Portable Agricultural Steam Engines at Newcastle. 705 
page 728, gives the difference between the temperature of the 
air, which was about 70", and the temperature of the engine at 
the times when the observations were made. Column 13 gives 
the difference between these temperatures and 124°, the lowest 
reading. If now we make the ordinate of the observation at 
6 h, 17 min. equal to 144* 5°, and set off upon it the other 
differences in column 12 to the same scale, and draw horizontal 
lines so as to cut the curve, it will be seen that the points of 
intersection coincide very nearly with the points indicating the 
Fig. 3. 
l37-° 
/OO." 
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7 top 
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0" 
1 
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;::O.DOa. 
250.000. 
200.000. 
100.000. 
50.000. 
0 
6 7 8 9 io II li TTa 3 4 5 6 7 
P.M. ^ A.M. 
units of heat, thus showing that the loss of heat follows the 
same rate as the decrease in the difference of temperatures. 
But there still remains the question, What is the rate of cooling 
at the working pressure 95 lbs. ? The number of units of heat 
in the engine at 95 lbs. was 321,105 above the lowest observation. 
Plot this value on the ordinate of the 6 h. 17 min. observation, 
and draw a horizontal line till it cuts the curve at c, and draw 
the ordinate AC. This ordinate meets the base at 6 h. 30 min. : 
hence the steam fell to 95 lbs. at that time. Next from the 
