434 report— 1878. 



of air is required to produce a spark than at higher pressure. This is not incon- 

 sistent with Sir William Thomson's result, namely, that a greater electromotive 

 force per unit length of air is required to produce a spark at short distances than 

 at long.* 



5. On the Absorption Spectrum of Chlorochromic Anhydride. By Gr. 

 Johnstone Stonet, M.A., F.B.S., Secretary to the Queen's University 

 in Ireland, and J. Emerson Reynolds, M.D., F.C.S., Professor of 

 Chemistry in the University of Dublin. 



The authors described and exhibited the absorption spectrum produced by the 

 vapour of chlorochromic anhydride (C 2 O 2 01 2 ). This spectrum is of pecidiar 

 interest from its having supplied information as to the duration and character of a 

 motion within the molecules of the vapour (see ' Philosophical Magazine ' for 

 July, 1871). The spectrum consists of lines very regularly distributed over the 

 orange yellow and green, bordering a more refrangible region of complete absorp- 

 tion. The lines are found to be equally spaced when plotted down on a map of 

 oscillation-frequencies, and the intensities of the successive lines are such as to 

 form definite patterns. From the positions of the lines, of which 105 had been 

 examined, it has been ascertained that they are to be referred to one motion in 

 the molecules of the gas, since they are all either its harmonics or such quasi- 

 harmonics as, for example, occur when an elastic rod is made to vibrate trans- 

 versely, most of which cannot be distinguished by observation from true 

 harmonics. We further learn from the positions of the lines that this motion, 

 on the more probable supposition that the lines are its true harmonics, is 

 repeated 810,000,000,000 times every second in each molecule. And from the 

 succession of intensities we learn that this molecular motion is in some way 

 related to that of a particular point on a violin-string when vibrating under 

 the influence of the bow, viz., a point nearly but not quite two-fifths of the 

 length of the string from one end (see ' Philosophical Magazine,' loc. cit., p. 47). 



6. On the Floio of Water in uniform regime in Rivers and in Open Channels 

 generally. By Professor James Thomson, LL.V., D.Sc, F.B.S. 



The communication of which the following is an abstract was made to the 

 Mathematical and Physical Section, in order to offer for consideration there a new 

 theory of some of the phenomena of the flow of water in rivers and in open 

 channels generally, which formed the subject of a paper just before sent to the 

 Royal Society. The author stated that during a great part of the present century 

 experimental investigations had been accumulating, made on some of the largest 

 rivers in the world, and on aqueducts and on small artificial channels, which tended 

 strongly to show that usually the velocity of flow at and near the surface of a 

 river is less than the velocity at some depth between the surface and the bottom. 

 They seem to show that for descent from the surface towards the bottom the 

 velocity first increases till a maximum is attained, and farther down goes on di- 

 minishing for approach towards the resisting bottom. Now this has appeared very 

 generally, even to the investigators themselves who have carried out the experi- 

 ments, to be a very perplexing result, because it has seemed, through various modes 

 of consideration, that the surface and upper portions of the current, being farther 

 distant from the resisting bed, should be expected to be less influenced by its 

 resistance, and should, therefore, flow faster than the parts intervening between 

 them and the bottom. The principal object of the paper was to offer a new theo- 

 retical view explaining and accounting for the observed phenomenon which now 

 appears to be highly probable, if not absolutely ascertained through experiments, and 

 which these theoretical considerations of the author go to confirm. He says that 



* Papers on Electrostatics and Magnetism, § 323, p. 248. 



