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Royal Society : — 



all crevices closed, by pasting paper over them, the floor carpeted, 

 the door double, and the inner door surrounded with list. The 

 outer wall, having a north aspect, was so sheltered by surrounding 

 buildings that the direct rays of the sun never fell upon the window. 

 Discs of delicate tissue-paper were suspended in several parts of the 

 room, to indicate currents of air, if any existed, and observations 

 were taken only when these were perfectly quiescent. 



Mason's hygrometer was first employed in these experiments, to 

 test the presence of a current of air in the tube ; on the principle 

 that as evaporation produces cold, and as evaporation is increased by 

 a current of air, the wet-bulb thermometer would show a greater de- 

 pression if any current existed, than if the air were perfectly quiescent 

 within the tube. The tube (fig. 1) was placed in the middle of the 

 room, and isolated from the floor by a cylinder of thick glass laid 

 under it. 



It was found that in ninety-one observations of the hygrometer, 

 suspended in the free air of the room, the mean depression of the 

 wet-bulb thermometer was 3°'9 Fahr., while in ninety corresponding 

 observations, with the hygrometer at the lower aperture, O, of the 

 tube, the mean depression was increased to 4°"9 Fahr., clearly indi- 

 cating the existence of a current of air within the tube. 



Partial closure of the upper orifice of the tube, by placing a piece 

 of fine muslin upon it, produced a sensible influence on the hygro- 

 meter. In seventeen observations with the tube thus partially ob- 

 structed, the mean depression was 2°' 5 Fahr. ; but in an equal num- 

 ber of comparative observations, with the tube perfectly free, the 

 mean depression was increased to 3°"12 Fahr. ; showing a consider- 

 able-diminution of the force of the current within the tube, as a 

 result of the partial obstruction of its upper aperture. 



Fig. 1. 



Fig. 2. 



Fig. 3. 



I 



M 



/ 



Fie:. 4. 



L. A ledge or step to place the hygrometer upon. 

 O. Orifice against which the bulhs were placed. 



