188 On Solar Radiation. 



But the following extract from the report of the Com- 

 mittee of the Royal Society may be useful : — 



" This instrument consists of a large hollow cylinder of 

 glass, soldered at one end to a thermometer-tube, terminated 

 at the upper end by a ball drawn out to a point, and broken 

 off, so as to leave the end open. The other end of the 

 cylinder is closed by a silver or silver-plated cap, cemented 

 on it, and furnished with a screw, also of silver, passing 

 through a collar of waxed leather, which is pressed into 

 forcible contact with its thread by a tightening-screw of 

 large diameter enclosing it, and working into the silver cap, 

 and driven home by the aid of a strong steel key or wrench, 

 which accompanies the instrument. 



" The cylinder is filled with a deep blue liquid, (ammonio- 

 sulphate of copper,) which ought to have been prepared 

 some months beforehand, as it deposits a sediment when 

 fresh, however clear or carefully filtered. This sediment, 

 if deposited in the interior of the instrument, may be wash- 

 ed out with weak muriatic acid, which should itself be 

 removed by water before refilling the instrument, and the 

 ball at the top being purposely left full of air, and the 

 point closed with melted wax, it becomes, in any given posi- 

 tion of the screw, a thermometer of great delicacy, capable 

 of being read off on a divided scale attached. The cylinder 

 is enclosed in a chamber blackened on three sides, and on 

 the fourth, or face, defended from currents of air by a thick 

 glass, removeable at pleasure. 



" The action of the screw is to diminish or increase at 

 pleasure the capacity of the hollow of the cylinder, and 

 thus to drive, if necessary, a portion of the liquid up into 

 the ball, which acts as a reservoir ; or, if necessary, to draw 

 back from the reservoir such a quantity as shall just fill it, 

 leaving no bubble of air in the cylinder. 



" To use the instrument, examine first whether there be 

 any air in the cylinder, which is easily seen by holding it 



