By 1800，hand printers could not supply enough books to satisfy [使满意] the demand．Books were not printed fast enough when hand-made paper and wooden hand presses were used．Fortunately [幸运地]，about this time, many machines were invented that aided[辅助的] in the mass production of such products as cotton and woolen cloth．The invention of new types of printing machines and paper machines followed on the heels[后跟] of these inventions and helped to speed up the production of books．The paper machine produced paper in rolls instead of sheets; and the iron hand press enabled the printer to use larger pieces of paper on which more text could be printed in less time．Question： Printing was speeded up when ______________.