Letter to Pictet. 737 



a long time at a distance of half an inch from the bulb 

 of the thermometer, to my great surprise the instru- 

 ment gave no indication of being sensible of the pres- 

 ence of the ice ; while on presenting my hand to the 

 thermometer, at the same distance, the calorific rays 

 which escaped set the index in motion almost imme- 

 diately. The bulb of this thermometer is a spiral tube 

 of very thin glass, filled with alcohol, and placed in a 

 vertical position. Its diameter is about half an inch, 

 and the tube makes five revolutions about the centre 

 of the spiral. The diameter of the disk formed by this 

 spiral is about five inches. The piece of ice which I pre- 

 sented to it was circular : it was about six inches in 

 diameter and four inches thick. As the front of the disk 

 of the thermometer is vertical, and the flat surface of 

 the piece of ice was placed parallel to the disk, and 

 directly in front of it, the descending current of air, 

 which was cooled by contact with the ice, did not affect 

 the thermometer at all ; when, however, the ice was 

 held immediately above the instrument, the index 

 moved backwards immediately, as might naturally have 

 been expected. I was surprised that it did not affect 

 it at all when placed side of it ; and I should have been 

 surprised even if I had never heard of your experiment 

 so strongly was I impressed with the idea of the effect 

 which proximity ought to produce. If you have made 

 any new researches on this curious subject, I shall be 

 obliged to you if you will kindly inform me of the 

 results, or will indicate to me other experiments which 

 have been made on the same subject 



As to the success of my efforts to perfect chimney 

 fire-places, you will be able to get an idea of the econ- 

 omy of fuel effected when I inform you that, under the 



VOL iv. 47 



