MEASURING THE DIRECT HEAT OF THE SUN, 19 



of the scientific principle of the instrument, to alter it in 

 such a manner as to remedy this defect. Without altering 

 the size of the bulb, I should propose for a permanent 

 instrument a stem, say, i8 inches long, with a bore of such 

 diameter that the stem shall embrace a range of tempera- 

 ture between 20° Fahr. and 92° Fahr. Thus somewhat less 

 than five degrees will go to the inch. The stem might be 

 protected from the risk of accident by an appropriate shield. 

 Let such a thermometer be heated for two minutes and 

 the size of the lens be somewhat increased. In this case 

 a rise of something like 5° Fahr. will be obtained ; and this 

 heating effect might be very easily estimated to one hun- 

 dreth of the whole, while the same thermometer will serve 

 for all the temperatures likely to occur in these islands 

 during the course of the year. 



I ought to add that a pasteboard cover, gilded on the 

 outside, is made to surround the chamber, and also that 

 between the lens and the chamber there is a pasteboard 

 shield with a hole in it to permit the full rays from the lens 

 to pass — the object of this shield being to prevent rays from 

 the sun or sky from reaching the instrument. 



In such an instrument r, or the change taking place in 

 the thermometer before exposure to the sun, will in all pro- 

 bability completely disappear, while r' will be extremely 



r + r' 

 small. At any rate we may be quite certain that R -\ — - 



<o 



will accurately represent the heating effect of the sun. 



We may probably suppose that in the same instrument 

 the lens (which must always be kept clean) will always stop 

 the same or neai'ly the same proportion of the solar rays. 

 But the lens of one instrument may not stop the same pro- 

 portion as that of another instrument. This^ however, is 

 no objection if it be borne in mind that the instrument is 



a differential one. In practice there would be some stand- 



r *> 



