54 



NA TURE 



[November 16, 1905 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of N A TURE. 

 No notice is taken of anonymous communications. \ 



British Mosses. 



I\ the review of Dr. Braithwaite's " British Mosses 

 which appeared in your number of August 31 (vol. lxxii. 

 p. 425), I attributed the finding of Catharinea tenella to 

 Lord Justin- Stirling, and 1 did so on the authority of a 

 passage relative to the plant in Dr. Braithwaite's supple- 

 ment. The Lord Justice is, i find, desirous that it should 

 be known that the entire merit of the discover) is duf to 

 .Mr. E. S. Salmon. " I had the good fortune," says the 

 learned judge, " to be his companion when the little plants 

 were gathered, but his eye detected them in the field, and 

 by his acuteness the true name o! them was discovered." 

 I am glad to second the Lord Justice in his desire that no 

 mistake should be made in this matter, and 1 beg your 

 •courtesy to insert this short note. E. F. 



November 8. 



Border occasionally seen between Light and Dark 



Regions on Photographic Prints. 

 Since my recent brief note on a photographic appear- 

 ance, Mr. Burke has informed me that the subject attracted 



the attention of Sir George Stokes, and was thought 

 worthv of a communication from him to the Royal Society 

 in May, 1S.S2 (I'roc. Roy. Soc, vol. xxxiv. p. 63). 



Had I been aware of that I should, of course, have 

 referred to it. It seems to me now that there may be 

 more than one explanation of such an appearance. 



Oliver Lodge. 



Halation. 



When a photograph is taken of a dark object with a 

 bright object or the sky some distance behind it. blurring 

 occurs where the images of the objects meet. 



That part of the bright object from which only a part of 

 tlie lens is visible (the rest of the lens being cut off by the 

 dark object) forms an image of varying brightness in the 

 shape of a band which covers the edge between the images 

 of the dark and light objects. If the bright object be at an 

 infinite distance from the lens, the breadth of the band 

 will bear approximately the same ratio to the diameter "I 

 the lens as that which the focal length of the lens bears 

 to the distance of the lens from the near object. 



It seems probable that many cases of halation are due to 

 this cause. J. A. Conn. 



108 Church Road, Richmond, Surrey, November 13. 



The Engineer's Unit of Force. 



I am much indebted to " The Reviewer " lor hi- cour- 

 teous answer to my letter on the subject of the engineer's 

 unit of force in your issue ol November 2. 



I readily admit that the engineer's unit of force may be 

 so defined as to make it a constant quantity independent 

 of locality; but does the engineer in actual work-a-day 

 practici make use of this invariable unit? In problems 

 involving the derived unit of work, does he not. as a 

 matter of fait, estimate the work done or the potential 

 energy, as the case may be, by multiplying together the 

 distance factor and the weight (i.e. the force) factor with- 

 out making any allowance for the variation of tin- loiter 

 with latitude? 



The question at issue, it seems to me, narrows itself 

 down to this: — is the title "engineer's unit of force" to 

 be appjied to tin variable unit in actual use by the 

 engineer, or is it to be restricted to the absolute gravi- 

 tational unit, which may be defined, bul which in nine 

 cases out of ten is no! actually applied in engineering 



It is quite true that in engineering practice a correction 

 for latitude is seldom made in regard to the gravitational 

 energy of a raised weight, the reason being that other and 

 very much larger sources of error are usually present. 

 But under sufficiently refined conditions this small correc- 

 tion is actually made. A Bourdon pressure gauge registers 

 pounds per square inch in absolute measure the same 

 everywhere. If -Mr. Carnegie considers the pound force 

 to vary with locality, what is his value, in foot pounds, 

 for Joule's equivalent at the centre of the earth? 



But surely even Mr. Carnegie himself must use the foot- 

 pound unit, and hence the pound-force unit, in an absolute 

 sense, when applied to such quantities as the kinetic 

 energy of a relating fly-wheel, 'he strain energy of a 

 stretched spring, the work done on the piston of a steam 

 or gas engine, the energy of motion or of position of a 

 planetan body, &< . 



The present case is an illustration of the apparent in- 

 ability of academic writers to understand the engineer's 

 position in this matter, and of the confusion which inevit- 

 ably arises from the combination of two closely related 

 systems of units. In any dynamical system the magnitude 

 of unit mass is quite arbitrary, and tin pound mass 

 possesses no intrinsic merits over any other unit. Indeed, 

 the choice of the pound unit has proved to lie a most un- 

 fortunate one, for the conception of inertia or mass, coming 

 as it does after thai ol force, finds the pound force already 

 established and ingrained in the mind, forming an effective 

 barrier against the practical adoption of the derived 

 poundal, and being a fruitful source of error on account 

 of the new and old meanings attached to the word pound. 

 People do not, and never will, think in poundals, and so 

 custom litis compelled its advocates to incorporate into 

 their system the pound force and the foot-pound unit of 

 work, a tacit admission of its practical failure. Engineers 

 contend that this duplex system with its overlapping terms 

 is harmful and quite unnecessary. They advocate a single 

 system which, so far as possible, shall adopt units in 

 common use. The system used by them fulfils all require- 

 ments. It is an absolute dynamical system. Its termin- 

 ology is not divorced from common thought and speech. 

 It gives an exact and absolute meaning to the pound force 

 and consequently to the foot-pound unit of work, and its 

 unit of inertia has a distinct name of its own, never used 

 in the sense of force, thus avoiding the conflicting nomen- 

 clature of the present mixed system. The engineer's and 

 the C.G.S. -isiems an- sufficient for all purposes, and it 

 would be a great gain if the academic British system could 

 be abandoned. The Reviewer. 



practn 



i'ton Abbot, November 6. 

 NO. I 88 I, VOL. y$] 



I). J. Car: 



THE EXPLORATION OF THE ATMOSPHERE 



OVER THE TROPICAL OCEANS. 

 '"PHE study of the trade-wind region by the use of 

 *■ kites was first proposed by .Mr. A. L. Rotch 

 al the- meeting of the British Association at Glasgow 

 in 1901, after he and his assistant, Mr. Sweetland, 

 had obtained a scries of observations with them 

 during a voyage across the North Atlantic. This 

 method of investigation was later adopted by other 

 meteorologists, notably by the French Scandinavian 

 expedition to explore the atmosphere, which, under 

 the direction of Si. 1.. Teisserenc do Bort, flew kites 

 in 1902 on the Baltic, and last year by Prof. 

 Herg'esi II, who communicated to the Aeronautical 

 Conference at St. Petersburg the interesting results 

 of a cruise on the yacht of H.S.H. the Prince of 

 .Monaco in the vicinity of the A/ores and Canaries. 

 During'- this voyage fourteen kite-flights, some of 

 which reached a great height, were made, and in a 

 communication to the French Academy ol Sciences 

 on January 30 Prof. Hergesell said : — " Un courant 

 de S.W., qui correspondrait an contre-alize' thc-orique, 

 n'a jamais etc trouve par les cerfs-volants bien qu'ils 

 aient plusieurs fois depasse 1 la hauteur du Pic de 

 TenerilTo. Plusieurs constatations m'amenent a 

 ponser que les vents de S.W. observes au Pic par 

 plusieurs observateurs sont d'origine locale et dus a 

 ['influence de 1M . " 



