On Solar Radiation. 197 



" The dimensions of the instruments which are used in 

 these reductions are, 



" 1. The external diameter of the cylinder containing the 

 coloured liquid, i. e. its mean diameter, if on measurement 

 with fine callipers its two ends be found to differ. 



" 2. The length of that portion of it which receives the 

 sunbeam. The product of these two data gives the area of 

 the section of the sunbeam effective in raising the tempera- 

 ture, and which, though not all equally effective, by reason 

 of the cylindrical form of the glass, is yet effective in the 

 same ratio in all of them by reason of their general similarity 

 of figure. 



" 3. The contents (in water grains) of 100 parts in length 

 of the capillary tube used for the scale. This may best be 

 determined by gauging it with mercury before it is soldered 

 to the cylinder, and ought always to be so determined by 

 the maker; but when fitted this is impracticable, and the 

 measurement of the element in question must be performed 

 as follows : — 



" The instrument being placed horizontally, and allowed to 

 attain the precise temperature of the apartment, let the 

 liquid be brought to zero by the motion of the screw ; after 

 which let the screw be turned precisely one revolution, 

 or half revolution (as the scale may require) in and note 

 the rise of the liquid in parts of the scale. This must be 

 done several times, alternately screwing in and out. The 

 screw must then be taken out; its thread counted, and the 

 weight of water displaced from a narrow vessel exactly 

 full, by the immersion of the whole length occupied by 

 the thread exactly ascertained by a nice balance; after 

 which a very simple calculation will give the value of the 

 parts of the scale in water grains required; this process 

 was followed in the case of the instruments above men- 

 tioned, and if carefully conducted, is susceptible of great 

 precision. 



