no 



NATURE 



[Marci 



1911 



LETTERS TO THE EDITOR. 



;;/_.•.;. t.V--V 



: ; j^rr."?; i'Xli'' 



IThe 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 f>ar< 0/ Nature. 

 No notice is taken o^ anonymous ccmmunications.'\ 



The Flow of Thin Liquid Films. 



Whilst obsorvinj^ the " Brownian " movement of 

 particles of gambojji' in water with the aid of a micro- 

 scope (maf^nifuntion, about 360 diams.), it occurred to me 

 to press j^ently on the cover-jjlass of the slide, so as to 



cause a mo\ement of 



...._.._. ._ the water containing 



;. 1 _"_"_'. ~ . ir^ ri'.-;;_ the suspended matter, 



; :_— T "s'i •**-. •. '\'-\ "Z J^-^- and to note the paths 

 5 Z l"". ''i''''s-' -r* ^C - '-• .* i •" .1 " "— ~~ of these in the 

 vZ-'O/''- '*•'.'-*•- vVVl:" .^..''o --"«' — vicinity of some 

 : ' * '--/■'.''. ■ - •' '•■.■":.:•.""••; '.* *I *^^3* larger stationan,- 

 ;V-'-~. ."..*, masses, as one would 

 :•:':': r.- 1'. '. then be approaching 

 ,?'V:.\:;V^.:.v the Condition set 

 i-rivJVv:';'-;" forth by the late Sir 

 y"-- -."" G. G. Stokes, namely, 

 •'•' ".'.'. ' that liquids in thin 



.. --".Zl.1'. ^7.- - -'-~--*-*— films behave as fric- 



r l-'I '. • '-'7- iJJrr ."'.""." " ~". ."_ tionless fluids. The 



•••".•_,;■■■■ I_~— IT r. results fully con- 



1 "..'."-• ■• ~-~ • - — - - firmed the behaviour 



Pij, , of such thin films of 



liquid. The moving 

 particles, as they rushed by the stationary masses, showed 

 no trace of eddy currents, passing along the edges of the 

 obstacle and leaving it without any swirls, as shown in 

 Fig. 1. The moving particles next to the obstacle had a 

 high velocity, and were in greater numbers per unit area, 



than those further re- 

 . ■_- -. -. -. T .--■•-. - -:•■ moved; the obstacle 

 '- 2 zr~~ — ■- - -" ~ ^."^Z~7~E.'.~ ^^^ "° effect upon 

 ~— -^ *_ r - "- "• -'- • ~ Z ' - distantly removed por- 

 - '_£z '• ~~-~ •'•'-'- -^ ~ I -~ tions of the liquid — 

 {"jr.'-'. '- - 7{.-^-f-r~~~-". £" ~ they moved in straight 

 ---l-T^" lines. For very low 

 '-2 ~.S.V: . velocities the course of 

 "•.-r ■.■."."'.: the particles was ex- 

 f.r.-.r.- ".•,.' ceedingly in accordance 

 . . I "-V-f •." with the motion of a 

 %■.-.:-'■■' frictionless fluid. With 

 :.^-^-^"-_'~M high velocities, a cone 

 • • ' "_ of slow-moving liquid 

 ■"."... . formed both in front 

 - - -'".' and behind the ob- 

 stacle, as shown in 

 Fig. 2. When two 

 masses are in the same 

 line of flow, it is difficult to prevent a certain number 

 of particles, mapping out a stream-line, from cross- 

 ing over from one side to the other between the 

 obstacles, as shown in Fig. 3. We have here a hydro- 

 dynamical analogy to the circuit of a Wheatstone bridge 



Fig. 2. 



when determining the value of an unknown resistance. 

 The liquid represents the metallic circuit, the particles of 

 gamboge the current — or the corpuscles if preferred, the 

 two obstacles the insulation between R,, R, and R„ R„ 

 and the fluid between them the galvanometer circuit. 



NO. 2160. VOL. 86] 



When equilibrium i» established, no current flows through 

 the galvanometer : no fluid passes across the inter\'cninfi 

 space between the two obstacles. Vary any one o( 

 resistances, and equilibrium is upset, causing a curr< 

 flow through the galvanometer; caust; an u"' ' 

 pressure on one side or the other of the line j' 



two obstacles, and a current of fluid flows from .... , 



of greater to that of lesser pressure. 



Very interesting effects are produced by introducing air 

 bubbles into the liquid instead of solid obstacles. On 

 pressing the cover-glass, the bubble appears to increase in 

 size, while at the same time a rush 

 of liquid passing it is noticed ; on 

 releasing the pressure, the bubble 

 contracts, the liquid moving in the 

 opposite direction. One of the most 

 striking effects is seen when a 

 bubble moves of its own accord 

 through the liquid. The effect is 

 ditlicult to produce, but well repays 

 the effort. As before, gamboge is 

 used to define the course of the ' 



surrounding fluid. As the bubble 

 moves forward, the fluid next it is seen to be ni' 

 along its edge in the same direction, while ;. 

 little distance it is moving in the opposite di. 

 that in which the sphere moves. This effect is 

 the arrows in Fig. 4, the heavy arrow denoting i... 

 tion in which the sphere is moving; .At the pole t 

 fluid seems to appear, passes with a high velocity tc 

 pole d via the surface of the bubble, and disappears, 

 effect of the moving bubble on the surrounding I 

 extends for a great distance compared with the case v 

 the liquid is in motion and the obstacle stationary 

 ante). W. G. Royal- D.\ w so :>. 



4 Montague Street, London, W.C., March 6. 



Water- Vapour on Mars. 



I NOTE in N'.ATiRE of February 9 (p. 486) an account of 

 a recent unsuccessful attempt to verify the existence of 

 water-vapour on Mars, already demonstrated by means of 

 other methods by Dr. .Slipher and myself (see The .4. ' 

 physical Journal, vol. x.wiii., p. 397, December, 1908, 

 I^oweil Observatory Bulletins, Nos. 36, 43, and 49). \. ... 

 you allow me to point out that the method employed by 

 Director Campbell was proposed several years ago by Dr. 

 Percival Lowell, and was actually tested by Dr. Slipher at 

 the Lowell Observatory in 1905, with a result similar to 

 that which Director Campbell has now obtained in his 

 repetition of the experiment? The details may be found 

 in Lowell Observatory Bulletin No. 17. 



The reason for the failure perhaps lies in the insensitive- 

 ness of the method. The spectrum of a body no brighter 

 than Mars cannot be obtained with the utmost fineness of 

 detail under a high dispersion, because a relatively wide 

 slit has to be used, or el.se a very long exposure must be 

 given to the photographic plate, either of which is fatal 

 to sharp definition of fine spectral lines. In these circum- 

 stances it is not easy to distinguish between the terrestrial 

 and planetary components of a fine absorption line with 

 the high dispersion which is absolutely necessary to 

 success of the experiment. 



It still seems to me that the best method of measi 

 the Martian aqueous vapour which is at present avai 

 consists in the observation of the little a band wiiii .t 

 spectrograph of low dispersion, which gives the band a» 

 a shading in which individual lines cannot be dis- 

 criminated, but the integrated intensity of which can be 

 measured photometrically. The method is also applicaMp 

 to those diffuse bands discovered by .\bney and Fe^ 

 in nearly saturated aqueous vapour, which apparently 

 not composed of fine lines, but which are sometimes mucn 

 more intense than the linear groups. 



Frank W. Very. 



.\strophysical Observatory, Westw-ood, 

 Massachusetts, March 6. 



The F^-x and »h- Fl^as. 



I HAVE just been told a very interesting story by Mr. 



James Day of this town. Many years ago he and his 



father, both then engaged in agriculture, were sitting 



with their backs to the straw'-covered hurdles which haff 



