Page:Miscellaneous Papers on Mechanical Subjects.djvu/26

 18 happen, unless the comparison be constantly made between them all.

In the use of the surface plate, care should be taken to prevent unnecessary injury, whether superficial or from straining. It should also be occasionally submitted to careful correction, and should invariably be supported on three points. In no other way can a high standard be maintained.

It will be found convenient to set apart one plate for the purpose of comparing others, allowing it to remain entirely undisturbed. It would otherwise be necessary, at every revision, to repeat the process for obtaining an original surface, and a considerable loss of time would thus be occasioned.

A mistaken idea prevails that scraping is a dilatory process, and this prejudice may tend to discourage its introduction. It will be found, however, to involve the sacrifice of less time than is now wasted on grinding. Were the fact otherwise, it would be no argument against the preference due to the former. But it is worthy of observation that, in this instance, as in many