190 



NATURE 



[June 21, 1888 



New York, by George H. Williams. With this paper the 

 author concludes for the present his elaborate petrographic 

 studies of the extremely varied massive rocks of the " Cortlandt 

 Series," as it has been designated by Prof. J. D. Dana. He 

 treats in detail the gabbro, diorite, and mica-diorite varieties of 

 norite occurring chiefly in the south-western portion of the area. 

 — Three formations of the Middle Atlantic slope (continued), by 

 W. T- McGee. In this concluding paper the whole subject of 

 the Columbia formation is recapitulated, the general conclusion 

 being that it is much older than the moraine-fringed drift-sheet 

 of the North-Eastern States, and that while th evertebrates of its 

 correlatives suggest a Pliocene origin, both stratigraphy and the 

 invertebrate fossils prove that it is Quaternary. Thus the 

 Columbia formation not only enlarges current conceptions of 

 Quaternary time, and opens a hitherto sealed chapter in geology, 

 but at the same time bridges over an important break in geo- 

 logical history, between the Tertiary and Quaternary epochs.— A 

 comparison of the elastic and the electrical theories of light with 

 respect to the law of double refraction and the dispersion of 

 colours, by J. Willard Gibbs. The main object of this paper is 

 to show the great superiority of the electric over the elastic 

 theories of light as applied to the case of plane waves propa- 

 gated in transparent and sensibly homogeneous media. The 

 phenomena of dispersion here studied corroborate the conclusion 

 which seemed to follow inevitably from the law of double refrac- 

 tion alone. — Mr. Henry J. Biddle contributes some valuable 

 notes on the surface geology of Southern Oregon, visited by him 

 during the summer of 1887. 



SOCIETIES AND ACADEMIES. 

 London. 



Royal Society, June 7. — "An Additional Contribution to 

 the Placentation of the Lemurs." By Prof. Sir Wm. Turner, 

 Knt, M.B.,LL.D.,F.R.S. 



In 1876 the author contributed to the Royal Society a memoir 

 " On the Placentation of the Lemurs," which was published in 

 the Philosophical Transactions of that year (vol. clxvi. Part 

 2). The gravid uteri which he examined and described were 

 from specimens of Propithecus diadema, Lemur rufipes, and Indris 

 brevicaudatus. 



In April of the present year he received from Mr. F. E. 

 Beddard, Prosector to the Zoological Society of London, the 

 gravid uterus of a Lemur, which was Lemur xanthomystax. 



The examination of this gravid uterus confirmed the conclusions 

 to which both Alphonse Milne Edwards 1 and the author had 

 arrived independently from previous investigations, that the 

 placenta in this important group of animals is diffused and non- 

 deciduate, and that the sac of the allantois is large and persistent 

 up to the time of parturition. In these important respects, 

 therefore, the Lemurs, are, in their placental characters, as far 

 removed from man and apes as it is possible for them to be. 



Although the author is not disposed to attach too much weight 

 to the placenta as furnishing a dominant character for purposes 

 of classification, yet he cannot but think that animals which 

 are megallantoid, non-deciduate, and with the villi diffused 

 generally over the surface of the chorion, ought no longer to 

 be associated in the same order with animals in which, as in the 

 apes, the sac of the allantois early disappears, and the villi are 

 concentrated into a special placental area, in which the foetal and 

 maternal structures are so intermingled that the placenta is highly 

 deciduate. Hence he is of opinion that the Lemurs ought to be 

 grouped apart from the Apes in a special order, which may be 

 named either with Alphonse Milne Edwards Lemuria, or with 

 Victor Cams and others Prosimii. 



The foetus possessed an imperfect covering, external to the 

 hairy coat, and quite independent of the amnion, composed of 

 a cuticular membrane. It corresponded with the envelope 

 named by Welcker epitrichium, and described both by him and 

 by the author as present in Bradypus and Cholopus. But it 

 occurred in the foetus both of Lemur xanthomystax and Pro- 

 fit hecus diadema in flakes and patches, and not as a continuous 

 envelope as in the Sloths. 



Physical Society, May 26.— Mr. Shelford Bidwell, F.R.S., 

 Vice-President, in the chair. — The following communications 

 were read: — Note on the governing of electromotors, by Profs. 

 W. E. Ayrton and J. Perry. In a paper read before the Society of 



1 "Histoire Naturtlle des Mammiferes de Madagascar," forming vol. vi. 

 chap. ix. ofGrandidier's " Histoire de Madagascar." 



Telegraph-Engineers in 1882 the authors deduced the conditions 

 of self-regulation of electromotors for varying load when sup- 

 plied either at constant potential or with constant current. The 

 conditions involved "differential winding," i.e. the use of a 

 shunt motor with series demagnetizing coils. With this arrange- 

 ment fairly good regulation has been obtained, but owing to 

 want of economy the methods have not been developed further. 

 Since then another arrangement, in which a simple shunt motor is 

 used, and a few accumulators placed in series with the armature, 

 has been devised for working in a constant current system. By 

 means of a suitable switch, the accumulators can be charged 

 when the motor is at rest. On the assumption that the 

 E.M.F. of motors is given by E = n(f + tZ), where 11 = speed, 

 Z = number of turns on magnets, and p and t are constants, 

 it is shown that the speed at which a motor will govern is 

 given by 



and the constant current 



z + a +a' 



,-, _ e - np 

 a + a' 



where 2 and a are the resistances of the shunt and armatuie 

 respectively, and e and a' the E.M.F. and resistance of the 

 accumulators. Since a and a' may be small and tip not large, the 

 value of e need not be great to give a considerable value for C, 

 and thus only a small number of accumulators will be required. 

 — On the formulae of Bernoulli and Haecker for the lifting-power 

 of magnets, by Prof. S. P. Thompson, read by Prof. Perry. 

 The formulas referred to are P °c ^/ \\T2 and P = al/^i re- 

 spectively, where P = lifting-power, W = mass of magnet, and 

 a a constant depending on the material and shape of the magnet. 

 These formulae, the author shows, are equivalent to saying that 

 the lifting-power of magnets in which the magnetic induction, 

 B, has been carried to an equal degree, is proportional to the 

 polar surface, and that Haecker's coefficient a is proportional to 

 B- through the surface. Assuming the induction uniform over 

 the surface, it is shown that 



p = 1bsa, 



Sir 



where A = area of surface, and this gives a very convenient 

 method of determining B from measurements made upon the 

 pull exerted at a given polar surface. If P be measured in 

 kilogrammes and A in square centimetres, the formula for B 

 becomes 



B = 5000 



V a' 



and if the measurements be made in pounds and inches, the 

 constant becomes 131 7. It will be readily seen that the greater 

 power of small magnets in proportion to weight does not require 

 for its explanation the sometimes alleged fact that small pieces of 

 steel can be more highly magnetized than large ones, for if B 

 be the same, the lifting- power will be proportional to the polar 

 surface, and not to weight, and hence must necessarily be greater 

 relatively to weight in small magnets. In the case of electro- 

 magnets for inductions between 6000 and 16,000, between which 

 the permeability, /x, is approximately given by 



16,000 - B 



fj. = -, 



3 '2 



the lifting-power is shown to be 



\S» + 2-56// ' 



where P is in kilogrammes, A in square centimetres, Si = ampere 

 turns, and / = mean length of the magnetic circuit. — Experi- 

 ments on Electrolysis ; Part ii., Irreciprocal Conduction, 1 by Mr. 

 W. W. Haldane Gee and Mr. H. Holden. An abstract was 

 read by the Secretary. The authors have observed, when strong 

 sulphuric acid is used as an electrolyte, the electrodes being of 

 platinum, that the decomposition nearly ceases, if, by decreasing 

 the resistance in circuit, it is attempted to increase the current 

 beyond a certain maximum. When this condition (called the 

 insulating condition) is arrived at, reversing the current imme- 

 diately restores the conductivity. Experiment shows that the 

 current density is an important factor, and that the composition, 



1 Irreciprocal conduction is said to occur if a reversal of tl.e direction of a 

 current causes any change in its magnitude. 



