There was no way of telling in advance what would be the costs of producing these more important supplies. Many of them were of types and designs entirely new to American manufacturing experience. It was hard enough for the government agents to induce manufacturers to undertake these contracts even on a cost-plus basis. Had the War Department attempted to advertise the specifications of such a mechanism as the French hydropneumatic recuperator, it would never have received a bid. The only possible terms on which any sane manufacturer would take such a contract would be the payment of his costs, plus a profit. Many of the contracts for the more difficult sorts of supplies were bound to continue over an extended period before all the deliveries could be made. It was often impossible for such contractors to make commitments in advance for all their raw materials. Therefore they faced a rising market and prices which they could not predetermine. They also faced almost certain increases in the wages paid to their employees; yet here again they could not anticipate what these increases would be. The costs of a maker of optical instruments for the Army depended partially upon what he should have to pay for optical glass, but the glass was to come from a new war industry which had not yet begun production and therefore could not estimate what it would charge for glass. With its designs for war implements the War Department did the best it could, founding its specifications upon the latest and best information at its command. Yet so rapid was the evolution of war materials resulting from their intensive use that sometimes the Department found a design obsolete before its production was fairly begun. Its designers therefore made changes in the specifications at the factory, and these changes involved heavy manufacturing costs. Every contractor knew that his work was to be subject to such changes in the specifications; yet no one could foretell whether changes would be made or what they would cost.