1894.] Atmospheric Temperature, especially during Fohn. 127 



thermometers of German pattern, it takes little part in the exchange 

 of heat with the outside. Another method of obtaining the exact 

 circumference of the bulb, which is a little easier and perhaps more 

 exact, is to wind fine thread round it, each turn touching its neigh- 

 bour closely until, say, ten turns have been taken. The thread is 

 then unwound and measured. The tenth part of the length is the 

 circumference of the bulb. By measuring the axial space occupied 

 by the ten turns, the correction for " pitch " can be ascertained, but 

 if anything but very coarse thread is used it is negligible. The 

 active superficial area of the bulb is given by adding to the hemi- 

 spherical end surface the product of the mean circumference into 

 the total length of the cylindrical part of the bulb. In like manner, 

 the volume of the bulb is obtained by adding to the hemispherical 

 volume of the end the product of the mean circular area into the 

 length of the cylindrical part of the bulb. The volume, multiplied by 

 0'457, gives the thermally equivalent volume or weight of water. 



Air thermometers of the simple kind described above, are very 

 easily made so as to give calorimetricai results. It is only necessary 

 to weigh and measure the piece of glass tube before blowing the 

 bulb. The shortening of the straight part of the tube after blowing 

 gives the length of it which has been expanded into a ball, and from 

 the known length and weight of the original piece of tube, the weight 

 of the bulb is found. By carefully gauging the volume of the ball its 

 volume can be obtained, and from that the thickness of the glass. 

 When the specific heat of the glass is known, the water value of the 

 bulb is given ; if the air contained is taken into account, the value is 

 increased by from 1 to 2 per cent. The surface of the ball divided by 



TableVI. Particulars of Calorimetric Air Thermometers made of 



Lead Glass. 



