340 On Measuring the Thermal Intensity of the Sun. [Feb. 2, 



one foot square and of any convenient height. A slot is cut in the table 

 of suitable size to permit of the passage of the (J -tube and stopcock, 

 the aperture being subsequently closed by a slip of wood level with the 

 top of the table. This table is painted on both sides with " flatted " 

 zinc-white, and its legs are firmly fixed in the ground, so that it cannot 

 be disturbed by the wind. The branch tube, d, should also be 

 anchored to the ground by elastic bands, so as to prevent any move- 

 ment of the instrument from the same cause. It is, moreover, desir- 

 able to surround the table with wire netting. 



By means of a funnel and flexible tube, mercury is now introduced 

 cautiously into the (J -tube through the stopcock, d, until it reaches 

 the zero of the scale, care being taken that both bulbs are at the same 

 temperature (which should be noted) during the operation. The 

 pressure upon the air in the bulbs must then be determined, and 

 employed, together with the temperature, in calculating the length of 

 each degree upon the scale. Once charged, the instrument must 

 thenceforward be kept in its normal position, or nearly so, otherwise 

 the mercury will get into the bulbs, whence it can only be dislodged 

 with difficulty. In transporting the instrument from place to place it 

 is therefore advisable to withdraw the mercury. 



The following comparative determinations of solar intensity were 

 made with this instrument, and with the ordinary blackened bulb in 

 vacuo read on white paper synchronously with a shade thermometer. 







Solar intensity. 



Blackened bulb 



Shade 







in vacuo. 



temperature. 



Result of 



Self-registration 







duplex 



of differential 







observation. 



instrument. 











48°3 C. 



29°-0 C. 



19°3 C. 



20° C. 



49 -0 



29 -2 



19-8 



20-5 



49 7 



29 '2 



20-5 



20 -5 



46-3 



28-7 



17-6 



16 9 



45-9 



28 -4 



17 5 



17-5 



48-4 



29 -3 



19-1 



20 -4 



50 -0 



29 -7 



20-3 



20 -3 



47 -0 



29-5 



17 5 



17 7 



43-2 



27 -4 



15 -8 



16 1 



