OF COAL AND RATE OF EVAPORATION. 13 
three small boilers placed in series were heated from 65° 
to 212°; the rate of the first boiler being represented by 
the line , the rate of the second boiler by ----— : 
and the rate of fie third by —---—----— : 
To be heated from 65° to 212°, the first boiler required 
40 minutes, the second boiler required 92 minutes, the 
third boiler required 161 minutes; or the heat acquired in 
equal times by the three boilers in series was by the first 
boiler 100°, by the second boiler 43.5°, by the third boiler 
24.9°. 
By comparing the amount evaporated from the third 
boiler, Table A, with its comparative rate of heating as 
above, it will be seen that flue space acts much more use- 
fully in heating cold water up to 212° than in boiling it off, 
namely in the proportion of 24.9 to 16, the third boiler in 
both cases being considered as flue surface only. 
The progressive manner in which heat is communicated, 
when the temperature of the body to be heated is not 
greatly under that of the heating body, is illustrated 
practically for steam as the vehicle of the heat by the fol- 
lowing Table. The apparatus used was a square wooden 
cistern with an iron bottom, double-cased for the purpose 
of admitting steam. The cistern being filled with water 
at 60°, steam of the temperature of 218° was admitted. 
Table C. 
Time in Temperature of Increase of 
Minutes. Water in Cistern. Temperature. 
0! 60° 
10’ 100° 
20’ 134° 
30/ 158° 
40/ 174° 
50’ 183° 
60’ 192° 
70’ 198° 
80’ 201° 
90’ 206° 
100’ 210° 
