Jan. 7, 1875] 



NATURE 



187 



as in tliis case, the results are entirely at variance with those of 

 profound and elaborate researches in the same direction which 

 have preceded. We propose, therefore, to examine brielly only 

 a veiy few points in the reasoning from which these results have 

 been deduced. 



The author states in the commencement that equilibrium is 

 disturbed by the three following causes : — 



(i7.) Alteration of the specific gravity of the water or air. 



(/'. ) The rotation of the earth on its axis. 



(c. ) The attraction of the sun and moon, 

 lie accordingly treats the subject under these three general 

 heads. Under the first two he endeavours to show that none of 

 the usual causes to which the currents of the ocean and the atmo- 

 sphere have been usually referred can have much, if any, effect 

 in producing them, and that they must, therefore, be due to 

 some other cause. This seems to be designed to make way for 

 the introduction into this subject of tlie new disturbing forces 

 contained above under the last head (<-). Much might be said 

 with regard to what is stated under the first two heads in dis- 

 paragement of the forces upon which these currents have been 

 heretofore supposed to depend, but we shall confine ourselves 

 here to a very few steps merely in the reasoning under the last 

 head. 



The author sets out under this head by assuming that the 

 equilibrium theory of the tides is applicable to the real case of 

 nature, and with this assumption he endeavours to show that the 

 flood-tide rises higher above the plane of static equilibrium than 

 the ebb-tide sinks below it. Now, it is well known by all who 

 are familiar with tidal theories, that this theory is entirely worth- 

 less as a representative of the real tides of the ocean. Here, 

 then, there seems to be a weak place in the very foundation of 

 the whole reasoning, and any results based upon it should be 

 received with much distrust, if even all the following steps in 

 the argument were regarded as valid. In the second place, he 

 attempts to show, by a method which is very unscientific and 

 inconclusive, that the forces of the sun and moon tend to produce 

 a current from the east towards the west in the flood-tide, but 

 the reverse of this in the ebb-tide. This is then followed by 

 another assumption in the following language : — " Since, as we 

 have shown, the flood rises more above the normal level of the 

 sea than the ebb sinks below it, we think we can assume, as an 

 hypothesis, that the force of the flood-current will be greater 

 than that of the ebb-current." From this he infers that the dif- 

 ference in these forces must produce a constant current in the 

 ocean in the torrid zone from east to west, but, for reasons which 

 do not seem clear, the reverse of this toward the poles ; and in 

 this way, taking into account the deflections of the continents, 

 he accounts for all the ocean currents without the aid of any of 

 the usual causes assigned. In the case of the atmosphere he 

 thinks that the same reasoning must hold, but admits that in 

 this case the alteration of the specific gravity by heat toward the 

 equator may produce some additional and modifying effects. 

 Saying nothing with regard to the steps in the argument, these 

 results are based upon a confessedly doubtful hypothesis, and 

 therefore should not be received without further proof. 



This is not a question to be settled by authority, but after the 

 profound investigations of Laplace and Airy upon the tidal forces 

 and the solution of the tidal problem, from which no constant 

 currents around the earth were obtained, we would scarcely 

 expect that such results would be legitimately obtained in a lew 

 pages of verbal reasoning without the aid of mathematics. It is 

 true that more recently a very small effect of that kind has been 

 obtained, tending to produce a westward current in all latitudes, 

 from which, by means of friction, the earth's rotation on its axis 

 is supposed to be slightly changed, but this effect is of an order 

 almost infinitely small in comparison with those under considera- 

 tion, and not at all contemplated in the author's reasoning, 

 referred to above. Wm. FERRiii. 



Washington, D.C., Nov. 7, 1874 



Mud Banks on Malabar Coast 



The phenomenon of the " mud banks and of tracts of mud sus- 

 pended in the sea " on certain parts of the Malabar coast, is not, as 

 you suppose (vol. xi. p. 135), unexplained. ThelateCapt. Mitchell, 

 curator of the Madras Museum, some years ago submitted a 

 quantity of the mud to microscopic examination, and published 

 the results in the Madyas Journal of Literature and Science (\ 

 have not the work at hand, or I would give you volume and 

 page). He found it to consist almost entirely of Diatomaceo;, of 



wliich he detected and distinguished sixty-two species. In the 

 paper in the Madras Journal Capt. Mitchell gives a list of the 

 genera and a numerical list of the specific fonns. 



The causes that have determined this local development of 

 Diatomaceoe remain for investigation. They appear sometimes 

 to shift their place. Thus, a Dutch navigator (Stavorinus, I 

 believe) described two such banks as existing to the south of 

 Cochin in 1777, but these no longer exist. 



Richmond, Surrey Henry F. Bl.^nford 



Ring Blackbird 



Every morning a brown bird (apparently a female blackbird) 

 feeds at my library window. She has a white spot oa the breast, 

 and a large white ring, in the exact position of that on a Bai'bary 

 dove, not meeting under the chin. Is this an unusual variety ? 

 I see no mention of such a peculiarity in any of the books at 

 hand, as Lewin, Bewick, Mudie, &c. C. M. Ingleky 



\'alentines, Ilford, Jan. 4 



ON THE MORPHOLOGY OF CRYSTALS* 



pROFESSOR MASKELVNE, in introducing his sub- 

 -l ject, said that in the assembly-room of the Chemical 

 Society he should have to treat of Crystallography as the 

 Science of Chemical Morphology. To the chemist the 

 crystallisation of a substance is a familiar marvel ; so 

 familial", indeed, that he hardly sufficiently considers its 

 importance in relation to his own science. For the 

 physicist, on the other hand, the instinct with which the 

 molecules of a substance obey the laws of a sublime 

 geometry — sublime because simple and universal — is a 

 theme the contemplation of which has guided him to some 

 of the most subtle and almost metaphysical conceptions 

 that he has formed regarding the constitution of matter, 

 and has afforded him invaluable insight into the working 

 of the laws that control the pulsations of heat and hght 

 and other manifestations of force. But, although the mor- 

 phological relations of the crystal are the external expres- 

 sion of the more subtle physical properties which underlie 

 thern, he stated that the purpose of the lectures he was 

 about to deliver would be confined to the consideration 

 only of the former. 



Placing a large and very perfect crystal of apophyllite 

 from the Ghats of India on the table, the lecturer pointed 

 out that certain faces carrying peculiar striations were 

 repeated four times ; that again others of a triangular 

 form, planted on the angles of the latter, were repeated 

 eight times, and that these had a lustre of their own ; while 

 again a plane of octagonal form was repeated only once 

 on the top and at the bottom of the crystal, and carried 

 a peculiar roughened surface, whic'n was seen to be made 

 up of innumerable small square pyramids in parallel posi- 

 tions. He further showed that by turning the crystal 

 round about an axis perpendicular to the last planes, the 

 relative situations of the planes, as viewed from any 

 point, came always to be the same at any revolution 

 through a quarter of a circle. A group of faces repeated 

 with similar properties was defined as a form, the crystal 

 in question thus exhibiting three forms ; the repeated 

 faces of each form retaining the same general aspect so 

 long as they were not moved round through an angle 

 greater or less than 90°. Then taking crystals of quartz 

 which presented the s^-mQ/onus, he pointed out that faces 

 that corresponded to one another on the different crystals, 

 and even on the same crystal, have very different relative 

 magnitudes ; and that, in fact, these magnitudes were 

 controlled by no rigid geometrical law. On the other 

 hand, the angles which measured tlie inclination of corre- 

 sponding faces on each other were in every case identical ; 

 hence angular inclination, that is to say, the direction in 

 space, not relative position, that is to say, precise mutual 

 distance, in the faces, has to be recognised as a principle 



* Some note-! of the Lectures delivered at tlie Chemical Society's rooms 

 in Burlington House, on the Morphology of Crystals, by Frof, N. S. 

 Maskelyne, F.R.S. 



