I9I4.] ATMOSPHERIC PRESSURE. 77 



apparatus, such as eddy-currents in the confined air, caused by air- 

 churning in the pressure-tank. In other words, it is uncertain whether, 

 if a more spacious pressure-tank could have been used, with a longer 

 fork radius, the square-root relation might not have held better than 

 it did at the upper and lower air-pressures. 



3. The efifect of moisture in the air, upon the forced convection 

 of heat from a thin wire, seems to be small, and has not yet been 

 determined. 



4. The forced convection of heat from a thin wire at constant 

 temperature in air of variable pressure may be regarded as vary- 

 ing approximately with the square root of the mass of air displaced 

 by the wire per second, or as ^va, where v is the wind-velocity of 

 displacement and a the density of the air. 



5. When the air-pressure p remains constant, (Pc/O) " or the square 

 of the linear convection per deg. C. elevation, plotted against wind- 

 velocity V, gives a straight line for a thin metallic wire. When, how- 

 ever, the air-pressure p varies, then it seems that, at least to a first 

 approximation, {Pc/OY/p plotted against wind-velocity v, gives one 

 and the same straight line, for that wire, for all values oi p from 

 0.5 to 2.0 megabars.. 



6. A thin vertical platinum wire about 25 cm. long, after being 

 calibrated in a motor-driven fork, can serve as a recording wind meas- 

 urer or anemometer. The record requires to be corrected both for 

 the temperature and pressure of the air. The degree of precision 

 obtainable is srreatest at low wind-velocities. 



