24? 



NATURE 



[October 21, 1920 



Ewing's "Thermodynamics." 



The very appreciative review of my book on 

 "Thermodynamics for Engineers," which appeared 

 in Nature of September i6 over the well-known 

 initials " H. L. C," points out what is certamly a mis- 

 statement. Will you kindly allow me Space to cor- 



On p. 123, in speaking of the MoUier chart, the 

 co-ordinates of which are the total heat 1 and the 

 entropy ^, I should have said that the critical point 

 is "near," not "at," the point of inflection of the 

 boundary curve. It is, as "H. L. C." suggests, a 

 little above the point of inflection. The isothermal 

 line which passes through the critical point suffers 

 inflection in touching the boundary curve there. The 

 line of constant pressure which passes through the 

 critical point also has a singular point there, namely, 

 a point at which 



('^^ -o, and f^') =o. 



\d(t) 



Without undergoing inflection, it has a " stationary 

 tangent" at the critical point. These features 

 of the curves of constant temperature and of 

 constant pressure which pass through the critical 

 point are apparent in the I^ chart for carbonic acid, 

 given on p. 148. It will also be apparent from that 

 chart that the point at which both these singularities 

 occur lies a little above the point of inflection of the 

 boundary curve itself. 



The following corrections should therefore be made 

 on p. 123 of my book : 



In line i, for "at" read "near." 



In the footnote, delete the last two sentences and 

 substitute : " Hence also, at that point, 



/dn\ (dr\ . (dH\ (dn\ „ 



{W)r\d^)r°' ^"'^ \d^-).-\Wlr' ' 



since on the T^ chart the constant-pressure line 

 through the critical point runs level and suffers 

 inflection there. Thus on the \(p chart the constant- 

 pressure line through the critical point has zero 

 curvature there, though it does not suffer inflection." 



Also, on p. 149, line i, for "coincides with" read 

 "is a little above." J. A. Ewing. 



The University, Edinburgh, October 13. 



A Diver's Notes on Submarine Phenomena. 



Sir Ray Lankester in one of his delightful popular 

 papers describes how he found that the glowing light 

 produced by rubbing quartz pebbles together could 

 still be got when the rubbing was done under water, 

 as, for example, when holding the pebbles submerged 

 in a bucket. On the supposition that if the effect 

 were connected with entangled or surface-adhering 

 air it should be enhanced under a high pressure, an 

 experiment was made. My occupation involves a 

 good deal of diving work, and'on a recent occasion 

 I took down a few suitable pebbles to a depth of 

 21 fathoms. On rubbing them together sparks were 

 produced to just about the same extent as when tried 

 in a few inches of water. The lighl on the bottom 

 was dim enough to allow of the sparks being seen, 

 yet visibility was relatively good. 



Wave-action at this depth can be very violent. 

 The wreck of a freshly sunk .-Xtlantic liner at the 

 same place was battered to pieces in the course of a 

 single gale, and the large stones strewing the bottom 

 are flung about amongst her remains whenever there 



NO. 2660, VOL. 106] 



is a big, long swell running. As many of these stones 

 are quarU, there may probably be considerable 

 illumination on such occasions. 



In connection with light under water, I may men- 

 tion that if, as often happens, one's hand gets cut 

 when working at a fair depth the blood streaming 

 out into the water looks quite black, like ink, at the 

 source, thinning out to a bluish cloud as it gets more 

 diluted with sea-water. No trace of redness can be 

 made out. Similarly, an abrasion looks like a dab 

 of tar on one's hand. I presume that this effect is 

 caused by the absence of red rays, which are cut off 

 by the upper layers of water, and that it could be 

 predicted ; but I do not understand why in the same 

 circumstances crabs (C. pagurus) look as red as they 

 do on deck. 



The volume of air in his dress at a given time is 

 a matter of much importance to a diver. He has to 

 control his buoyancy to suit the work in hand, and 

 on a change of depth must rapidly adjust the valve 

 to maintain the same volume during and after the 

 change of pressure. Failure to do so results in a 

 "squeeze" or a "blow-up," either of which may be 

 fatal. 



Fish with swim-bladders must have a similar 

 practical interest in this application of Boyle's law, 

 and a diver who considers the matter will be able 

 to sympathise with them in the difficulties they must 

 often encounter from their relatively slow means of 

 adjusting volume. 



Usually when a mine or similar explosion takes 

 place under water numbers of swim-bladdered fish 

 float to the surface. Stunned or injured, they have 

 risen above the depth for which they were adjusted, 

 and the resulting expansion of the bladder gases then 

 overcomes any efforts they may make and surges 

 them upwards, hopelesslv out of control. In addition 

 to the distension so often described, many of them, 

 as I have found, are killed by rupture of the bladder 

 and resulting internal haemorrhage. It is curious that 

 the escaped gas from the bladder often finds its way 

 into the heart and great vessels, producing a condition 

 like that due to compressed-air illness. 



The diver going down to a wreck where blasting 

 has been going on generally finds that numbers of fish 

 have fallen to the bottom through a converse process 

 and are lying there dead. On a recent occasion 

 where blasting was going on almost dally the 

 accumulation of dead pollack, pout, horse-mackerel, 

 etc., on the bottom attracted swarms of spotted and 

 spiny dogfish, which could always be seen cruising 

 about among the wreckage, often w-ith dead fish in 

 their mouths. The blasting, of course, went on just 

 the same, and heavy charges were often fired in the 

 midst of these shoals, but I have never been able to 

 find a dead dogfish on the bottom (having constant 

 negative buoyancy, they always sink when dead). On 

 the contrary, I have (from the ship) seen them rise 

 to the surface immediately after an explosion in 

 pursuit of the stunned swim-bladdered fishes and tear 

 them to pieces as if nothing had happened. Dogfish 

 have no swim-bladder, and its absence enables them, 

 as in this case, to dash from a hydrostatic pressure 

 of 57 lb. per sq. in. to atmospheric pressure and back 

 again with their prey without ill-effects. Moreover, 

 it is probably this absence of an included gas space 

 which renders them so immune to submarine 

 explosions. 



Another point about swim-bladders. In the North 

 Atlantic sunfish and basking sharks idling at the 

 surface of deep water, with positive buoyancy yet 

 without swim-bladders, are familiar and somewhat 

 puzzling objects. W'ith such means as one has 

 aboard ship I find the specific gravity of skates to 



