Prof. Forbes on the Refraction a?td Polarization of Heat. 157 



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

 the use of it, I have found it more simple, manageable, and 

 comparable, than I could previously have imagined. Not- 

 withstanding its delicacy and the promptitude of its action, a 

 few precautions suffice to prevent any derangement from with- 

 out. The only inconvenience which I experienced, was in 

 the determination of the zero of the scale, which appears lia- 

 ble to some fluctuations, which may be considered as acci- 

 dental. 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 temperature (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 deli- 

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

 to ascertain very minute differences of effect, which no other 

 thermometric instrument possesses. When we wish to ascer- 

 tain 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 deviation (at which, after several oscil- 

 lations, it is finally to settle) could afford. Not only does the 

 acquired velocity carry it through double the space due to the 

 statical effect; but I have observed that the action of the 

 thermo-electric pile so far resembles that of the voltaic, that 

 we appear to have an excess of effect at the first moment of 

 action, which gives a greater deviation than can be afterwards 

 obtained*. It is therefore to be recollected, that, in speaking 

 confidently of effects, which, statically speaking, are exceed- 

 ingly small, the experimentalist has a species of evidence far 

 stronger than the mere numerical expression of the deviation 

 of the needle, but the degree of which must be taken on the 



• This remarkable effect, which may be described as an increase of ten- 

 sion by confinement, seems generally to exist where the conductors of im- 

 ponderable agents oppose considerable resistance to their passage. It is fa- 

 miliar in voltaic electricity, and I have often observed it in magnetic elec- 

 tricity. It is similar to the action which I have attempted to demonstrate 

 in the passage of heat from good to bad conductors (see Lond. and Edinb. 

 Phil. Mag., vol. iv., p. \b^etseq.)y where we have the full advantage of the 

 dynamical effect ; whilst the existence of statical tension in heat seems like- 

 wise to be proved (as we might have anticipated) by the beautiful experi- 

 ment described by Professor Powell in the Philosophical Transactions for 

 1834, and noticed in the last number of this Journal, p. 58, 

 Third Series, Vol. 6. No. [)2, Feb, 1835. T 



