220 Description of a Differential Thermometer. 



perfection cannot be obtained by the most skilful artist; there 

 always remains behind, notwithstanding all care to prevent 

 it, a small residuum of air, which is sufficient to make a difier- 

 ence in the height of the two columns. To obviate this incon- 

 venience, before the scale is adapted, the liquid is all to be 

 brought into one ball, and the instrument is then reversed and 

 left for a considerable time in that position, that both balls may 

 acquire an equal temperature, and the small portion of air may 

 be equally diffused through them. It is then to be restored to 

 its proper position, and the point at which the liquid finally 

 settles, is to be marked as the commencement of the scale. 

 The same operation is to be repeated whenever the instrument 

 has been deranged by transportation or other causes. 



If it were possible to employ constantly ether or spirit of wine 

 of exactly the same degree of strength, it is plain from the laws 

 investigated by Mr. Dalton, that the scale would be constantly 

 uniform ; but as this is not easily obtained it is arbitrary. I 

 have hitherto used the division of the millimetre of France *. 



* The best mode of constructing the above instrument, is to bend the 

 tube previous to the introduction of the ether, a considerable portion of 

 which shouUl be boiled out of the tube, in order to ensure the expulsion of 

 atmospheric air ; it is also convenient to tinge the ether of a red colour, 

 by the addition of a drop of tincture of cochineal. 



I have constructed upon the same principle a photometer, and an ethrio- 

 scope, both of which, though liable to some objections, are most curiously 

 sensible to the impression of light, and to the frigorific emanations of the 

 heavens. I have also employed a modification of the same instrument as 

 a photometric thermometer, which I have found useful in comparative 

 experiments upon the light of different flames. For this purpose, the in- 

 strument is constructed as shewn in the above wood-cut, by Dr. Howard ; 

 the upper ball is then covered by a thin coating of Indian ink, and the 

 other with gold leaf, applied by a dilute spirit-varnish ; it is then cov^ered 

 by a thin glass shade. Upon bringing a candle near the black, or sentient? 

 ball, that is withm the distance of 14 inches, or one foot, it produces an 

 instantaneous depression of the column of liquid. Placing this instrument 

 at the distance of 16 inches from the flame of a wax candle, it fell 1° in V. 

 A gas flame which I had previously ascertained, by a comparison of sha- 

 dows, to give the light of eight wax candles, caused a depression of lO" in 

 1', when placed at the same distance from the instrument. 



W. T. B. 



