S56 Prof. Silliman on Natural Ice-Houses in Connecticut 
top of a ridge of trap in Connecticut. The excavation wa^ 
made simply, by removing the loose pieces of trap rock, which 
are here piled in enormous quantities, but composed of frag- 
ments of very small size. These loose pieces of stone, with the 
air in the cavities, are better non-conductors of heat than the 
ground which usually surrounds ice-houses; for the ice keeps 
remarkably well in this elevated ice-house. Perhaps this will 
aid us also in explaining the phenomena of the natural ice-houses 
that have been mentioned. 
It may not be useless, before dismissing this article, to men- 
tion, that the roof of an ice-house should be painted white, and 
that it should be thatched with straw, beneath the ordinary 
wood-roof The surface of the roof thus becomes reflecting, and 
non-absorbing, and the substance non-conducting in relation to 
heat. We can speak from experience of the eflicacy of this ar- 
rangement. 
Art. XX. — Description of a New and Portable ^thrioscope^ 
By John Murray, Esq. F. L. S., M. W. S., and Lecturer 
on Chemistry, &c. 
The .^thrioscope which I propose to describe in the follow- 
ing paper, is not only extremely sensible, but is also remark- 
able for the simplicity of its construction, and for its perfect 
portability. 
It consists of a lower glass-ball B j Plate X. Fig. 1 0., re- 
posing in the cavity of a stands This ball contains alcohol or 
ether tinged red, and is only about half filled with fluid. A 
glass-stem CD, screws into this ball, or is adapted to it^by 
grinding, and its lower orifice passes into the coloured fluid, 
say |ths of its depth. This hollow glass-tube, into which the 
linged liquid ascends, carries the scale as represented in the 
figure. The hollow brass spherical cup A, screws on at top, 
ind appears in the sketch protected by its lid. The small glass- 
ball 6, which surmounts the tube, is screwed on in the cup, 
or ground to fit its extremity. The instrument is then com- 
plete, and so soon as the lid is removed, and the sentient-ball 
included in the cup, exposed to the sky, the coloured fluid will 
