Chemical and Physical Notes 99 



heat at the rate of n gr. C. per second, and / = - is this rate 



<^> 



for a bundle of rays of I square centimetre section. It will be 

 seen that all the values of/ given by thermometer B are lower 

 than those given by A. In line q we have the ratio of /A :/B, 

 and it will be seen that the values of q agree very well with 

 each other. The mean value is 0-873, the extremes being 

 O'86o and 0-896; that is to say, thermometer B indicates 

 13 per cent, less heat than thermometer A for I square 

 centimetre of sun's rays. Yet A has a silvered glass bulb 

 and B has a plain uncoated glass bulb. It is possible that 

 to this difference is due the difference in the results obtained. 

 The silvered bulb dissipates by reflection some of the heat 

 which strikes its metallic surface, but whatever is not reflected 

 by this surface passes into the bulb of the thermometer. In 

 the case of the uncoated glass bulb there are two surfaces of 

 reflection, namely that separating glass from air and that 

 separating glass from mercury. The heat which has passed 

 through the outer glass surface has still to pass the inner 

 surface, where some of it is rejected by reflection. 



Taking the maximum value of /, namely 0-00889 gr. C. 

 of heat supplied per square centimetre per second, and 

 multiplying it by 60, we have 0-533 g r - C. per square centi- 

 metre per minute. This is the heat actually taken up by the 

 silvered bulb of a thermometer from I square centimetre of 

 the rays of the winter sun, which have passed obliquely 

 through a glass window. The sky was quite clear and 

 cloudless, but the sun's zenith distance was 57. In order 

 to allow for this, we use the formula given by Sir John 

 Herschel 1 , from which we obtain the value of the solar constant 



A = Tj^y = ri 1 14 gr. C. per square centimetre per minute, 



\3/ 



where is the transmission coefficient of the air for heat and 

 1-84 = sec 57. 



Two-thirds of this, or 0-7409, would then be the heating 

 power of the rays of the vertical sun at the surface of the earth 



1 Meteorology, by Sir John Herschel, Bart. Edinburgh, 1861, p. 10. 



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