on the terrestrial Rays that occasion Heat. 449 



be brought to an equality, and are always liable to change, 

 especially in very quick exposures. 



Transmission of Solar Heat through colourless Substances. 

 Q/tfb Experiment. 



I laid a piece of clear transparent glass, with a bluish-white 

 cast, upon one of the holes of the transmitting machine : the 

 faces of this glass are parallel, and highly polished. Then, put- 

 ting the cover over both holes, I placed the machine in the 

 situation where the experiment was to be made, and let it remain 

 there a sufficient time, that the thermometers might assume a 

 settled temperature. For this purpose, an assistant thermometer, 

 which should always remain in the nearest convenient place to 

 the apparatus, will be of use, to point out the time when the 

 experiment may be begun ; for this ought not to be done, till 

 the thermometers to be used agree with the standard. In order 

 not to lose time after an experiment, the apparatus may be 

 taken into a cool room, or current of air, till the thermometers 

 it contains are rather lower than the standard ; after which, 

 being brought to the required situation, they will soon be fit 

 for action. 



All these precautions having been taken, I began the experi- 

 ment by first writing down the degrees of the thermometers ; 

 then, opening the cover at the time that a clock or watch shew- 

 ing seconds came to a full minute, I continued to write down the 

 state of the thermometers for not less than five minutes. The 

 result was as follows. 



