G. Jounstone Sroney on the Penetration of Heat across Layers of Gas. 21 
Om will represent the rate due to convection in the cylinder. The observations 
recorded by De la Provostaye and Desains enable us to fix the points m, r, and n, 
corresponding to the tensions 70, 45, and 15 mm., nearly in a horizontal line. They 
also state that near the tension of 6 mm. the rate of cooling diminished. with 
“excessive rapidity,” but that nevertheless at a tension of 2°8 mm. it still exceeded 
by a large amount that which presented itself at the same tension in their largest 
receiver, and which is represented in figure 2. These statements indicate that the 
observations, if plotted down, would have given a curve like the thick line of 
figure 4, Itis hardly necessary to point out that the larger development of the 
phenomenon and its exhibiting itself at higher tensions with each diminution of 
the size of the receiver, as shown in the foregoing diagrams, are in the most 
satisfactory accordance with the Gey, presented in this memoir. 
13. If we suppose the [as oe 
shaded portions of the 
ordinates in fig. 3 to 
be moved vertically 
downwards till they 
abut upon a horizontal 
axis of abscissas, we 
shall obtain the curve in fig. 5, the ordinates of which represent the rate at which 
heat escaped by penetration in De la Provostaye and Desains’ cylinder, separated 
from the effects of radiation and convection. as ap 
We can only compare this figure in a very general way with the 
formula given above for the escape of heat by penetration, viz.— 
AQ = Vop, eu e ° ° ° (8), 
1 
because too little is known of V, p,, and 40, to enable us to plot 
down a curve from this expression.* But we can, at all events, 
see that V will be only moderately affected by alterations of 
tension, that p, will vary nearly as the tension, and that as the 
tension is diminished, Ad, will gradually rise from 0 to a value 
which is nearly 4 (6,-6,). Hence, the curve must be one some- 
what like that of fig. 6+, whose ordinates first rise gradually to a 
maximum at a certain tension, after which they fall away to 
cypher, if the exhaustion is continued indefinitely. This descrip- 
tion agrees with the form determined from the observations and 
which is plotted down in fig. 5; so that the comparison, though 
* Just as the parabolic curve of convection could not have been plotted down 
from equation (2) § 7, owing to the vagueness of some of the quantities which 
appear in it, viz., Q, v, and ‘D6. 
t In fig. 6, OB is intended to represent a portion of the curve y=Vop,4 
6, # Os and OnA the result 
1 
of shortening its ordinates in the ratio a 
2 
): E 
