i8 
METEOROLOGY OF 1 897. 
or more below the air temperature. Hence it is not uncom- 
mon to find frost occurring, even when the lowest tempera- 
tures of the night, indicated by a thermometer at the 
ordinary height above the ground, has not descended 
below 40*. 
^33. The precipitation is measured by the use of a 
standard rain gage exposing 50 square inches of surface. 
If the precipitation occurs as snow, the measurements are 
more unreliable, as the snow drifts and the rain gage 
does not give reliable results. Iletter results are obtained 
by distributing boards Hat on the ground, and after the 
storm choosing one which seems not to be affected by air 
currents, obtaining from it, by inserting the rain gauge, 
the amount falling on a surface the same as the gage and 
melting the snow. There is a great difference in the 
amount of moisture in a given amount of snow according 
to its condition, whether light and powdery, or damp and 
heavy. On the average, the amount of moisture is i-io the 
depth of the snow. 
S34. The wind is measured 
by the Robinson Anemometer, 
d'he standard instrument is 
placed on top of the tower of 
the college building, and is con- 
nected by wires with the office 
of the Meteorological section, 
so that every mile of wind that 
passes is registered on the 
recording cylinder. The instru- 
ment is about 60 feet above the 
ground. Trees surround the 
campus with an elevation of 
about 40 feet. The tower ex- 
tends about 20 feet above the 
roof of the adjacent building. 
§35. Solar radiation is meas- 
ured by means of the actinome- 
ter. This instrument is of the Arago-Davy form, consisting 
of two thermometers with spherical bulbs one centimeter 
in diameter, surrounded by a glass enclosure from which 
the air has been exhausted. These are placed with their 
bulbs uppermost exposed to the sky. One of the two has 
its bulb covered with lamp black and absorbs the radiations, 
and indicates a highe rtemperature. The readings of these 
instruments are given in Centigrade degrees, which may be 
