ach 60 
a ton of coal will make over 50 tons of ice, except for the 
practical difficulties. 
The practical difficulties are many, and greatly differing 
from each other; that they are formidable is apparent, when 
we observe that in practice, instead of getting 56 tons of ice 
by the use of one ton of coal, we think we do pretty well 
when we get ten per cent. of that amount, and the coal fur- 
nishes only a moiety of the energy consumed; we waste 
nearly all our force in meeting practical difficulties. "We may 
well be abashed, when we observe that these practical difficul- 
ties, and indeed all others, are incident only to the ignorance 
and want of skillof men. The only one of these which the 
author proposes to discuss, was that pertaining to the degree 
of volatility of the vaporizing substance of the ice machine. 
Aside from practical difficulties, one volatile liquid is as good 
as another ; it would cost no more of the direct force to make 
ice by the evaporation of Mercury or water, than by the 
evaporation of Ether or Ammonia. 
What is the precise degree of volatility, of a liquid best 
suited for use in the practical machine? Mercury and liquid 
Nitrous Oxide may be taken as the extremes, quite unsuited 
for use and for opposite reasons; what liquid is the proper 
mean? The author here exhibited a chart of curves, repre- 
senting the tensions and temperatures of the saturated vapors 
of water, Ether, Sulphurous Acid, Ammonia and Carbonic 
Acid. The ordinates of the curve represented temperatures, 
and the abscissas the tensions. 
In order that the curves might be easily comparable by in- 
spection, they were all commenced from the same point of 
tension, namely, atmospheric pressure, 760mm. The chart 
was drawn on engineers’ profile paper, about a yard long, and 
the data of the most recent experiments were used. 
The author continued the discussion by going considerably 
into detail, all illustrated by the chart, or having direct 
reference to it. This part of the paper is unsuited for 
abstract, without the assistance of engravings. The conclu- 
sion arrived at, was that a liquid boiling about midway of 
