Intelligence and Miscellaneous Articles. 87 



on the plate a space void of powder and having exactly the shape 

 of the disk. 



With electrodes of a conical form with an angle of 60° or of 30°, 

 or in the shape of a tapering needle, M. Schneebeli ascertained that 

 the electrical figure is as much larger as the angle at the summit of 

 the cone is smaller. 



Lastly, the diameter of the figure is greater when the discharge is 

 effected in a rarefied gas than at the ordinary pressure. — Archives 

 des Sciences Physiques et Naturelles, vol. xlvi. p. 269. 



NOTE ON GASEOUS PRESSURE. BY ROBERT MOON, M.A., HONO- 

 RARY FELLOW OF QUEEN^ COLLEGE, CAMBRIDGE. 



As Mr. Strutt appears indisposed to continue the controversy 

 between us*, it may perhaps be permitted me to recapitulate its 

 results. 



Mr. Strutt impugned my views as to gaseous pressure on two 

 grounds t : — 



1. That an expression for the pressure involving both velocity 

 and density leads to absurd results. 



2. That the analytical argument upon which I based such an ex- 

 pression fails, "the question at issue" being "a purely physical 

 one " — the pressure prevailing in a fluid in motion being, as he con- 

 sidered, settled by Boyle's law, which he assumed to hold in that case. 



How far the first part of this criticism was justified I remit to 

 the judgment of those who may think it w T orth while to refer to 

 my papers in the Philosophical Magazine for August and February 

 last how far the second part is consistent with fact appears from 

 the second paragraph of Mr. Strutt's recent paper. 



6 New Square, Lincoln's Inn, 

 June 2, 1873. 



NOTE ON THE USE OF A DIFFRACTION-" GRATING " AS A SUBSTI- 

 TUTE FOR THE TRAIN OF PRISMS IN A SOLAR SPECTROSCOPE. 

 BY PROF. C. A. YOUNG. 



Since the diffraction-spectrum differs from a prismatic spectrum 

 of the same length in having the less-refrangible rays more widely 

 dispersed, it some time ago suggested itself that a so-called " Gitter- 

 platte" or " grating " of fine fines, might advantageously replace 

 the prisms in spectroscopes designed for the observation of the solar 

 prominences through the C line. In this idea I was strongly con- 

 firmed on seeing last winter some of the beautiful gratings ruled 

 upon speculum-metal by Mr. Chapman, Mr. Eutherford's mechani- 

 cian. The spectra furnished by these plates far exceed in brilliance 

 and definition any thing of the kind ever before obtained. 



* See paper " On the Law of Gaseous Pressure " in the Philosophical 

 Magazine for June 18/3. 



t See paper " On Mr. Moon's Views on Gaseous Pressure " in the Num- 

 ber of the Magazine for July last. 



