1825.} Mr. Powell ofi Terrestrial Light and Heat. 369 



Article V. 



On Light and Heat from Terrestrial Sources, ^^^ on the Theory 

 of the Connexion between Light and Heat. By Baden Powell, 

 MA. FRS. 



(1 ) In all investigations on radiant heat, one of the principal 

 sources of difficulty consists in properly estimating the loss of 

 heat by radiation from the bulb of the thermometer on the s de 

 not exposed to the radiant influence, and which depends on the 

 rate orcommunication of heat through the bulb, and on the 

 radiating power of its surface. Externa circumstances regulate 

 the amo'unt of this effect; the proximity of a/-^«^,««^f^^.f 

 lower temperature increases it as we have already had occasion 

 to notice; and independently ^f.^^^iation, there mu^ be a 

 triflino- loss by conduction to the air m contact ; but in all these 

 cases'it is evident that the loss will be very different, according 

 Swhe ther';e are observing the rise of the thermometer in a 

 Svin short time, in a longer time, or its stationary indication. 

 S cfmmumcSion of belt through the bulb will f - be ve^^ 

 different in a mercurial and in an air thermometer : in the latter 

 also the expansion of the glass will be hkely to produce considei- 

 able error from the lower conductmg power of the inclosed a r 



All these circumstances, and perhaps others, have a gieat 

 tendency to perplex the experimental results ; and I have been 

 the more inducek here to allude to them, because I am inclined 

 o thmkthat 1 have not given some of my former arguments the 

 advantage they might hSve had from attributing too great au 

 tnflu end to tl/e loss by radiation. This probably need not have 

 been taken into consideration in the formula, since it would 

 seem that a greater length of time would be necessary m orde- 

 o the communication of heat through the bulb so as to produce 

 any sensible loss of heat. It would be easy to u^vestiga e a 

 more general and correct formula ; but upon reconsidering those 

 experiments (28) to which the formula applies, I am by no means 

 su^e whether they are of a nature sufficiently susceptible ot pre- 

 cSion to deternnne with any exactness the P.^o^tion mair. 

 tained between the heating and illumiuatmg intensity ot the 

 •ays. In fact, until we possess that important desideratum a 

 photometer u^on the principle of illumination, tins part ot the 

 subject must remain involved in considerable uncertainty 



(2 ) The consideration above adverted to will apply to the 

 expenments on the solar heat (8), and the remark upon them 

 mI) The conclusion is in fact thus very mucu strengtliened ; 

 Ind the effect of simple heat, if any were added by the i-emov.t 

 of the screen, would be to diminish the ratio of the white to the 

 black effect, by addition of quantities to Us terms m the ratio ot 



