332 Intelligence and Miscellaneous Articles. 



ON THE DEPENDENCE OF THE COEFFICIENT OF FRICTION OF AT- 

 MOSPHERIC AIR ON TEMPERATURE. BY ALBERT VON OBER- 

 MAYER, CAPTAIN OF ARTILLERY. 



Of the two hypotheses from which the dynamical theory of gases 

 starts, the older gives the coefficient of friction of gases propor- 

 tional to the square root, the newer (Maxwell's) gives it proportional 

 to the first power of the absolute temperature. From the retarda- 

 tions of vibrating disks by the friction of the air, Maxwell found 

 experimentally the power 1, 0. E. Mayer the power | ; by experi- 

 ments on currents through capillary tubes O. E. Mayer found the 

 power f , J. Puluj the power f, of the absolute temperature. i 



For the more certain determination of the ratio, experiments on 

 currents through four capillaries of glass and one of brass were 

 undertaken ; and together with the temperature of the room those 

 of boiling water, congealing paraffin, and a mixture of salt and snow 

 were employed. A first series of experiments, less accurate, were 

 carried out with the difference of pressure variable ; a second, con- 

 siderably more exact, with the difference constant. 



The results of the two series agree very well, and confirm those 

 of Mayer's experiments in a perfectly satisfactory manner. For 

 the coefficient of friction /z at the temperature t there were found : — 



According to the first series, /u = 0-0001706 (1 + 0-002735 ; 

 „ second „ ^ = 0-00016747 (1 + 0-002723*). 

 — Kaiserliche Akademie der Wissenschaften in Wien, Sitzung d. math- 

 naturw. Classe, Feb. 4, 1875. 



ON 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, General Post Office, 



March 2, 1875. 



Camacho's concentric-tube electromagnet seems to be identical 

 in principle with one lent me when I lived in Manchester about 

 1852, by the late Richard E/oberts, so well known in connexion 

 with spinning-machinery. 



His electromagnet was an iron cube sawn nearly through by a 

 number of parallel cuts, crossed at right angles by other cuts, so as 

 to form a series of cores connected at one end. 



Each of the cores was wrapped with wire, forming a series of 

 electromagnets, connected by a common breech piece in metallic 

 connexion with all the cores. 



This arrangement was very powerful for its size; I regret that I 

 have no note of its performance. 



Tours faithfully, 



R. S. CULLEY. 



