BOOH II. J APH0RISM8. 471 



at the distance of five inclies (for instance), and be tlien gradually 

 aud slowly withdrawn to the distance of ten inches. The cone 

 and focus of the rajs, however, are the same, but the mere 

 motion increases the effect of the heat. 



29. Conflagrations, which take place with a high wind, are 

 thought to make greater way against than with the wind, be- 

 cause when the wind slackens, the flame recoils more rapidly 

 than it advances when the wind is favourable. 



30. Flame does not burst out or arise unless it have some 

 liollow space to move and exert itself in, except in the exploding 

 flame of gunpowder, and the like, where the compression and 

 confinement of the flame increase its fury. 



31. The anvil becomes so hot by the hammer, that if it were 

 a thin plate it might probably grow red, like ignited iron by re- 

 peated strokes. Let the experiment be tried. 



32. But in ignited bodies that are porous, so as to leave room 

 for the fire to move itself, if its motion be prevented by strong 

 compression, the fire is immediately extinguished ; thus it is 

 with tinder, or the burning snuff of a candle or lamp, or even 

 hot charcoal or cinders; for when they are squeezed by snuffers, 

 or the foot, and the like, the effect of the fire instantly 

 ceases. 



33. The approach towards a hot body increases heat in pro- 

 portion to the approximation ; a similar effect to that of light, 

 for the nearer any object is placed towards the light, the more 

 visible it becomes. 



34 The** union of different heats incrtas&s heat, unless the 

 substances be mixed ; for a large and small fire in the same spot 

 tend mutually to increase each other's heat, but lukewarm water 

 poured into boiling water cools it. 



35. The continued neighbourhood of a warm body increases 

 heat. For the heat, which perpetually passes and. emanatex 

 from it, being mixed with that which preceded it, multiplies the 

 whole. A fire, for instance, does not warm a room in half an 

 hour as much as the same fire would in an hour. This does not 

 apply to light, for a lamp oy candle placed in a spot gives no 

 more light by remaining there, than it did at first. 



36. The irritation of surrounding cold increases heat, as may 

 be seen in fires during a sharp frost. We think that this is 

 owing not merely to the confinement and compression of the 

 heat (which forms a sort of union), but also by the exasperation 

 of it, as when the air or a stick are violently compressed or bent, 

 they recoil, not only to the point they first occupied, but still 



* The fires supply fresh heat, the water has only a certain quan« 

 r.ty of heat, which being diffused over a fresh supply of cooler K^atw, 

 must be on the whole lowered. 



