134 Professor Forbes on the Refraction 



simple observation, and to obtain results which otherwise I must 

 have been unable confidently to announce. 



7- For the precautions to be employed in the use of the 

 thermo-multiplier, I must refer to the first of M. Melloni's 

 very original papers in the Annales de Chimie (for May 1833), 

 but I may state, once for all, that when habituated to the use 

 of it, I found it more simple, manageable, and comparable, than 

 I could previously have imagined. Notwithstanding its delicacy 

 and the promptitude of its action, a few precautions suffice to 

 prevent any derangement from without. The only inconveni- 

 ence which I experienced, was in the determination of the zero 

 of the scale, which appears liable to some fluctuations, which may 

 be considered as accidental.* It rarely happened, however, that 

 these affected the results of my experiments, because, as I have 

 said, these were always confined to small variations of tempera- 

 ture (indicated by a deviation generally under 15° on the scale) 

 when such fluctuations did not appear ; and the results produced 

 by the same cause under the same circumstances were admirably 

 constant, as well as the position of the zero point. 



8. There is one circumstance which gives a degree of delicacy 

 to the indications of the thermo-multiplier, when we wish to as- 

 certain very minute differences of effect, which no other thermo- 

 metric instrument possesses. When we wish to ascertain the 

 existence, not the measure, of some cause of heat or cold, if we 

 watch the needle of the multiplier at the instant at which the 

 change of circumstances intended to produce the effect takes 

 place, we shall perceive, in the instantaneous effect on the needle, 

 an evidence of a far more decisive character than the merely statical 



* This fluctuation appears to have been wholly, or almost wholly, owing to the 

 imperfect communications established between the pile and the galvanometer, which, 

 when I received the instrument from Paris, were in a very rude state, and the want 

 of continuity gave rise to some curious phenomena. Since I had the more import- 

 ant junctions soldered, these anomalies disappeared. 



