490 ON IMPRESSIONS OF COLD 



fourth, &c. surfaces, will at last acquire the same energy as if 

 the aggregate difference of seven degrees had been all exerted 

 at once. Thus, the under surface of the stratum G (see fig. 8. 

 PI. XL) darts pulses downwards, which, being augmented in 

 succession at the under surfaces of the strata F, E, D, C, B, and 

 A, may have finally the same intensity as if they had originated 

 from the opposition of the extreme strata G and A. Accord- 

 ingly, having planted a large screen immediately before the 

 fire, and placed a delicate pyroscope about the middle of the 

 room, with a broad circular piece of metal suspended a few 

 inches above it ; on withdrawing this canopy after some time, 

 the instrument indicated a small impression of heat, seldom 

 exceeding, however, one degree. But the effect may be ren- 

 dered more sensible, by a moderate concentration of the power 

 excited. Thus, the hemispherical pendant asthrioscope (fig. 6.) 

 will, in the same situation, mark a very sensible calorific impres- 

 sion, amounting, at least in ordinary cases, to three or four 

 degrees. Hot pulses are, therefore, actually shot downwards 

 from all the upper strata of the confined air of a room in which 

 a fire is kept steadily burning. 



The experiment can be likewise reversed. Let an inverted 

 gethrioscope, composed of a pendant differential thermometer, 

 have its sentient ball fitted with a small hemispherical cup which 

 is turned downwards, (see fig. 7.) This instrument being set on 

 the floor, will remain at zero ; but if lifted only a few feet, it will 

 indicate a visible impression of cold received from below, which 

 will increase to three or four degrees when the aethrioscope is 

 suspended near the top of the room. Wherefore, the upper 

 surfaces of the successive decumbent strata, being comparative- 

 ly colder, send upwards a series of chilling pulsations. Each 

 of the conterminous boundaries appears thus to perform a 

 double operation, shooting downwards impressions of heat, and 

 darting upwards equal impressions of cold. Such a mutual 



exchange 



