484 ON IMPRESSIONS OF COLD 
third part of the whole height of the cavity, the diameter of the 
sentient ball having likewise nearly the third part of the diame- 
ter of the orifice of the cup. In order to separate the balls of 
the pyroscope, the gilt one may be carried somewhat higher 
than the other, and lodged in the swell of the cavity, its stem 
being bent to the curve, and the neck partially widened, to 
prevent the risk of dividing the coloured liquor in carriage. 
The instrument. is adjusted to its position by a clasp which 
slides along the stalk. A lid of the same thin polished me- 
tal as the cup itself, is fitted to the mouth of the ethrioscope, 
and removed only when an observation is to be made. The 
scale may extend to 60 or 70 millesimal degrees above the 
zero, and about 15 degrees below it. 
2. Sectoral.—To ascertain and measure the cold pulses shot 
obliquely, as well as those in the vertical direction, the zethrio- 
scope may be constructed to turn towards any portion of the 
sky, (see fig. 1. and 2. Pl. XI.) To effect this, the form best fitted 
for reflexion would be that of an hyperbola, whose asymptotes 
have an inclination equal to the visual angle of the space to be 
explored. Fig. 1. may give an idea of the construction. But 
to obtain accurate results, the focal ball must be small, and the 
hyperbolic conoid wide and much extended. It will answer 
nearly the same purpose, however, to adopt a truncated sphe- 
roid, of great eccentricity. Let the height of the focus, for in- 
stance, be one inch, that of the entire cavity nine inches, and, 
consequently, the widest diameter six inches. The shape re- 
presented in fig. 2. is rather more distended, its extreme 
width being equal to double the eccentricity, and the focal ball 
dividing the height of the orifice in the ratio of 1 to 3+ 8,or 
of 6 to 35 nearly. The pyroscope inserted has a peculiar twist- 
ed form, and receives its adjustment from a moveable socket. 
While the sentient ball remains always in the same position, 
the 
