The panic, followed by five years of economic prostration, was only occasioned by the failure of Cooke. Its real causes lie throughout the period of Civil War expansion. Never had the daily necessities of the United States equaled its production, and the resulting surplus, available for permanent improvements, was larger than ever in the sixties because of the growing use of machinery. Funds for investment, produced at home and increased through the strong foreign credit of the United States, tempted and aided the speculative development of the North and West. Yearly greater sums were sunk in municipal improvements that brought in no return, or in railroads that were slow in paying, or in errors that were a dead loss. The loss from the Civil War was an added charge upon the surplus. Great fires in Boston and Chicago consumed more of it. By 1870 the United States was using surplus at a rate that threatened soon to exhaust it. When the limit should be reached, new enterprises must necessarily cease, and all that were not wisely planned must fall, dragging down others in their ruins. For months before the failure of Jay Cooke, business had been dangerously near this margin. His failure, caused by his inability to find a market for Northern Pacific, merely precipitated the inevitable crash. The faulty currency, outstanding since the war, and adding to the business uncertainty, now aggravated the panic when it broke. The greenbacks were slowly rising in value. They profited by the growing credit of the United States, and received a special increase because of the development of business. After 1865 business transactions grew in number and volume more rapidly than the amount of available money, and this, driven to greater activity in circulation, rose in value from the increased demand. As the purchasing value of the dollar increased, prices, measured by the greenbacks, necessarily fell, while the equivalent of every debt that had to be paid in a specified number of dollars as steadily rose. Indeed, so great was the increase of production from the new farms, reached by the new railroads, and supplying raw materials for the new factory processes, that prices fell, even when stated in terms of gold. In a period of falling prices and appreciating currency, the gap between the poor and the rich was widened. The debtor carried a growing burden while the creditor harvested an unearned increase. Persons who lived on fixed salary or income profited by the fluctuations, but commercial transactions were made more difficult for the debtor.