SCIENCE. 



Baldwin, 33 South street, New York. The Hopkins and 

 Dickinson M'f'g Co,, 76 Reade St., N. Y., and Darlington, 

 N. J., have the exclusive specialty of metalined sliding 

 door sheaves and builders' hardware generally. But these 

 illustrations need not be extended. 



THE SUN. 

 By Professor C. A. Young. 



To the Editor of" SCIENCE." 



Dear Sir, — May I avail myself of the columns of your 

 journal to correct a few serious errors which have come to 

 light in my recent book on the Sun. 



P. 16, near bottom. — The interval from the vernal equi- 

 nox to the autumnal is 186 days, instead of 184, as 

 stated. Of course the remaining part of the year is 179 

 days, not 1 81. 



P. 44.— The earth would fall to the sun in about two 

 months, not four. 



P. 240, 241, and 279. — The candle power of the sun is 

 given just four times too great. The figures printed 

 express the number of candles which, distributed over 

 the surface of an opaque globe, would give the same 

 amount of light the sun does, each flame being considered 

 as a small FLAT radiating surface.' But this does not 

 express the true ratio between the sun's light and that of 

 a candle radiating freely in all directions. 



P. 271. — In the formula for the number of calories of 

 heat generated by the stoppage of a moving body, the 

 denominator ought to be 8338 instead of 850. The fac- 

 tor g (9.8i m ), having been accidentally omitted. In 

 consequence, a few lines below, another 850 becomes 

 8338 also, and 300 c becomes about 30' 



There are a number of other minor errors, which it is 

 hardly worth while to notice here, though they will be 

 corrected in the second edition. C. A. Young. 



To the Editor of " SCIENCE." 



A friend of mine who is a reliable observer relates an 

 incident which forcibly illustrates the power of parental 

 affection to overcome fear. The gentleman found a nest 

 of young mice and removed them to the ground nearby. 

 The mother mouse made her appearance and carried 

 away one of her young and while she was gone the gen- 

 tleman took the remaining mice in his hand. When the 

 mouse again appeared and could not find her young she 

 seemed to hesitate a moment and then ran up the gentle- 

 man's clothes, took one of the young and carried it away. 

 This was repeated until all the young were removed to a 

 place of safety. J. H. PILLSBURY. 



Springfield, Mass., Dec. 27, "8i. 



BOOKS RECEIVED. 



A Treatise on Comparative Embryology, by 

 Francis M. Balfour, LL. D.. F. R. S. Vol. II. 

 Macmillan & Co., New York, 1881. 

 An extended notice of this admirable work will appear 

 later, we now simply announce that Messrs. Macmillan 

 are ready to supply the second volume which completes 

 the work, and we leel sure that every Biologist and Anat- 

 omist will avail himself of the mass of information in- 

 cluded in Professor Balfour's book, which in competent 

 hands must prove one of the most valuable aids to 

 original work in this direction. 



Elementary Lessons in Electricity and Mag- 

 netism, by Silvan us P. Thompson, Professor of 

 Experimental Physics in University College, Bris- 

 tol. Macmillan & Co., Bond St. New Yo.'k and 

 London. Price $1.25. 



An Elementary Treatise on Electricity. By 

 J ames Clerk. Maxwell, Professor of Experimental 



Physics in the University of Cambridge, England. 



Clarendon Press Series, Oxford, 1881. Price $1.90. 



Imported by Macmillan, Bond Street, New York. 

 Students, and the many practical men who are now 

 studying Electricity with a view to its application to the 

 manufactures and arts, will find that these two books will 

 exactly meet their requirements, in being comprehensive 

 thoroughly practical and reliable. Those who can- 

 not purchase both works, should commence with that by 

 Professor Thompson, and follow with Professor Maxwell's 

 as being more advanced. 



The doctrine of the Conservation of Electricity, now 

 growing into shape, but here first enumerated under that 

 name, is thoroughly explained in Professor Thompson's 

 book, and may be studied with profit by all interested in 

 the science of electricity. This theory teaches us that 

 we can neither create nor destroy electricity, though we 

 may alter its distribution. According to this view all 

 our electrical machines and batteries are merely instru- 

 ments for altering the distribution of electricity by run- 

 ning it from one place to another, or for causing elec- 

 tricity, when accumulated or heaped together in one 

 place, to do work in returning to its former level dis- 

 tribution. 



Ideality in the Physical Sciences. By Benja- 

 min Peirce. Messrs. Little, Brown & Company. 

 Boston. 



This work by the late Professor Benjamin Peirce is an 

 admirable illustration of the fact, that a man of individu- 

 ality and sound judgment may pursue the highest sci- 

 entific work and still find himself in harmony with the 

 religious sentiments of his fellow man. 



A great portion of this work is devoted to a review of 

 past astronomical research, and will be read with interest 

 as a reliable exposition written for those who require sci- 

 entific work explained in simple language. 



Photographic Exhibition. — The substitution of a film 

 of dried gelatin for the thin layer of wet collodion, which 

 the photographer formerly employed as a vehicle to retain 

 the sensitive salts of silver in a suitable condition on his 

 glass plate, has involved considerable alterations in the 

 mechanical appliances used in photography. For out of 

 doors work, or work away from home, the photographer 

 no longer requires to carry what was practically a portable 

 laboratory. Not having to " develop " his pictures on the 

 spot, he need take with him neither dark tent nor chemi- 

 cals. On the other hand, he must have some provision by 

 which his store of dry plates can be placed, one after the 

 other.in the camera and properly "exposed" without the risk 

 of the slightest particle of light reaching their sensitive sur- 

 face, other than the light properly directed upon them by the 

 lens. As he wishes to carry an ample supply of plates with 

 him, and as the glass plates themselves make an appreciable 

 burden in a long walk, it is essential that the apparatus for 

 carrying them should be as light as possible ; hence have 

 arisen considerable improvement in the camera and its 

 " slides." Again, the increased sensitiveness of the gelatin 

 films makes it possible to give exposures shorter than can 

 be affected by the hand uncapping and re-capping the lens ; 

 hence the invention of numerous "instantaneous shutters," 

 by which exposures of a few hundredths of.a second can 

 be given, and pictures of moving objects readily secured. 

 These are but instances of the many novel appliances which 

 recent progress in photographic science has originated, 

 and, besides these, there has been, during recent years, 

 many and important improvements in the application of 

 photography to the production of permanent illustrations 

 for books and newspapers. All these varied applications 

 of the art are to be illustrated by an exhibition of photo- 

 graphic appliances which the Council of the Society of Arts 

 announce will be held during January and February next, 

 in connexion with a course of Cantor lectures to be given 

 before the Society by Capt. Abney. Full particulars of 

 this exhibition are given in the Journal of the Society of 

 Arts for last week. 



