490 ON IMPRESSIONS OF COLD 
fourth, &c. surfaces, will at last acquire the same energy as if 
the ageregate difference of seven degrees had been all exerted 
at once. Thus, the under surface of the stratum G (see fig. 2. 
Pl. XI.) darts pulses downwards, which, being augmented in 
succession at the under surfaces of the strata I’, 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 athrioscope (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 
zthrioscope, 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 zethrioscope 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 
