to the Antarctic Regions. 207 



The surface temperature of the water of the sea or of rivers 

 may be conveniently obtained by taking up a bucket-full of water 

 and stirring round the thermometer in it. 



The temperature of the water of deep wells may be ascertained 

 in the same way, and should be taken monthly, if near the residence 

 of the observer. The temperature of rain should also be attended 

 to at times ; it may be determined by receiving the rain in a linen 

 funnel, totally enclosed in a tin case to prevent cooling by evapora- 

 tion from the linen. 



The temperature of the soil at different depths is a point of con- 

 siderable importance. For this purpose excavations should be 

 made in a dry sheltered situation, 3, 6, and 9 feet deep, and lined 

 with brick or earthenware tubes. In the bottom of these excavations 

 earthenware quart bottles may be carefully placed, filled with water, 

 spirit, or brine, and corked. They must be carefully covered with 

 tow or cotton, and drawn up on the 21st of every month (being the 

 day of horary observation), and their temperatures taken by an ac- 

 curate thermometer, and registered apart. 



As a general caution it may be mentioned, that the standard ther- 

 mometer should never be exposed to risk by application to such 

 purposes, but thermometers which have been compared and cor- 

 rected by comparison with it. 



3. ACTINOMETERS. 



Amongst the observations of highest importance must be ranked 

 those of the force of solar and terrestrial radiation. The most per- 

 fect means of observing the former is afforded by the actinometer. 



This instrument consists of a large hollow cylinder of glass, sol- 

 dered 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 for- 

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

 and the point closed with melted wax, it becomes, in any given po- 

 sition 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, de- 

 fended 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. 



