May y, 1877] 



NA TURE 



83 



je crois aussi plus faciles, ont ^te ouvertes, et c'est I'ceuvre 

 de ceux qui veulent servir la science et leur pays de dis- 

 cerner ce que les elements peuvent recevoir de I'immense 

 elaboration qui s'est accomplie depuis Gauss jusqu'h. 

 Riemann." 



There is in the present tract a clear exposition of the 

 elementary applications of Trilinear and Triangular 

 Coordinates, and just a passing glance at Polar reci- 

 procals. 



In such a work we do not look for anything original, 

 but for clearness and correctness. These ends, we think, 

 have been attained, and we wish Dr. Wright health and 

 leisure to enable him to carry out his design. 



Grundriss der chcDiiscIien TccJuiologic. Von Dr. Jul. 



Post. (Berhn : Robert Oppenheim, 1877.) 

 Dr. Post, who is known to chemists as an able Privat 

 Docent at the University of Gottingen, has, like many other 

 teachers, felt the great necessity of a manual of chemical 

 technology, suited to the requirements of students who 

 desire a general training in that branch of applied 

 chemistry. A considerable number of exceUent treauses, 

 as, for example, those of Knapp, Wagner, Bolley, Kerl, 

 and Stohmann, already exist in German, and some of 

 them have been translated into English, but no one of 

 them is exactly adapted to the class-room. Their excel- 

 lence consists in their completeness as works of reference ; 

 indeed as such they may be said to be invaluable to the 

 chemical manufacturer ; but the mode of their arrange- 

 ment renders them of comparatively little value as aids to 

 systematic study. Dr. Post has succeeded in producing 

 a work which, within the compass of some of our smaller 

 chemical manuals presents a complete outline of the 

 present position of chemical technology. His book thus 

 serves as a fitting introduction to the larger and more 

 special treatises above mentioned. 



LETTERS TO THE EDITOR 

 \The 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. 

 A'o notice is taken of anonymous communuations. 

 The Editor urgently requests correspondents to keep their Utters as 

 short as possible. The pressure oti his space is so great that it 

 is impossible otherwise to ensure the appearance even of com- 

 munications contai}iing uiterestifig and novel facts."] 

 Colour-Sense in Birds 



I HA\'E been lately watching, with great delight, two gold- 

 finches building their nest. They placed it nearly at the end of 

 an outside branch of a young sycamore tree, so that there was 

 nothing but sky above it, and the gravel path below. Thn 

 window from which I observed them, being never opened, and 

 well covered with flowers in pots and a blind, seems to have 

 caused them no alarm, although not more than two yards dis- 

 tant from them ; and their object appears to have been to make 

 their nest invisible from below. To this end they chose their 

 building materials with such skill and such colour-matching 

 power that if one had not seen the nest built it would be quite 

 impossible to discover it ; to match the tree they took its long 

 flexible blossoms, and to match the sky the equally long and 

 flexible stalks and flowers of the garden forget-me-not, of which a 

 bed was close at hand in full bloom. I watched them carefully, 

 and, as far as I could see, they used no other materials than 

 these flowers, though I saw one of them attempting to get the 

 dirty-white cotton tie off a budded rose-tree. At all events 

 the nest was mainly built of them. The blue of the forget-me- 

 not has of course faded, but the general effect from below is 

 that of a scarcely visible grey-green thickening of one of the 

 bunches of sycamore leaves. fhey seemed to enjoy flinging 

 their flower-wreaths .-ibout. And that leids to the question 

 whether birds — who are in many ways like chddren — do not 

 often out of mere playfulness and love of colour, pull to pieces 

 yellow crocuses and other bright flowers. While my pen is in 

 my hand I may mention, with reference to Dr. Muirhead's 

 communication on the subject of noise causing a sensation of 

 colour, tha*: I have fiequently observed whilst tuning a harp, 



that the sudden breaking of a string will cause a curious taste 

 and sensation in the mouth, like that produced by a piece of 

 silver and one of zinc placed above and beneath the tongue, 

 when they are made to meet. J. 



A Simple Wave-Motion Apparatus 

 It has been suggested to me that I should publish a descrip- 

 tion of a simple and portable wave-motion apparatus, devised 

 by me a year or two ago, which has given satisfactory results to 

 others as well as to myself. I therefore send the description. 



In the figure A A represents an ordinary wooden lantern slide, 

 with a rectangular aperture, which may vary in size according 

 to the size of the lantern condensers, the sketch being half size 

 for 4-inch condensers. A small winch, r>, is fitted into the slide 

 at one end of the aperture, and held in its place by the tongue, 

 T. The spindle, K D, is milled or otherwise roughened near the 

 end, D. A brass stud similar in shape to the milled end of the 

 spindle, but smooth and slightly smaller, is fixed in the opposite 

 side of the aperture at c. A helix of 25-gauge hard brass wire 

 is wound on a spit of the same size as ttie smooth stud, taking 

 ciie to wind the coils close together ; about fifteen turns of the 

 liclix arecut off, and the middle five turns drawn out till they 

 firm a perfect wave similar to the figure when held up to the 

 light. The length of the helix should then be the same as c D. 

 One end of the helix is pushed tight on the milled end, D, and 

 the oilier end is slipped loosely over the stud, c, so as to work 



on it like a swivel, to keep the end of the helix true when the 

 winch is turned. A little bead of wax is melted on each crest 

 and hollow of the wave to represent particles, and the essential 

 parts of the apparatus are complete. On placing it in front of 

 the lantern, and focussing, a distinct and striking image of a 

 moving wave with its vibrating particles is produced by turning 

 the winch. 



If the helix is not perfectly straight the image of the wave will 

 rise up and down more or less as a whole ; the helix should then 

 be straightened or " set " with the fingers till true. When once 

 set thin glass plates may be placed on each side to protect it 

 from injury. An index, I, of wire, may be fixed so as to give a 

 means of proving that the particles only move up and down. 



A modificaiion I have tried by using a dark wire with bright 

 silver beads, on a velvet back-ground in the aphengescope, is 

 more difficult to make and use. I therefore prefer the apparatus 

 as sketched above. 



Of course the amount of finish depends on the taste of the 

 user, &c. A pasteboard frame instead of mahogany, a wire bent 

 twice at right angles instead of a finished brass winch, and tied 

 to the frame by two bits of wire instead of let in, &c., may be 

 used, thus reducing the cost to a few halfpence. 



In use it will recommend itself. W. Jesse Lovett 



Birmingham 



Atmospheric Currents 

 A CONTROVERSY was recently waged in your columns as to 

 the course which is pursued by ths hot water-laden air of the 

 equatorial regions in its journey to the poles. Bjth combatants 

 seem to adopt what I may call the sheet-theory, which regards 

 the winds as moving in sheets or strata, and gliding over and undtr 

 each other at the polar and equatorial sides of the calms of 

 Cancer and Capricorn, a process which would inevitably result 

 either in both opposing winds being torn to tatters, or in their 

 cjmmixture and neutralisation. Surely the truth is that like all 

 other moving fluids, the air will seek equilibrium in the direction 

 of least resistance, and will carve out for itself wide channels in 

 accordance with local conditions froai the poles to the equator. 



