224 



NATURE 



{Jan. 20, 1876 



the left-hand page, and so above the plates, which are 

 immediately under the pupil's eyes. The printing and 

 the plates (the only figure that does not please us is the 

 oval on Plate II.) leave nothing to be desired. 



We proceed to point out a few matters which we think 

 admit of improvement. Plate II. in the definition of a 

 circle invarying is used ; why not " constant ? " The 

 construction of Fig. 6 (Plate IV.) is hardly satisfactory to 

 our view, though it is one very frequently given ; the 

 tangent to the two' arcs is not obtained ,by a legitimate 

 method. We cannot make out the definition of an 

 harmonic mean given on Plate VII., but\the mentis are 

 correctly constructed. In Fig. 31 (text), for GH:HA, 

 read vice versa. We may remark that it is a curious fact 

 that the approximative constiuction given in Fig. 87 is true 

 in the cases of regular figures of three, four, and six sides. 

 In Fig. 99 (text) read "through f and E." In Fig. 112 

 (text) arcs " cutting in c," not G. Constructions to Figs. 

 123, 125 give particular ellipses ; so ,in the case of the 

 parabolas in Figs. 138, 139, we note that certain figures 

 are stated to be (r(?-centric and certain curves have assym 

 ptotes. In Fig. 271 (text) read to cut in " I'and H." We 

 object, on pure geometric grounds, to the constructions 

 in Figs. 278, &c., where a line is found equal to the semi- 

 circumference of a circle, &c. ; also the inscribed circle 

 of a square and the inscribed triangle are stated as being 

 in the ratio, triangle : circle : square, as 2 : 3 : 4. In Fig. 

 279 (text) the two last a's should be D. The construction 

 to Fig. 297 (to draw a line to bisect any triangle from a 

 given point within it) is new to us, and on a cursory 

 examination of it we have not satisfied ourselves of its 

 correctness. In Fig. 314, for x Y, read ZY. In Fig. 316, 

 " the square on," or some such words have been omitted. 

 In Fig. 323 the limitations have not been laid down. In 

 Fig. 329, '■'join point x," &c. ; in 331, for "rectangle" 

 read " parallelogram." These trivial oversights will serve 

 to show how correctly the text has been printed. 



OUR BOOK SHELF 



Observaciones Magneticas y Meteorologicas del Colegio de 

 Belen de la Compaiiia de Jesus en la Habana, 1873 j 

 1874. (Habana, 1874 and 1875.) 



The observations made at the College of the Society of 

 Jesus, Havana, are peculiarly valuable for the fulness and 

 care with which they are made, and for the completeness 

 with which the observations themselves and the monthly 

 means and extremes are given in each monthly table and 

 its accompanying diagram. The diagrams, which have 

 been published in their present improved form since June 

 1873, and which exhibit on one sheet the two-hourly 

 observations as made daily from 4 A.M. to 10 P.M. of all 

 the meteorological and magnetical elements, will very 

 much facilitate the study of those inquiries which deal 

 with the inter-relations of these elements. To these 

 observations are added the daily amounts of the rainfall 

 and evaporation — the latter being of great interest as 

 contributing to our knowledge of the evaporation in inter- 

 tropical regions, of which so little is known. Whilst only 

 the daily amounts of the rainfall is given, each hour 

 during which rain falls is noted, together with the hour of 

 occurrence of thunder and other irregularly recurring 

 phenomena. As regards the diurnal variations of the 

 wind it changes from about S.E. in the early morning, 

 through E. and N.E. to N.N.E. its most northerly point, 

 which is usually reached^about 2 p.m., and thence in the 



reverse direction through N.E. and E. to E.S.E., which is 

 reached about 10 p.m. The diurnal velocity is at the 

 minimum at 4 a.m., rises to the maximum at 2 p.m., and 

 thence falls steadily to the minimum. The N. and N.E. 

 winds are decidedly the strongest, and the S.E. the 

 weakest, the ratio being as two to one ; in other words, 

 the sea-breeze blows with double the velocity of the land- 

 breeze at this station. 



LETTERS TO THE EDITOR 



\lhe Editor does not hold himself responsible for opinions extressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond "with the ■writers of, rejected manuscripts. 

 No notice is taken of anonymous communications. ] 



Blowpipe Analysis 



Mr. Humpidge (vol. xiii. p. 208), on the entirely gratuitous 

 assumption that I use "commercial reagents" — whatever that 

 term may mean — says that there is probably iron in my soda. 



To this I only reply that I will undertake to show pyrologi- 

 cally the presence of o'Oi per cent, of iron oxide in a fragment 

 of a salt the size of a pin's head; and that, when Mr. Hum- 

 pidge can do as much without using the dangerous test potassium 

 ferrocyanide (which itself contains iron), I will admit his right to 

 assume that he knows his tools better than other workmen. 



No one has ever doubted the proportional relativity in precipi- 

 tating power between a drop and a gallon of water, but if Mr. 

 Humpidge will only do me the justice not to mutilate my state- 

 ments in the reproduction, he will repeat that 2i pi-ccipiiate could 

 not be shown in a drop of water ^^ on a fused mass upon an 

 aluminium plate." W, A. Ross 



Shepherd's Bush, W., Jan. 14 



The D-line Spectrum 



Will Prof. Stokes give us the reason of his now holding that 

 his first — to all appearance, extremely rational — conclusion, that, 

 in consequence ol " the powerful affinities o{ sodium, it could not 

 exist in &free state in the flame of a spirit-lamp," is "erroneous"? 



Shepherd's Bush, W., Jan. 8 W. A. Ross 



I 



The Difference of Thermal Energy transmitted to the 

 Earth by Radiation irom different parts of the Solar 

 Surface, 



The tenor of certain letters received from scientific persons on 

 the above subject induces me to lay the following stalemtrit 

 before the readers of Nature : — 



I. Previous to undertaking a systematic investigation of the 

 mechanical properties of solar heat, I examined thoroughly 

 the merits of Laplace's famous demonstration relating to the 

 absorptive power of the sun's atmosphere, proving that only one- 

 twelfth of the energy developed by the. sun is transmitted to the 

 earth. The demonstration being based on the assumption that 

 the sun's rays emit energy of equal intensity in all directions, 

 my initiary step was that of testing practically the truth of that 

 proposition. It has been asserted that Laplace did not propound 

 the singular doctrine involved in such a proposition, I therefore 

 feel called upon, before proving its unsoundness, to quote tlie 

 words employed by the celebrated mathematician. (See "Me- 

 chaniquc Celeste," tome iv. page 284.) Having called attention 

 to the fact that any portion of the solar disc as it approaches the 

 limb ought to appear more brilliant because it is viewed under a 

 less angle, Laplace adds: — "Car il est naturel de penser que 

 chaque point de la surface du soleil renvoie une lumiere egale 

 dans tous les sens." Let abed, in the annexed diagram, Fig. i, 

 represent part of the border of the sun, and b a, cd, small equal 

 arcs; a a', b b' , cd, dd', being parallel rays projected towards 

 the earth. Laplace's theory asserts that owing to the concen- 

 tration of the rays the radiation emanating from the portion dc 

 txaxisrahs greater intensity towards the earth than ba, in the pro- 

 portion of cdiofc. The proposition is thus stated in " Me- 

 chanique Celeste " : " Call the arc of a great circle of the sun's 

 surface, included between the luminous point and the centre 

 of the sun's disc, the sun's radius being taken for unity ; a very 

 small portion a of the surface being removed to the distance 9 



