8 
METEOROLOGY OF i8q7 
or decreases. The pen records on a paper wound ori a 
cylinder. The cylinder is caused to revolve once a week by 
clock work inside the cylinder, working through pinions at 
the base. The record of temperature by the instrument 
has been satisfactory, but the clock work in this and other 
similar instruments has not been a good time keeper. 
THE REGISTERING PSYCH ROMETER. 
Fis?. 4. 
§12. I'his instrument, shown in Figure 4, is similar to 
the thermograph except that there are two thermometer 
bulbs instead of one, and two pens which record on the 
sheet. One of the bulbs gives the temperature of the air, 
while the other, covered with muslin moistened in water, 
records the temperature of evaporation. The two bulbs 
correspond to the wet and dry bulb t hermometers^ of the 
ordinary psych rometer. To prevent interference with each 
other, the pen of the wet bulb is set about 10^ lower than 
the other. The record of this instrument has been main- 
tainded since 1891. The record of the wet bulb thermom- 
eter is not as satisfactory as that of the dry bulb,^ but 
affords an interesting indication of the changes in humidity 
between the regular periods of observation. Many times 
the changes which take i)lace are sudden ; sometimes cur- 
