In essence, a portable engine comprises a 4 wheeled chasis (here with modern towbar fixtures) carrying a boiler and firebox with a single cylinder on top driving a pair of flywheels. The belt to be driven simply goes round the flywheel. This is quite a large example of the breed and, at the time under consideration, they would have been quite a bit smaller, probably with only one flywheel. The funnel is usually articulated such that it can be laid flat along the body during transportation to avoid low-hanging obstructions.
The engines usually also feature a small pump so that they can keep themselves supplied with water from any convenient nearby source, typically a barrel carried for the purpose.
The primary motivation for having a portable steam engine on a farm was to permit the use of a whole range of small cutting and crushing machines and, in particular, the newly-developed threshing machines. These amazing constructions of iron and timber (also known as thrashing machines) take the harvested wheat in at the top and separate the grain, chaff and straw to three different outputs. For some reason they are invariably painted a soft pink colour. The more advanced specimens would even grade the grain into different sacks for small, medium and large. Chaff appears to have been discarded, but the straw could be fed onto a separate baling machine seen here in the foreground. In this particular configuration, the motive power for the whole rig is a steam traction engine. This configuration would often be used by contractors, using the traction engine to tow the threshing machine from farm to farm in their area and then supplying power on arrival. This whole chain of machines is now incorporated into the modern combine harvester.