202 
jipplicalioiL of' S'eani Power 
from field to field, or with its threshing-mill from farm to farm, 
I must look back to the gradual nature of the steps by which the 
object has been so nearly, if not completely, attained. In the 
year 1841, ^portable steam-engine was first exhibited as "the 
great novelty," at the Society's Show held at Liverpool, though 
others may have been in use before that time. At Bristol, the 
next year, the same engine was shown, with additional me- 
chanism, by which it propelled itself, and was readily steered in 
any direction over uneven ground. There were also two portable 
engines by other makers. At Derby, in 1843, three portable 
engines were present, and steam-engines there figured for the 
first time as a class in the prize-list. At Shrewsbury, in 1845, 
there were only two ; at Newcastle, in 1846, only one portable 
engine. The succeeding- meetings showed a continual increase until 
1851, ten years after the first was exhibited, when the number 
of portable threshing engines in the kingdom was computed at 
8000, and the multiplication has proceeded at a far more rapid 
rate since that periotl. Of course, it is not possible to ascertain 
every instance of attempted locomotion of an agricultural steam- 
engine, but the following are those with which I am acquainted. 
Though the engine was made locomotive at so early a period, 
the principle found little favour, and it is only very recently that 
we have ventured to trust our expensive motive-agents to propel 
themselves (with the added weight of water and fuel and steering- 
wheel) over rough roads, which we find they are quite strong 
enough to do, dragging a load at the. same time. In 1851, Mr. 
Usher's steam-cultivator, weighing 6^ tons, travelled 5 miles to 
a trial-field and 5 miles back, along a common road, and passed 
to and fro over the surface of fields, doing some very rough work, 
without the jolting causing any injury to the machinery or boiler. 
I need not enter into the history of the traction-engine, fitted 
with Mr. James Boydell's "endless rails" (patented in August, 
1846, and February, 1854), which has excited so much attention 
ever since the Carlisle Meeting ; but shall refer to some of its 
performances. Rail-pieces hang upon the circumference of its 
carriage-wheels, fitting each to the other as they successively 
form the feet of the wheel in its revolution, so as to produce a 
continuous line of rail over which it revolves. They bridge over 
hollows and lay inclines over eminences, thus averaging a com- 
paratively level line of roadway. The mechanical advantage upon 
soft ground is obvious from a mere consideration of the difference 
between bearing the weight of the carriage upon the convex 
periphery of a broad wheel and bearing it upon a flat platlbrm 
like a " shoe," or rail-piece of similar breadth. Theoretically, 
a wheel-tire touches the ground so that its surface of contact 
forms only a mathematical " line," and it must sink into the 
