lviii Introduction to Makerstoun Observations, 1844. 



ter, with a black bulb, by R. Adie, of Liverpool, was placed, in the end of May 

 1844, within the enclosed space occupied by the Observatory rain-gauge, exposed 

 to the sun, for the purpose of obtaining the maximum amount of solar radiation ; 

 another self-registering alcohol thermometer, with black bulb by the same maker, 

 was placed near the other in September 1844, with its bulb in the focus of a para- 

 bolic metallic reflector, for the purpose of obtaining the minimum of terrestrial 

 radiation. The observations of the self-registering thermometers are given pages 

 310-312; they have all been corrected for the scale errors of the thermometers. 



93. Another thermometer was employed for the determination of the tempera- 

 ture of the water in two pump-wells, which are within about 200 yards of each 

 other; the pumps are nearly on the same surface-level, the depth of the cottage- 

 well being 10 feet, — that of the garden-well 21 feet. On one occasion, it was found 

 that there was one foot of water in the cottage-well, and two feet of water in the 

 garden-well. In obtaining the temperature, the water was pumped till the reading 

 of the thermometer remained constant. All the observations have been corrected 

 for the scale error of the thermometer used. 



Actinometer. 



94. The actinometer was made by Stevenson of Edinburgh; it consists of a 

 hollow cylinder of glass filled with ammonio-sulphate of copper. One extremity of 

 the cylinder is joined to a thermometer tube, terminating in a hollow bulb ; the 

 other extremity is cemented to a metallic cap, through which a screw, working in a 

 collar of leather, passes into the cylinder ; a scale of 100 divisions is attached to 

 the thermometer tube ; the whole is inclosed in a larger glass cylinder of two 

 inches diameter. A portion of this cylinder, opposite the liquid, is inclosed by a 

 segment of a metallic cylinder, blackened within. In making an observation, the 

 inner cylinder was exposed to the sun's rays at a perpendicular incidence for 60 

 seconds, the scale readings of the fluid in the tube being observed at the beginning 

 and end of the minute. A screen was then interposed for one minute, or for one 

 minute and a half; if for one minute only, the last observation in the sun was also 

 noted as the first in the shade ; if for one minute and a half, the first reading in the 

 shade was not made till the instrument was shaded half a minute. At the end 

 of 60 seconds the scale reading was again observed, and the screen was removed, 

 that reading being also noted as the first in the sun. When the liquid mounted 

 near the top of the thermometer tube, the screw was withdrawn nearly half a revo- 

 lution, when the liquid fell to near the bottom of the tube. The times were noted 

 from a box-chronometer by Dent, No. 1665. In February 1844, the liquid in the 

 cylinder was frozen while the instrument was in the Observatory, and the cylinder 

 was broken. The following were the dimensions of this actinometer : — liquid cylin- 

 der, 5^ inches long; mean external diameter, 1*013 inch; mean internal diameter, 



