THEORY OF THE STEAM ENGINE. CXXXIX, 
such as the area exposed to admission steam, the range of tem- 
perature during expansion, the relative times of duration of 
admission and expansion, and so forth. Altering the value of the 
ratio of expansion is however found to affect the amount of 
cylinder condensation in a fairly regular manner, and consequently 
the waste may be satisfactorily expressed as a function of the 
ratio of expansion. What this function is has not yet been satis- 
factorily determined ; it can only be stated in general terms that, 
within the limits of the ratios of expansion usually adopted, the 
waste increases as the ratio increases. 
(2) Proportions and size of cylinder. These two conditions 
must necessarily havea very important influence on the percentage 
of steam in a cylinder which is condensed, but the exact relation- 
Ship has not been determined. The difficulty of making series 
of experiments upon many engines of different proportions and 
sizes under otherwise exactly similar conditions is very great. It 
is known of course in a general way, that the heat loss is less pro- 
portionally in an engine of large power than in a small one, and 
that the less clearance surface per pound of steam the better. 
(e) Condition of surface exposed to the incoming steam. With 
regard to the effect which this condition has on the amount of the 
loss by initial condensation, the experimental evidence is not very 
large, and is on the whole opposed to what would be naturally 
anticipated. It has usually been assumed that certain conditions 
of the clearance surface would exert a marked influence in modify- 
ing the amount of the heat loss, that, for instance, when the 
clearance surface is coated with grease, its effect in causing con- 
densation is small,! but tests made of engines in which the piston 
surfaces and cylinder heads were coated with lead, with porcelain,® 
and with other substances of different heat conducting powers 
than those of iron, appear to show practically no difference in the 
1 Cotterill—« The Steam Engine,” p. 339. 
2 Proc. Inst. M. E., 1887, p. 524. 
3 Thurston—* Manual of the Steam Engine,” p. 495, note. 
