344 On the Objections to Mr. Prevosl's Explanation [May, 



of increase or decrease of the actual quantity of free caloric in tlie 

 surrounding sphere ; but that where there is such cause, such given 

 point will be affected by the form and nature of the reflecting sur- 

 faces which change the direction of rays that fall on, or proceed 

 from, the point where such active cause exists. 



4. That where heat is increased by more reflection on one spot, 

 it must be diminishea somewhere else, and vice vena. 



5. That when any body whose temperature (arising from extra- 

 neous causes) viuies from that of others in its neighbourhood, if 

 hotter, it affects a body placed in any given point, by throwing on 

 it a more intense radiation than that which it gives out ; if colder, 

 by intercepting and absorbing rays of free caloric from some part of 

 the sphere by which it is surrounded, and thereby depriving it of its 

 due and usual compensation. 



. If, then, by means of the conical tube, or by any other means, 

 a thermometer indicates decrease of temperature from its relative 

 position with that of a cold body, we have only to see where and 

 how are radiations that would have fallen on the thermometer inter- 

 cepted by that cold body. 



The mechanical form of the polished interior of this tube will 

 effect this according to the known laws of optics. The effect of 

 this reflection differs from that of the mirrors ; for instead of a 

 single focus at one point, there is an indefinite number of foci all 

 along the axis. The incident rays which are parallel to the axis are 

 reflected in the form of cones, and cross in the axis : however, the 

 largest hollow cylinder of incident rays will form the focus of 

 greatest intensity, and a multitude of other rays will be brought by 

 complex reflections from the polished interior to the same spot. 

 The tube is of course a truncated cone ; for the smaller end has a 

 diameter. The principal focus is not at the apex of the cone, but 

 short of it ; and we suppose the thermometer to be placed at this 

 spot near the aperture. I need not trespass more on you by speci- 

 fying all the circumstances of the situation ; but it is easy to see 

 that a large portion of the rays of the surrounding sphere is inter- 

 cepted from this point by the tube, and reflected from its exterior 

 surface in foreign directions. The loss of these rays is compensated 

 by the concentration of the cylinder of rays brought to this point by 

 reflection from within the tube, else the thermometer in this place 

 must indicate decrease of temperature without the presence of the 

 cold body, contrary to the rule No. 3. But when the bottle of ice 

 is brought to the wide aperture, all this cylinder of radiation is 

 intercepted and absorbed, and the radiation from the thermometer 

 is uncompensated. A hot body in the place of the ice wuuld have 

 its radiation condensed on the thermometer. A visible object would 

 be magnified in an eye placed in the room of the thermometer, and 

 a dark hole in a white surface would have its diameter apparently 

 increased ; and thus a perfect analogy subsists between this experi- 

 ment and those of Mr. Pictet. 



Mr, Pictet's theory appears also capable of accounting for the 



