Nov. 8, 1888] 



NA TURE 



31 



from Boohtarminsk, on the right bank of the river, just at the 

 entrance into the Kozlovskee gorge, and is known among the 

 local inhabitants under the name of Slepoy Borok (Little Blind 

 Forest). But the illustrious travellers have made rather a 

 blunder in defining, after only a rapid examination from the 

 deck of an Irtyshian karbaz, the mutual relations of the rocks. 

 A section at right angles to the stream sliows that the granite 

 lies not on the slates, but /// them, and that it occurs as a main 

 vein, with some secondary ones, all having the same strike and 

 dip as those of the slates. The main vein is some 30 or 40 metres 

 thick, the secondary o'5 metre, or still less. From the river one 

 can see only the lower limit of granite, and as the joints of this 

 rock are nearly horizontal, whilst the bedding of slate stands 

 almost vertical, the appearance is presented which suggested the 

 original inference that the granite has been poured out over the 

 ends of the slates. 



My measurements gave the following results : — On the level 

 of the current the strike of the slates varies between Qh. 45m. 

 and loh., dip = 82°-85° north-north-east ; lower limit of granite 

 strike, 7h. 30m., dip = 73° north- north east ; upper limit of the 

 same, strike 8h. 45m., dip 43°-45° north- north-east. The con- 

 tact of the two rocks can be observed occasionally exceedingly 

 well. In the vicinity of the granite the slate loses all traces of 

 fissility, and becomes a very compact rock, with abundant scales 

 of muscovite. Generally the slates are schistose, phyllitic, and 

 chiastolitic. Both rocks are welded together. 1 he secondary 

 veins of granite somewhat differ in structure from the main vein, 

 which is of a normal fine-grained variety, with little scales of 

 biotite. The main vein is covered with young forest, consisting 

 f pine, birch, and aspen trees, while on the slates nothing 

 ^Mows but rare bushes of gooseberry, honeysuckle, some species 

 uf horse-tail, and some grasses. 



The conclusion of Humboldt and Rose, that some Altaic 

 granites are younger than local schists and slates, remains in- 

 disputable. I wish to add that they are also younger than some 

 local greenstones, as may be seen at Beeshbanovskee crags on 

 the Irtysh, near Oostkamenogorsk, where a dioritic breccia is 

 cemented with granitic matrix. A. Bialoveski. 



Oostkamenogorsk, Western .Siberia, September 20. 



Rankine's Investigation of Wave Velocity. 



The investigation relating to the propagation of waves con- 

 tained in chajiter xv. of Maxwell's " Theory of Heat," and 

 based on that of Rankine (pp. 530-31 of " Collected Papers") 

 presents peculiar difficulty to most readers. " The kind of 

 waves to which the investigation applies are those in which the 

 motion of the parts of the substance is along straight lines 

 parallel to the direction in which the w ave is propagated, and 

 the wave is defined to be one which is propagated wiih constant 

 velocity, and the type of which is not altered during its 

 propagation." 



Two cross-sections of unit area at a fixed distance apart are 

 conceived to travel through the substance with the velocity of 

 the wave, inclosing between them a cylindrical space within 

 which things are always in the .same condition though the matter 

 is continually changing. The momentum of the matter which 

 enters through the front section in the unit of time is duly 

 expressed, and also the momentum of the matter which escapes 

 at the rear section. The difference of these t"o momenta i< 

 then equated to the difference of the pressures before and 

 behind. The puzzle is to justify this quasi deduction from the 

 second law of motion ; and in connection with this puzzle, the 

 question of sign occurs. For instance, if the momentum of the 

 entering fluid exceeds that of the issuing fluid are we to attribute 

 the gain of momentum to the fact that the contents of the 

 cylinder are more strongly pushed forward behind than they are 

 pushed backward in front ? Such is the impression produced on 

 the reader's mind by Maxwell's words: "The only way in 

 which this momentum can be produced is by the action of the 

 external pressures " ; but it is not correct. 



The momentum included between the two travelling .sections 

 is changed in two distinct ways : first, by convection — that is, 

 by gain and loss of moving matter ; secondly, by pressure 

 before and behind. The change from pressure must be ftjuil and 

 opposite to the change from convection ; since, by hypothesis, the 

 momentum included hettveen the t7vo sections remains always the 

 same. 



Rankine merely says, " Then in each unit of time the diflfer- 



ence of pressure /._,-/j impresses on the mass the acceleration 

 «.2-«i," and give; no explanation, 



I remember being puzzled by this reasoning of Maxwell's 

 some years ago, when I was writing Note A in Part 4 of 

 "Deschanel," and getting over the difficulty by taking the two 

 sections very near together ; l;ut my attention has been drawn 

 to it afresh by the receipt of a paper by Prof. MacGregor, of 

 Nova Scotia, in which the difficulty is pointed out, aid evaded 

 in the same manner in which I evaded it. Prof. MacGregor 

 points out that Maxwell obtains a correct result only by help of a 

 mistake in the algebraical work — the sign of a difference b^ing 

 changed in obtaining equation (7) from equation (6). This is 

 certainly true as regards the fourth and fifth editions, which are 

 now before me. In a later edition, Prof. MacGregor remarks, 

 the sign of the difference is changed in equation (6), thus 

 making the algebra right, but at the expense of making equation 

 (6) inconsistent with what goes before it. The explanation 

 contained in the sentence printed in italics above clears up the 

 difficulty. J. D, Everett. 



Belfast, November 2. 



Alpine Haze. 



Prof. Tyndall's letter in Nature about Alpine haze 

 induces me to say that as a non-scientific observer I have 

 never, I think, during a residence of many years here, seen so 

 much local fog or haze as this autumn. 



On October 29— a perfectly clear and cloudless day here (Vevey 

 and La Tour), with no appearance of fog, haze, or cloud, any- 

 where in the distance — I received a letter from Lausanne saying, 

 "While I write (11.30) so dense a fog has suddenly come up 

 that we fear for the boats on the lak-;." Other friends took a 

 trip to Lavey les Bains. They were in perfectly clear air until a 

 little beyond Villeneuve, when they f >nnd the whole Rhone 

 Valley thick with fog, but on turning off at St. Maurice Station 

 to Lavey les Bains (ten minutes' walk, and on perfectly flat 

 ground) they again cime into a quite clear atmo phere. As no 

 fog whatever came here all day, I cannot say whether it was 

 aqueour, or not. We have both sorts here from time to time, 

 but most commonly dryf ; this year has been rather an exception. 

 I should say fogs had been more frequently damp than usual, and 

 by observing the grass morning and evening I have found that 

 there has been much more dew than is common in this locality. 



S:reaky hazes or "long horizontal strix," as Prof. Tyndall 

 calls them, have certainly been unusually prevalent this year. 



La Tour de Peilz, November 4. M. C. C. 



The Animals' Institute. 



The l.Dng-continued suffering of animals fatally injured in our 

 streets, before the services of a slaighterer can be obtained, or 

 the owner be found to give his permission, has often been re- 

 ferred to. Poor animals with incurable abdoaiinal wounds, or, 

 it may be, coaaplete fracture of a limb, not unfrequently lie in 

 the streets for hours before being put out of their misery. The 

 po'ice have no power to order their destruction until the person 

 in charge assents, and he frequently cannot do this until his. 

 master has been communicated with. I remember one ca«e 

 where eight hours elapsed. I have recently fou.id that complete 

 absence of pain can be easily induced by subcutaneous injection 

 of morphia, and perhaps you would allow me to publicly state 

 that the apparatus and drug are always here at the service of the 

 police gratuitously in cases of street accidents. 



John Atkinson. 



9 Kinnerton Street, Wilton Place, Knightsbridge, 

 November 5. 



N. M. PRJEVALSKY. 



A TELEGRAM from Vyernyi— one of those small 

 Russian towns which have grown of late in the 

 outspurs of the Tian-Shan Mountains —announces the 

 death of Prjevalsky, the bold and indefatigable explorer 

 of the wildernesses of Central Asia In .September last, 

 immediately after having terminated the work which em- 

 bodies the results of his fourth great journey to Central 

 Asia, he started on a new journey, the fifth, thus prosecut- 

 ing again what has been the aim of his life during the 



