288 



NATURE 



\_Aug. 3, 1876 



with the ovum, and possibly some of the vascular corium 

 in addition ; and even in Canidce and Pinnipedia less of 

 the decidua comes away at parturition than in the cat. 



4. That the secretion of the uterine glands is absorbed 

 by the intervillous parts of the chorion, and serves as 

 " uterine milk or chyle," the comparison originally made 

 by Harvey. 



The important bearing of these researches on the 

 classification of Mammaha is obvious, and they suggest 

 scarcely less important questions as to the nutrition and 

 respiration of the foetus. P. S. 



OUR BOOK SHELF 



An Elementary Treatise on Kinetnatics and Kinetics. By 

 E. T, Gross, M.A., Fellow of Gonville and Caius Col- 

 lege, Cambridge, &c. (London : Rivingtons, 1876.) 



Mr. Gross says, in his preface to the book before 

 us, that it "is intended to contain as much as is 

 required, under the head of Dynamics, of candidates 

 for honours in the first three days of the mathematical 

 tripos." This object has no doubt determined to a great 

 extent the form which the work has taken, and we see 

 no reason to doubt that it is well suited for the purpose 

 mentioned, and will prove useful to students working for 

 Cambridge examinations. The first five chapters are 

 devoted to the Kinematics of a point, the conception of 

 Velocity being taken up at the outset, along with that of 

 Motion ; motion as change of position, and the theorem 

 of the instantaneous centre is only briefly mentioned in 

 a short chapter (the sixth) chiefly devoted to the " Geo- 

 metry of the Cycloid." The remaining ten chapters of the 

 book are given to Kinetics. The author has taken great 

 pains to put the fundamental conceptions of his subject 

 clearly before his readers, and the parts of his book most 

 valuable to the general student will certainly be those in 

 which he endeavours to crystallise the vague notions too 

 often picked up, at the commencement of a study, as to 

 velocity, force, &:c. At the same time we must say that 

 the arrangement of the book is not such as to fit it for 

 general purposes as an elementary text-book on its own 

 subjects. Perhaps this was unavoidable, considering the 

 main object with which it was written, but it is certainly 

 to be regretted. For most purposes it seems better to 

 commence the study of Kinematics by considering motion 

 as change of position only, leaving velocity to be brought 

 in later. This certainly makes it more easy for the student 

 to realise the matter, and obviates such difficulties as 

 occur for instance at pp. 16 and 20, where "change of 

 velocity " means in one place a change of velocity both in 

 direction and in magnitude, and in the other a change in 

 magnitude only. The same treatment also would allow 

 of portions of the Kinematics of rigid bodies being taken 

 up in an elementary manner, while in Mr. Gross's work 

 this part of the subject, the most important one, is prac- 

 tically left untouched. No motion, in fact, is considered, 

 except the motion of a point in a plane. The treatment 

 by the method of instantaneous centres is merely men- 

 tioned, although the development of this method certainly 

 furnishes excellent means for the elementary treatment ot 

 the more important problems connected with the kine- 

 matics of rigid bodies. Similar remarks might be made 

 in reference to the second part of the work, but perhaps 

 it is not fair to criticise from this general point of view a 

 book written chiefly for a special and hmited purpose. 



Mr. Gross has used geometric illustrations freely and 

 with great advantage throughout his book. We regret 

 that he has adhered throughout to the parallelogram of 

 velocities, forces, &c. Surely it is more elegant and in 

 every way better to use three lines than five. Culmann's 

 science can be very little known in this country if we have 

 not yet got even as far as this. 



LETTERS TO THE EDITOR 



[The Editor does not hoid himself responsible for opinions expressea 

 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.'] 



The Direct Motion in the Radiometer an Effect of 

 Electricity 



I HASTEN to communicate to Nature some new facts which 

 are destined, I believe, to throw some light on the theory of the 

 radiometer : — 



1. The glass globe becomes negatively electrified upon the 

 whole of its exterior when the instrument is submitted to 

 solar, or even obscure heat radiations of sufficient intensity, and 

 this electricity is more intense upon the hemisphere facing the 

 radiant source than that opposed. It was by means of a proof- 

 plane of large surface and a Bohnenberger's electroscope that I 

 was able to determine the presence of this free electricity. By 

 touching the globe several times in different places with the 

 proof-plane, and then applying it to the electroscope, the effects 

 are very sensible. This electricity cannot be attributed to the 

 friction of the radiometer vanes with the rarefied air of the 

 globe, since the electroscopic indications are not modified when 

 the instniment is inverted, and the vanes thus prevented from 

 moving. Neither, as several experiments show, can it be attri- 

 buted to feeble evaporation on the exterior. This development 

 of electricity upon the exterior surface of the globe is of course 

 necessarily accompanied by the development of positive elec- 

 tricity upon the inner surface. 



2. When exposed to radiation, the black face of the vanes is 

 electrified positively, and the bright face negatively. This I 

 have proved in the following manner :— I took a strip of mica 

 two decimetres lone, and having coated one of the sides with 

 lampblack, I suspended it in a Coulomb's Torsion Balance, having 

 previously electrified the metallic disc of the balance-needle with 

 positive electricity. The blackened side faced the disc. I then 



allowed the radiations from a gas-flame to fall upon the black- 

 ened surface of the mica strip. Notwithstanding the light was 

 at some distance, and had to penetrate the thick glass shade 

 inclosing the balance, the needle was rapidly repelled several 

 degrees, showing that the blackened face was positively electri- 

 fied under the influence of radiation. I then turned the strip of 

 mica so that the bright side faced the disc and allowed the 

 radiation to fall as before upon the blackened surface. The 

 needle indicated an attraction between the disc and the mica, 

 proving that the bright surface was negatively electrified. 



3. To anticipa'e the objection that these electrical manifesta- 

 tions are too feeble to account for the rapid revolution of the 

 vanes, I gently rubbed the globe with a brush composed of glass 

 threads ; the electricity developed on the globe acting by induc- 

 tion upon the nearest mica disc of the radiometer caused a brisk 

 oscillation. I then measured the intensity of this electricity by 

 means of the proof-plane and electroscope, and there were no 

 indications of greater intensity then when the globe was electri- 

 fied by radiation. 



4. From the above facts the following theory, if I mistake 

 not, necessarily flows. The radiometer is electrified as repre- 

 sented in the figure. At c the black face of the vane is turned 

 towards the radiant source, and in this position the vane will be 

 forced to move in the direction a c B ; when it arrives at D, the 

 direction of the rotation which the attractive and repulsive forces 



