For the hauling of field pieces the truck with 4-wheel drive was employed. This was heavily built, with a capacity of 12 tons generally with two trailers carrying ammuni tion and supplies. A few of these had 6 cylinder engines, but the 4-cylinder type predominated, and was favored. The engines were generally of 454-inch bore and 6-inch stroke. These heavy trucks are capable of transporting a 75-millimetre gun, and in the French service the trucks were fitted with jacks to take the weight off the wheels when the gun was fired. For hauling the heavy artillery, the 14-inch and 16-incb, guns, the motor used was of the simple traction type with road wheels of large diameter driven by internal gearing. The caterpillar tractor of the agriculturist operated at a slow speed was found more expeditious and certain in the long run than any other machine. These heavy tractors hauled loads up to 30 tons distributed over two or three trailers, one or more of which might be dropped on grades or where the going was very heavy. One of the recorded accom plishments of an American-built tractor of this type was the transportation of a 20-ton gun 29 feet long a distance of 11 miles in PA hours. The muzzle end of the gun was supported by a trailer. For armored cars carrying machine guns representatives of nearly every type of motor vehicle have been employed, from the motorcycle with a sidecar to the big 12-ton truck carrying four machine guns and a field piece. The more common adaptation was based on a powerful 4-cylinder touring car upon which was built up a single compartment of steel plates about one-third of an inch thick, sometimes with a turret, and generally equipped with a wire-cutting device. The plating covered the engine and radiator, and the wheels were of the steel disc type, with solid rubber tires. These cars were supplied with flanged metal rims which could be quickly adjusted for travel at high speed on railroad tracks. Trailers were used behind nearly all cars, a favorite type be ing the two-wheeled float with canvas top. These were often fitted with seats to carry a large number of men, and operated up to a speed of 35 miles per hour. The so-called "tanks" of the British army are agricultural tractors of caterpillar model encased in half inch steel armor. They weighed 15 tons and had engines of 120 horse-power.
Owing to the wide variation in construction of the many makes of motor vehicles in use on the Franco-British battle front, it was a hope less task to attempt to supply parts to repair a damaged car, and in case of a breakdown the car was simply abandoned for a new one. The
loss was estimated at between 60 and 70 per cent of the car fleet, owing in part to the load ing far beyond the burden which the cars were planned to carry. On the German front the cars were largely standardized, and rescue cars loaded with parts cruised about gathering up and repairing even the abandoned cars of the enemy. Some of these rescue cars were fitted up as machine shops, with trailers carrying supplies and parts. Others were derrick cars which lifted the broken-down car to the deck of a trailer and took it to a repair shop in the rear.
Specialized automobile machines appeared in large numbers as the war grew older. Among these were motor plows of 200 horse-power which dug trenches three feet deep faster than could be done by 100 men; roller tractors which not only smoothed newly made roads, but drew a train of loaded trailers behind; observation cars fitted with extension ladders to be run up high into the air field laundries; sterilizing chambers for clothing; X-ray laboratories; per ambulating bath-houses; and even traveling theatres for the diversion of the wounded. One of the more interesting emergency uses of com mercial cars was the conversion of the London buses into refrigerator transfer cars for fresh beef. Another was the fitting up of a motor cycle with a side car with a complete wireless outfit.
In the United States regular army service there are battle cars fitted with rapid-fire guns; tank cars for gasoline, oil and water; anti aircraft cars withguns mounted; 4-ton trucks carrying a 3-inch field gun with caisson — not mounted on the truck, however, as with the French 75's; a machine shop for repairs in the field; and a coffee percolator mounted as a two-wheeled trailer. These are, of course, in addition to the customary ambulances, field kitchens, etc., in general use in all military establishments.
Actual experience in the European War has proved that with the aid of the automobile in all its forms a whole army with its supplies and field equipment can be advanced at the rate of 30 miles per day, and a retreat can be con ducted at the rate of 50 miles per day.