80 METHODS AND RESULTS OF TORONTO OBSERVATIONS. 



with its bulb in the middle of a cubical chamber of brass, the cham- 

 ber being so massive that its temperature will remain sensibly con- 

 stant for some time. This massive chamber is lined outside with 

 felt, and this again surrounded by polished brass plates. 



A lense is attached by means of a rod to the cubical chamber, 

 and in taking the observation the sun's rays are focussed by this 

 lense, and projected directly on the bulb of the thermometer. In 

 taking an observation the aperture in the cubical chamber through 

 which the sun's rays are projected is first kept closed, and say that 

 the time of exposure has been selected as two minutes, the thermom- 

 eter is first read exactly two minutes before exposure begins. It is 

 again read at the expiration of the two minutes, and the exposure at 

 once made by drawing out the slide which covers the aperture. 

 Exactly two minutes after it is again read and the exposure discon- 

 tinued. Again, at the expiration of another two minutes, the ther- 

 mometer is read. The comparative heating power of the sun is 

 deducted from the formula 



R + r -^ 

 where R is the amount of heat gained during exposure, and r and 

 r' the difference between the ist and 2nd, and 3rd and 4th readings 

 respectively, which indicate the heat which the instrument is losing 

 by radiation from itself. 



The great difficulty in the way of making these direct observa- 

 tions is the variability of the condition of the earth's atmosphere, 

 which forms a medium of ever varying opacity, through which we are 

 forced to view the sun. This source of uncertainty might be reduced 

 to a minimum, if these observations were made at some great alti- 

 tude, as on the top of a high mountain where the lower or grosser 

 strata of the atmosphere would be left behind. 



In Toronto we have determined to pursue the observations 

 regularly, and endeavor to allow for the effect of the condition of the 

 atmosphere at the time of the observation by careful simultaneous 

 observations of cloud and the hygrometric condition of the atmos- 

 phere, etc., etc. 



I am strongly of opinion that, when true observations have been 

 taken for a sufficient length of time to determine the effect of the 

 sun's altitude, light cirrus cloud, etc., we shall be able to detect the 

 existence of any variability in the direct heating power of the sun. 



