The distinguishing feature of the “thermal boot,” as it came to be popularly known, is an air space between the inner and outer layers of wool pile insulation, both of which are completely sealed off by latex from any contact with moisture. This air space, under pressure, produces a vapor barrier such that heat cannot readily escape when it is emitted from the foot. Thus the wearer of the boot supplies his own warmth, which is retained as long as he is active, regardless of prevailing temperatures. If, however, the walls of the air space are punctured and the insulation becomes wet, the moisture collected within the boot freezes at low temperatures if the wearer remains inactive. In such cases, severe frostbite may result. Some of the tests were spectacular. One subject poured water containing pieces of ice into his boots and donned frozen socks before putting on the footgear. After 10 minutes of walking, the ice in the boots had turned to warm water, and there was no harmful effect on the man. Another subject waded across a knee-deep creek at a temperature of zero. Before he had marched a mile in the snow, his feet had warmed the water in the boots, although his pants were frozen so stiff that he could scarcely walk. Seldom has a military innovation been tested so thoroughly and scientifically in such a short time. Colonels Ion M. Bethel and John F. Stamm of Marine Corps Headquarters took a leading part in the development and procurement phases along with Lieutenant Colonel Gordon A. Hardwick. Major Vernon D. Boyd and Captain David R. McGrew, Jr. were active in the troop acceptance tests. A good many “bugs” had to be eliminated before the boot met with complete Marine approval. The manufacturer’s modifications were effected with minimal delay. It is perhaps needless to add that the thermal boot was not foolproof. Protection continued in subzero weather for at least an hour after the termination of activity, but it was inviting frostbite to remain motionless much longer. Socks had to be changed every 12 hours, and foot cleanliness and hygiene could not be neglected. If a few such simple rules were observed, a man had virtually perfect frostbite protection in the coldest weather. In fact, it was seriously proposed that a Marine casualty of this sort should be charged with misconduct if he acquired his frostbite while provided with thermal boots and a change of socks. In view of the tests and negotiations with the manufacturers, it was a marvel of promptness when the first shipment of boots reached the 1st Marine Division in August 1951, long before the advent of cold weather.