December 30, 1920] 



NATURE 



569 



vance of agriculture — moles, for instance, and 

 field voles, besides both insectivorous and grain- 

 eating- birds. 



The author follows the late Dr. Gunther in 

 giving specific rank to what are now recognised 

 as no more than local varieties of the brown trout 



Flo. 2. — Cheviol khcc,- -. 

 l*roin 



L.ve brcetlinic (chanipton, Hiiihlalid Show, I9t4.) 



i h« inllucnce ul Man on ADiaial Ltfe in Scotland." 



— Salmo fario (misnamed 5. trutia on p. 27S). 

 The signal success which rewarded the ac- 

 climatisation of this fish in New Zealand was 

 first achieved with trout, not from Lochleven. 

 but from the VVooburn at High Wycombe. A 



few slips and misprints await correction in a 

 future edition. Loch Askaig (p. 134) should be 

 Loch Arkaig; Loch-an-Eilein on the same page 

 appears as Loch-an-Eilan on p. 192, the correct 

 Gaelic being Loch-an-eilain. Dr. Ritchie would 

 probably wince if we were to write about " North- 

 umberlandshire," just as we did 

 when we read " Sutherlandshire " 

 on p. 126. 



Few of the numerous illustra- 

 tions are worthy of Dr. Ritchie's 

 interesting treatise. The 

 heaver (Fig. 37) and the bit- 

 tern (Fig. 64) are mere cari- 

 catures, and poor at that. 

 Some of the figures, however, 

 serve well to illustrate the in- 

 fluence of domestication and 

 selective breeding upon primi- 

 tive types of mammal, as in 

 the case of the sheep. Although 

 it may not be possible to de- 

 fine with precision the various 

 species which have contributed 

 to produce the modern breeds, 

 sheep, though a race almost 

 exclusively Palaearctic and Ne- 

 arctic. are peculiarly liable to 

 modification by food and environ- 

 ment, and are more plastic in 

 that respect than cattle, horses, 

 or swine. 



While indicating some hesitation in accepting 

 all Dr. Ritchie's conclusions, we congratulate him 

 on his useful contribution to zoological literature, 

 and we are grateful for the excellent index to 

 the book. 



Some Problems of Lubrication.* 

 By W. B. Hardy, F.R.S. 



T N lubrication, a fluid or other body is used to 

 ^ decrease the friction between opposed solid 

 faces. The lubricant may act in one of two ways. 

 It may separate the faces by a layer thick enough 

 to substitute its own internal friction, modified by 

 the mechanical conditions in which it finds itself, 

 for that of the solid faces ; or it may be present as 

 a film, too thin to develop its properties when in 

 mass, which reacts with the substance of the solid 

 faces to confer upon them new physical properties. 

 In the latter case the solid faces continue to in- 

 fluence each other, not directly, but through the 

 intermediation of the film of lubricant. There are 

 indications that these two types of lubrication — 

 one in which the .solid faces intervene only 

 owing to their form, rate of movement, etc., and 

 not by their chemical constitution ; the other in 

 which the chemical constitution is directly in- 

 volved- - are discontinuous states in that one 

 cannot be changed gradually into the other by 



' A Friday anning <ll«ci> irt* ritlivtrxl «l ih« Royal laalitiltloii on 

 FaKruary a;, if xo. 



NO. 2670, VOL. 106] 



simply thinning the layer of lubricant. The 

 change from the one to the other is probably 

 abrupt. 



It may by no means be asserted that resistance 

 to relative motion is always least when the solid 

 faces are floated completely apart ; it would, in- 

 deed, probably be truer to say of the best lubri- 

 cants that friction is least when the "boundary 

 conditions," to use Osborne Reynolds's phrase, are 

 fully operative. 



This address is concerned wholly with 

 "boundary conditions," and we get directly to 

 the heart of the problem by certain simple ex- 

 periments. If a glass vessel, such as a bottle, is 

 placed upon an inclined pane of glass at a certain 

 angle it slips smoothly down. The glass plate 

 is an ordinary plate cleaned with a cloth. In the 

 usual sense of the word, the plate is not 

 lubricated; the surface is "dry." The lower 

 half of the plate is then wetted with water, and 

 the bottle is now found to slip on the unwcttcd 

 part, and to be pulled up sharply by friction when 



