Connexion between Viscosity and Density in Fluids. 51 



and if 6 is the angle of elevation, then 



D = 22 + Ssec0. 



Using this value of D in the formula above, and adding one 

 half to the strength of the light to make up for absorption, 

 the true intensity of the light in standard candles can be 

 ascertained. We find in practice that, if an electric light is 

 moderately steady, ten measurements may be made, with some 

 confidence in their accuracy, in two minutes ; and the light 

 may be measured in ten different positions, from an angle of 

 depression of 60° to an angle of elevation of 60°, 100 observa- 

 tions being taken, in less than half an hour. 



We may mention one very important result we have been 

 led to by the systematic employment of a photometer which 

 can be used close to the electric light ; and that is the large 

 amount of absorption that occurs on certain days when the 

 rays from strong electric lights, and especially the green rays, 

 pass through the air which appears to the eye perfectly clear. 

 At first we were inclined to think the higher results for the 

 candle-power of a lamp obtained with our dispersion-photo- 

 meter than those obtained with an ordinary distance-photo- 

 meter were due to some error in our photometer itself ; but 

 we have since ascertained that this is due to the absorption of 

 the air — because we find that, if simultaneous measurements 

 are made with ordinary Rumford's photometers, each without 

 lens or mirror, placed at different distances from the lamp in 

 the same azimuth and in the same horizontal plane, the nearer 

 one gives, as a rule, the highest readings ; and the difference 

 is the greater the stronger the light, and is greater if the light 

 be examined at each photometer with green glass. 



V. On the Connexion between Viscosity and Density in Fluids, 

 especially Gaseous Fluids. By E. Waebuec4 and L. v. 

 Babo*. 



THE laws according to which the elasticity and viscosity of 

 a body are connected with its density are of great sim- 

 plicity in the case of gaseous bodies. The elasticity of these, 

 i. e. the reciprocal of their compressibility, is given, according 

 to Boyle and Mariotte's law, by the pressure, and is propor- 

 tional to the density; the viscosity, measured by the coefficient 

 of friction, is, according to Maxwell's law, independent of the 

 density. 



It is known that the first of these laws, that which refers to 



* Translated from the Sitzungsberichte der K. Preuss. Akademie der 

 Wissensclwften zu Berlin, May 4, 1882, pp. 509-51-4. 



E2 



