METEOROLOGY OF 1 897. 
3 
Radiation Thermometers are shown in Figure 7. The 
solar instrument is an ordinary maximum thermometer 
with blackened bulb in a glass enclosure. The thermom- 
eter is measurably protected from the temperature of the 
air but without a corresponding instrument with a bright 
bulb, is of doubtful scientific value. We have found that 
it is difficult to obtain instruments of this pattern of suffi- 
cient range, the temperatures under our intense sunshine 
causing the breakage of the bulb. The principal reliance is 
now placed upon the Arago-Davy form of conjugate ther- 
mometers, or actinometer, consisting of two thermometers, 
each with a bulb one centimeter in diameter, placed in 
glass enclosures from which the air has been exhausted. 
The difference between the readings of the two instruments 
is not proportional to the radiation, but gives a means of 
determining the intensity after the constant of the instru- 
ment has been determined, as has been shown by Professor 
Ferrel. The convenience of these instruments is that they 
may be read as easily as an ordinary thermometer, and do 
not require the skill and special attention required by most 
forms of actinometers. 
§ 20. The terrestrial thermometer is an or- 
dinary minimum thermometer with the stem 
protected from radiation. It is placed near the 
ground, with the bulb exposed to the sky. The 
difference between its temperature and the 
readings in the instrument shelter, show to a 
great extent^ the effect of the cooling due to 
radiation. It is intended that it shall be placed 
over grass, but the changes in the location of 
the instrument plat have not always permitted 
this. 
HOOK GAGE. 
§21. The evaporation has been measured by 
the hook-gage shown in Figure 8. The 
hook is submerged, and the small elevation of 
the water surface produced as the hook ap- 
proaches the surface permits the height of the 
surface to be determined within less than i-iooo 
of a foot. Water is maintained in a tank three 
feet cube of galvanized iron, sunk flush with the 
ground. Observations are made twice daily 
during the summer season, April to September, 
6 
Fig. 8. 
