Cooling of Cylinders in a Stream of Air. 119 



Apparatus. 



The measurements were carried out on a series of copper 

 tubes all of the s-tme length, but with external diameters 

 ranging from *433 cm. to 5'06 cm. These tubes, of about a 

 metre in length, were placed vertically in the wind-channel, 

 and were heated by steam from a boiler placed on top of the 

 channel; after a steady temperature was attained, the weight 

 of the water which condensed in the cylinder in a given time 

 measured the heat which it lost through convection and 

 radiation. 



The wind-channel, fig. 2, consisted of a wooden tunnel 

 placed horizontally, which was of square section, of side 

 3 feet, and was 10 feet long. This terminated in a rectangular 

 box of greater section in which the fan rotated. 



The fan was made by attaching four narrow steel blades to 

 the axle of a three-phase motor, the speed of which could be 

 regulated (a) by a rotor rheostat, and (b) by varying the 

 voltage of the alternator supplying the current. A good 

 range of wind-velocity could thus be obtained. The velocity 

 of the air currents produced by the fan was measured by the 

 ordinary Pitot-tube and gauge. This consisted of two glass 

 tubes of ^q inch bore, the dynamic tube facing the wind, 

 and the static tube being placed with the plane of its orifice 

 parallel to the direction of the flow. These tubes were placed 

 close to one another in the centre of the channel, imme- 

 diately in front of the cooling cylinder. By means of 

 rubber tubing they were connected to a water-gauge, the 

 difference of level in which measured the difference of 

 pressure, which was proportional to the square of the 

 velocity as given by 



h being the pressure difference in metres of air, and v the 

 velocity in metres per second. The difference between the 

 two sides of the U-tube was measured by a cathetometer 

 microscope reading to T ^ of a millim. The gauge-tubes 

 were 2 cm. wide so as to avoid surface-tension errors. Large 

 oscillations were produced in the liquid in the gauge on 

 account of variation in the wind-pressure, and this caused a 

 difficulty in measuring the wind-velocity. The oscillations 

 were damped, and the amplitude reduced by loosely stopping 

 the bottom of the U-tube with cotton- wool. In some of the 

 later experiments, thick paraffin was adopted as the liquid in 

 the gauge. With this, the difference of level was increased 

 on account of the smaller density of the liquid, while the 

 oscillations were reduced by reason of its greater viscosity. 



