290 PHILOSOPHICAL TRANSACTIONS. [aNNO l/S/. 



I formed a cylinder of thin pasteboard, 6 inches diameter and l6 inches long. 

 The inside I lined with rabbit skin, laying the fur smooth; a thin ring of paste- 

 board was placed in the middle. One end was closed with a bottom of the same 

 pasteboard; the other was open. This cylinder was placed in the centre of an- 

 other wider cylinder, also of pasteboard, which had likewise a bottom of paste- 

 board. It was so placed, as that the outer cylinder was distant from the inner ]i 

 inch ; at the bottom and sides the space between was filled with eider down, suf- 

 fered to rise to as great a bulk as it would from its own elasticity. The two cy- 

 linders were even at top, and the space between them shut by a cover of paste- 

 board. In the side of the machine, a little below the middle of the inner cylinder, 

 a pasteboard tube was made to pass through the outer and open into the inner, 

 half an inch wide, for the insertion of a thermometer. A similar tube was 

 placed a little from the middle, towards the other end of the smaller cylinder. 

 A circular plate of pasteboard, 6 inches diameter, and about -f thick, weighing 

 1 oz. 102 gr, was pushed down the inner cylinder, till it was stopped by the 

 ring. A circle of flint-glass, ground flat and parallel on both sides, was fixed 

 over the mouth of the inner cylinder so as not to obstruct any part of it. A 

 similar apparatus, as exactly as possible, was formed, excepting that the circular 

 plate in the middle of the inner cylinder was iron, of the same dimensions with 

 the pasteboard one, and weighing 12 oz. 62 gr. These apparatuses were set in 

 a warm exposure^ for several months, to dry. The circular plates, destined to 

 receive the direct rays of the sun, were placed as nearly perpendicular to the 

 inner cylinder as possible. They were both covered with a black paint, sufficient 

 to prevent the rays of the sun from penetrating either to the iron or pasteboard. 



On July 28, 1786, the sun shining on a room facing about s.w. the air not 

 cloudy, but not very bright; the air in the room 71°; at a quarter after 12, ther- 

 mometers being passed through the tubes below 

 the plates of iron and pasteboard, after standing -P™^^^*« 9f t^'^ rising of the thermo- 



fnctcrs 



SL quarter of an hour, showed the heat 67° in both Under paste-board " . Under iron 



apparatuses. Both were now exposed to the sun, diaphragm. diaphragm. 



so that the rays fell per()endicular on the paint 75 .'.*.'.**' 70 



covering the plates, in equal quantity on each as so 75- ^, 



nearly as possible. If there was any diff^erence, qq ^^ 



rather more were thrown on the pasteboard dia- 95 94 



phragm. In 5 minutes the thermometer below jq^ '■"• JJJ^* 



the pasteboard diaphragm showed 72°; the ther- 1 10 115 



mometer under the iron had hardly risen half a ^o ^^l!'. ??."''".'''^' 121 

 degree. 



♦ At this time thermometers were put through tubes into the chambers of the apparatus,, betweea 

 the glasses and diaphragms. The apparatus with the iron diaphragm raised this thermometer to 12 1°^ 

 that with the pasteboard to 120°. — Orig. 



