and the Mode of its Communication. 27 



thicknesses of confined air, the relative warmth of 

 which I was very desirous of ascertaining. 



The means I employed for covering up the ends of 

 the instrument were as follows. Having provided two 

 thin cylindrical wooden boxes (like common pill-boxes, 

 but much larger), something less in diameter than the 

 body of the instrument, and i\ inches deep, I dried 

 them as much as possible ; and, after having varnished 

 them within and without with spirit varnish, I covered 

 them within and without with fine wove writing-paper, 

 and then gave the paper three coats of the same varnish. 

 I then perforated the bottoms of these boxes with 

 round holes, just large enough to admit the neck of the 

 instrument, and the cylindrical projection at its bot- 

 tom ; and then inverted them over the two ends of the 

 instrument, filling the boxes at the same time with 

 elder-down. 



These boxes were fixed and confined in their places 

 by means easy to be imagined ; and, in order to con- 

 fine the heat still more effectually, each of the boxes 

 was covered on the outside with a cap of fur, as often 

 as the instrument was used ; as was also that part of 

 the neck of the instrument which projected above the 

 box. 



Two of the instruments, which I shall distinguish by 

 the numbers i and 2, were covered up at their ends in 

 this manner; the othej- two instruments, No. 3 and 

 No. 4, were left in the state represented by the Figure 

 i ; that is to say, the ends of their cylindrical bodies 

 were not covered with permanent coverings. 



In each experiment, two similar instruments (No. i 

 and No. 2, for instance, or No. 3 and No. 4) were 

 used, the one naked, and the other covered; and, as the 



