of the driving wheels of a locomotive. By 1907, dog and pony show operator H. H. Linn abandoned his gas-and-steam-powered four- and six-wheel-drive creations and had Lombard build a motor home/traction engine run by an underslung four-cylinder Brennan gasoline engine to travel the unimproved roads of the day, with wheels at the front and tracks at the rear: the first payload-carrying half-track. By 1909 this was replaced by a smaller machine with two wheels at the front and a single track behind, since rural wooden bridges presented problems. Stability issues, together with a dispute between Linn and Lombard, led Linn to create the Linn Manufacturing Company, builder of the Linn tractor, for building and putting onto the market his own improved civilian half-track–style machines. Lombard attempted to follow but, for the most part, remained a pulling machine. Linn would later register "Haftrak" and "Catruk" as trademarks, the latter for a half-track meant to convert hydraulically from truck to crawler configuration. World War I Tractors used to tow artillery and designs with front wheels and tracks at the rear began to appear prior to the outbreak of war, often based on agricultural machines such as the Holt tractor. The basic half-track concept was originally showcased by the British during the war. With such tractors, the tactical use of heavier guns to supplement the light horse-drawn field guns became feasible. For example, in the British Army it allowed the heavy guns of the Royal Garrison Artillery to be used flexibly on the battlefield. In England, starting in 1905, David Roberts of Richard Hornsby & Sons had attempted to interest British military officials in a tracked vehicle, but failed. Holt bought the patents related to the "chain track" track-type tractor from Richard Hornsby & Sons in 1914 for £4,000. Unlike the Holt tractor, which had a steerable tiller wheel in front of the tracks, the Hornsby crawler was steered by controlling power to each track. When World War I broke out, with the problem of trench warfare and the difficulty of transporting supplies to the front, the pulling power of crawling-type tractors drew the attention of the military. With tanks coming onto the scene, however, the combination of tracks and wheels seemed impractical when fully tracked or six-wheel, four-wheel drive vehicles were available. The half-track saw a comeback in the 1930s, with development occurring in several countries that would use them in World