machines cannot be run slowly enough to give the required length of exposure. Some of the older machines can be changed at rather slight expense so as to have an extra belt-pulley reduction between the motor and the blue-printing machine itself. The newer types of machine can usually be run slowly enough to give the exposure necessary for this special paper. If a new blue-printing machine is to be ordered, however, it is well to make some preliminary tests with the cards.

The cost is not great for blue-printing a whole set of record cards once each month after the last data have been plotted. Blue-print paper of heavy weight should be carried on hand, already sensitized and cut to size, four by six inches and four by twelve inches. The original curve cards, each backed by a sensitized card of the same size, are simply fed into the blue-print machine as rapidly as the operator can put the two cards together in pairs.

There is a tremendous advantage in having all curve records made on the high-grade transparent card so that any card may be duplicated by blue-printing whenever desired. It is impossible for anyone to predict what conditions will come up in the future of a business, and the only safe plan is to install from the start such a curve-record system that any card may be duplicated by blue-printing in future years if desired. It frequently happens that an executive wishes to have a blue print made of a recent year's curve card to compare with the curve card of an earlier year, in order that the blue prints may be mailed to some higher official or to some branch -house manager to point out certain conditions which it would be difficult to describe fully if copies of the curve cards could not be sent.