94 Mr R. Adie's Experiments with Thermo- and 



A familiar illustration of the force derived from small uprising currents 

 of heated air, is shewn by two equal weights in a delicate balance, where 

 the beam stands at zero when the weights are of equal temperature. Heat 

 one of those weights 20° degrees above the other, and replace it in its 

 scale, then it appears 100th or 200ths of a grain lighter than at the first 

 weighing, occasioned by the streams of air now rising from its surface. 

 On the surfaces of both pieces of metal C C, this minute force maybe de- 

 veloped by the power of the solar rays, and by slightly inclining the slips 

 of metal out of the vertical plane, so as to get the force of the streams 

 of air, rising from each piece, to act in the same direction, the rotation 

 in question is produced. The experiment succeeded best under a large 

 shade, where the contained air is not soon heated by the sun's rays ; 

 with thin slips of metal the motion commences in 25" or 30" after un- 

 screening ; but with stout pieces of metal, which I have had as heavy as 

 35 grains each, it is some time after exposure to the sun until the motion 

 begins, and again the rotation endures, for a corresponding period, after 

 the needle is screened. 4 or 6 arms, each carrying a slip of metal like 

 C C, may be used with advantage, to give steadiness to the rotations. 

 With a G-arm arrangement I obtained regular rotations by the rays from 

 a clear coal-fire, but not under a glass shade ; for this experiment, side- 

 screens must be used to prevent the interference of currents of air. The 

 rotations produced by the fine bright sun of mid-summer form striking 

 and beautiful experiments. I took great interest in them in June last, 

 from the expectation that simple arrangements, like fig. 5, might serve 

 for actinoraeter measurements ; but at present I am afraid that the force 

 which turns them is too feeble to admit of being formed into a scale for 

 this purpose. Where the pieces of metal on the extremities of the arms 

 A A are magnetized, however much the forms or number of pieces of 

 those slips of metal may be varied, there will always remain in the whole 

 arrangement some degree of magnetic polarity. Several variations in 

 the action of Mr Watt's needles can be produced by changing the in- 

 tensity of this polarity ; when the magnetic force is very feeble, and the 

 metal surfaces well placed for the development of the uprising streams 

 of air, the rotation takes place ; but when either the magnets are so 

 placed to give rise to feeble uprising currents of air, or the magnetic 

 polarity is increased, then the needle is deflected out of its statical posi- 

 tion, like the galvanometer needles {22), and there is no rotation, the 

 change of the sun's place in the sky varying the amount of that deflec- 

 tion. 



Cold streams of air playing on the side of the shade act on the same 

 principle as the solar radiation, for the radiation on the suddenly cooled 

 side of the shade then gives rise to streamlets of air descending from the 

 surfaces of the metallic slips. On these views, a needle which I am in- 

 formed points to the direction of the wind, may be explained ; and in 

 all of the arrangements, I think it will be found that it is the streamlets 

 of air rising off, or falling fromy the metallic surfaces t which produce the rota^ 

 tions or changes in the position of the needles. 



