face and the one destroyed the other. But it is evident, that where only two surfaces are concerned, the variety of error in the general outline is not sufficient to afford any probability of mutual compensation.
It will further appear, that if the original error be inconsiderable, the surfaces must lose instead of gaining truth. It results from the nature of the process that certain parts are acted upon for a longer time than others. They are consequently more worn, and the surfaces are made hollow. Nor is there any possibility of obviating this source of error, except by sliding one surface entirely on and off the other, at each move, a method which, it need not be shown, would be impracticable.
It may be mentioned, as an additional cause of error, that the grinding powder collects in greater quantity about the edges of the metal than upon the interior parts, producing the well-known effect of the bell-mouthed form. This is particularly objectionable in the case of slides from the access afforded to particles of dirt, and the immediate injury thereby occasioned.
Another circumstance materially affecting the durability of ground slides is, that a portion of the emery employed becomes fixed in the pores of the metal, and causes a rapid and irregular wear of the surfaces.
