HOGG. — VISCOSITY OF AIR. 



G17 



O 





o 



wire about 5 cm. above tlie sphere, and is carefully fixed so that the 

 wire is at risht angles to its plane. The sphere also is adjusted so that 

 the wire passes along a diameter. The 

 adjustment is maintained by a small quan- 

 tity of beeswax. The containing vessel, 

 L, the top of which is removable, is a 

 double-walled tin vessel, having an inter- 

 space of about 5 cm. This space is filled 

 with water, while a heavy coating of hair 

 felt surroun Is the whole. An extra 

 thickness of felt is placed on the top, as 

 here there is no water protection. M is 

 a thermometer. N is the glass-covered 

 opening in the vessel, L, through which 

 the image of a scale can be observed in 

 a telescope by reflection from the mirror. 

 On the top of the containing vessel is a 

 card with its centre at the centre of the 

 lid, and to the prolongation of the cap, 

 D, is fastened an index which marks tlie 

 zero position of D, i. e., the poshion in 

 which it must be placed that the front of 

 the mirror may be directed towards the 

 opening, X. By giving D a slight turn 

 a start can be given to the vibrator. A 

 carefully constructed manometer is con- 

 nected to the tube R, which is also 

 attached to a mercury pump. The 

 connection to the pump has a branch in 

 which is placed a drying tube and 

 through wliich, if desired, air can be 

 admitted into the apparatus. 



Figure 2. 





Preliminary Experiments. 



Preliminary experiments showed clearly that, in order to obtain a 

 regular diminution in the arc of swing, great care was necessary to avoid 

 any convection currents in the apparatus. The water jacket and felt 

 covering described above gave entire satisfaction in this respect and also 

 maintained the temperature of the apparatus sufficiently constant. 



