natural Philosophy. Parti. Mechanics. By J. Alfred 

 Skertchley. Pp. 168. (London : Thomas Murby, 1873.) 

 This work belongs to a series of small manuals which 

 the publisher calls the " Science and Art Department 

 Series of Text Books." It is designed for students who 

 possess but little mathematical knowledge, and each of 

 the theorems discussed is explained in very simple lan- 

 guage. In some respects the work keeps pace with 

 modern text-books, in others it lags behind them. Thus 

 while we have chapters on Kinetics and Kinematics, 

 and on Actual and Potential Energy, we find some of the 

 units as primitive as possible, and the Metric system is 

 ignored. The unit of length is given as the yard, and the 

 unit of weight as the grain. The definitions leave much 

 to be desired : thus Mechanics is defined as "the Science 

 which treats of the laws of motion and force, especially 

 as applied to the construction of Machines;" Hydro- 

 statics " the science treating of the pressure of water." 

 Again we find the following very loose definition of the 

 force of gravity : " Every particle of matter has a ten- 

 dency to draw to itself every other particle, and this 

 tendency is called the force of gravity." The other 

 attractive forces are here ignored, the student is left quite 

 in ignorance as to whether the force acts through a sen- 

 sible or insensible space, whether it acts between particles 

 or masses, whether such particles or masses are neces- 

 sarily of similar or dissimilar substances. A screw is 

 defined as " an inclined plane revolving round a centre." 

 " Any body capable of moving freely about a fixed axis 

 is a pendulum." The chapter relating to Energy requires 

 to be carefully revised, as, indeed, does much of the work 

 so far as accurate and logical definition is concerned. 

 The examples are useful, and the questions at the end of 

 the book will be found of service in teaching elementary 

 Science, but the book can scarcely be recommended until 

 the definitions are more precise and absolute. 



LETTERS TO THE EDITOR 



[Tlie Editor does not hold himself responsible for opinions expressed 

 by his correspondents. No notice is taken of anonymous 

 communications.l 



The Dutch Photographs of the Eclipse of 1871 



In the account cf the proceedings of the meeting of June 13 

 last of the Royal Astronomical Society, as published in Vol. viii. 

 p. 175, of Nature, I read the following : — 



"Mr. Ranyard remarked that the paper copies of the Dutch 

 photographs which he had seen had been printed from enlarge- 

 ments on glass, in which the moon had been stopped out with 

 black paper or some other material. On measuring he had 

 found that the body of the moon, as given in the jihotographs 

 was by no means circular, and Mr. Davis had pointed out to 

 him that the irradiation under the prominences was perfectly 

 sharp at the edges, as it would be when printed through fpaper; 

 It was therefore unfair to institute any comparisons as to 

 the amount of the irradiation in these and in the other photo- 

 graphs." 



I beg leave to state, in opposition to Mr. Ranyard's and Mr. 

 Davis's remarks, that no stopping out with black or any other 

 paper has taken place. I enclose hereby copies on paper of the 

 originals and of one of the enlargements. In the first-mentioned 

 everyone may see that the moon is sufficiently dark to render un- 

 necessary every artifice before making a good enlargement. In 

 fact I have seen the enlargements myself, and in them, too, the 

 moon was as dark as the surrounding sky. 



I think Mr. Dietrich's merit to be especially this, that he has 

 directed the attention of astronomers again to a method, as it 

 seems already wholly abandoned, if ever earnestly tried, viz. 

 that of taking an image with a photographic lens of short focus 

 but great force, so that a very short exposure might be sufficient. 

 As to the profit his photographs brought to our knowledge of • 

 the sun. Col. Tennant says, almost every depression of outline 

 of the Indian photographs could be recognised in the Java 

 ones, and thereby it is proved that in the interval of time needed 

 by the moon's shadow to make the traject from India to Java, 

 say 50 minutes, almost no change whatever took place in the 

 solar corona. 



Of course the method could be improved by moving the 

 camera by clockwork. Then the exposure could last a little 

 longer, e.g., one second, and the exterior outline wouH reach 

 farther ; a larger cimera, with photographic lens of the same 

 force would without doubt give more details. 



As to the not-circular (in fact elliptical) form of the moon in 

 the photographs, I think it pleads more against than in favour 

 of Mr. Ranyard's remark, for if a disc of paper were to be used 

 to stop out the moon, of course a circular one would have been 

 made, and not an elliptical one. The fact is that the copies of 

 the original c/Zc/;/ present the same peculiarity, the difference 

 between the longest and shortest diameter being about J^th of a 

 millimeter, as is easily recognised with a lens and a measure of 

 half-millimeters. In the accompanying diapositive thedifferenc- 

 = \ mm. As in other photographs of total eclipses, the diae 

 meter corresponding to the poles of the sun is the longer. This 

 phenomenon is in our case only partially explained by the moon's 

 motion during the time of exposure ; perhaps a stronger impres- 

 sion at the equatorial regions of the sun, or a trembling ot the 

 camera-stand has done the remainder. 



In the glass photographs, of which I have sent a pair to Lord 

 Lindsay and to Messrs. Lockyer, Iluggins, Warren De La Rue, 

 and Main, the details are finer and sharper than in the paper 

 ones. J. A. C. Oudemans 



Bat.avia, Sept. 10 



[We have no doubt from an inspection of the photographs 

 sent, that no stop was used. — Ed.] 



Elevation of Mountains and Volcanic Theories 

 The accompanying letter from Captain Hutton is in acknow- 

 ledgmentof my paper on "Tlie Elevation of Mountains by Lateral 

 Pressure," which I read at Cambridge in 1S69. I sent it to him 

 in consequence of seeing his lecture on Mountains, in the Geolo- 

 gical Magazine. He could not have received my critique on that 

 lecture at the time of his writing this letter. In accordance with 

 his suggestion I forward it for publication in Nature without 

 comment. Osmond FisheK 



Harlton Rectory, Cambridge 



