132 



ROYAL SOCIETY OF CANADA 



Before constructing these special thermometers some experiments 

 T\'ere tried with the differential thermometers which are in nse at the 

 uMcGill College Observatory, to obtain the temperature at the top of 

 Mount Eoyal. These have already been described by Professor C. 

 II. McLeod and myself.^ The moimtain thermometer was brought 

 down to the observatory grounds and connected along-side of the low 

 level thermometer cage. Equality, or zero traces were carefully made 



/(5//7 /^e'jby 'û6. 



Fig. 3. Radiation Trace, Minimum Air Temperature O'' Fahr. Showing 

 Effect of Clouds. 



with the tliermometers serened, and as nearly as possible at equal 

 temperatures. One of the thermometers was then removed from the 

 screen, and placed just over the cage, and exposed to the sky. The 

 recorder at once showed that the thermometer was cooled, and the 

 trace was continued on several occasions all night. It was a matter 

 of considerable interest to observe the magnitude of the cooling, and 

 to note the effect of a cloudy, or j^artially cloudy sky. 







•^ .5^7 ^/ 



Fig. 4. Radiation Trace, Minimum Air Temperature 3'^ Fahr. Showing 



Cloud Effects. 



Fig. 3 shows one of these traces on a partly cloudy night. The 

 maximum cooling was 6° F. at 12.45 a.m. 



Fig. 4 is another trace showing similar effects. The thermometer 

 Avas taken out of the cage and exposed at about 5-15, where the depar- 



' Trans. Roy. Soc. Can., 10, 71 (1904). 



