Steam Culture. 
405 
that rate of charge, or else the horse must have credit for the value 
imparted to the manure, by its consumption. 
8thly. — The interest, deprec iation, and cost of repairs should 
be charged on the whole year, and not on those days (or weeks) 
only on which the implements are at work. If the year's work 
is but small, the repairs may be set at a lower rate, but the interest 
will remain the same, and the charge for depreciation will not be 
much affected. 
9thly. — Before framing a general estimate based on reports of 
different degrees of correctness and authority, a good deal of care 
must be exercised, in scrutinising these constituents, and elimi- 
nating their inaccuracies : otherwise, the least trustworthy report 
will tell most upon the result, and the greater its inaccuracies, 
the greater will be its individual influence. 
To return then to our subject. The elements of which the 
problem consists are not very complicated, and some points are 
pretty well determined. We have a large amount of evidence as 
to the work which may on an average be executed in a day of ten 
hours without straining the engine or over-taxing the agility of 
heavily-shod farm lads. As to the quality of this work there is 
no difference of opinion — be it ploughing or cultivating it is the 
best of its kind ; and, again, the cost of the labour of the men 
employed is well known, and deemed satisfactory, inasmuch as it 
differs little from that required to carry out horse-work. 
The cost of coals bears a pretty constant ratio to the amount of 
work done ; practically, when the engine is working at a high 
pressure, the consumption of fuel seems to be proportionately 
rather larger, although an interesting law Avith respect to the 
latent heat in steam might lead us to a different anticipation.* 
* The law of latent heat as regards steam is very interesting. It ■would appear 
first, that 5i times as much heat is required to convert water at 212° into steam as 
sufficed to raise it through 180 degrees, i.e. from 32° to 212°. 5| X 180 making 990 
or nearly 1000, the latent heat of steam at 212° is therefore estimated as 1000; 
or, to adopt Bourne's definition from his ' Catechism of the Steam-Engiiie,' — " The 
latent hoat of steam is 1000, by which it is meant that there is as much heat in 
any given quantity of steam as would raise its constituent water 1000 degrees if the 
expansion of the water could be prevented, or would raise 1000 times that quan- 
tity of water one degree." Under the common atmospheric pressure, the heat indi- 
cated by tlie thermometer being added to this gives a sum of 1212°, which sum it 
appears is a constant — that is, under any condition of pressure it represents the sum 
of the latent heat and of the indicated heat proper to that pressure ; in other 
words, at higher temperatures and pressures the latent heat diminishes exactly as 
the indicated heat increases. 
For example, if we work with a pressure of 45 lbs. on the square inch ( = 3 atmos- 
pheres), the heat indicated will be 275°, the latent heat 937°, making together 1212°; 
if with (iO lbs. pressure (=4 atmospheres), the indicated heat will be 294^, latent 
918°, and their sum, 1212°, still the constant quantity. 
These observations are adapted from an able account of the steam-engine in 
Stephens's • Book of Farm Implements.' Tha author continues : " From this is 
deduced the important truth that by no alteration of pressure will a greater eco- 
