iQM.] ATMOSPHERIC PRESSURE. 57 



the liquid, and to the square root of the specific heat, the thermal 

 conductivity, the wind velocity, the fluid density and the wire radius. 



Dr. Alexander Russell communicated an important paper on the 

 theory of the subject to the Physical Society of London in July 

 1910,^ developing and extending Boussinesq's formula. 



Professor J. T. Morris has recently successfully applied the 

 square-root law of forced-convection velocity to the measurement 

 of wind-velocities, using an ingenious form of Wheatstone bridge 

 for this purpose. His observations were communicated to Section 

 G of the British Association in 1912- and also to " Engineering"^ in 

 1913. His results have confirmed the application of the law for 

 wires of various metals up to diameters of 0.3 mm. 



The papers and deductions of Boussinesq were not known to us 

 at the time we presented our former paper in 1909; but since the 

 square-root law of velocity arrived at theoretically by Boussinesq in 

 1901-1905, for an incompressible non-viscous liquid, has been found 

 to hold within errors of observation for ordinary air, it became 

 desirable to ascertain whether the linear forced convection of air 

 varied as the square root of the air pressure, as suggested by Bous- 

 sinesq's formula. 



Method of Measurement Employed. 



The method followed and the apparatus used were respectively 

 the same as those described in the A. I. E. E. paper of 1909, above 

 referred to. A short length of the thin wire to be tested was held 

 in a fork, and was driven by an electric motor at successively varied 

 speeds in a large steel tank, the atmospheric pressure within which 

 was kept constant in each series of tests; but was different in differ- 

 ent series. 



Test Wire. 



The wire used in all of the tests here described was of good com- 

 mercial platinum. No. 36 B. & S. gauge, with a mean diameter of 



1 Proc. Physical Society, 1910, Vol. XXII., also Phil. Mag., October, 1910. 



2 Prof. J. T. Morris, " The Electrical Measurement of Wind Velocity," 

 The Electrician, Oct. 4, 1912, pp. 1056-1059. 



3 J. T. Morris, " Distribution of Wind Velocity about a Circular Rod," 

 Engineering, Vol. 96, pp. 178-181, Aug. 8, 1913. 



