Whatever plate may be selected as a standard, if a large one, it remains stationary, while the work is repeatedly applied to the plate; it is therefore necessary to frequently unfix and refix the work in the vice when a heavy surface-plate is used. But if a small plate or block is used as a standard, it may be rubbed upon the surface of the work while it remains fixed in the vice, and this mode of trial is especially advisable if only one end is being specially planed, without making it, just at that time, parallel to the surface at the other end. When, however, both surfaces are being made plane and parallel to each other, the work must be frequently removed from the vice for measurement.

A little care is required every time tange is applied to any standard surface. If the tange is thicker on one part of the surface than on some other part, the operator, especially if he is a learner, is liable to be misled, because the comparative thick accumulation of the marking matter will adhere to any part of the work that may be put into contact, whether concave or convex, and the portions of the surface to which the matter sticks may be hollows, and not projections. The layer of tange should be so applied that any portion of it shall be equally thick with any other portion, so that, while on the standard surface, both the outer surface of the tange and its inner surface which touches the plate shall be parallel to each other, and, consequently, parallel to the plate's surface. To obtain a near resemblance to this parallelism, only a very thin layer of the mixture should be on the plate, at the time the surface being adjusted is to be put into contact, the quantity of tange being only sufficient to be seen on the work after having been rubbed upon the standard. The nearer the surface being scraped resembles a plane, the thinner must be the layer of marking matter, so that at the beginning of a scraping process the composition may be in greater quantity, to roughly and quickly indicate the places of contact.

The mode of properly distributing the composition on the standard when the surface in progress is nearly finished, consists in rubbing off nearly all the tange that was on during previous trials of the work. This rubbing off is done with a clean smooth cloth, and is continued until a mere cloud of tange appears to remain on the plate's surface. During this final smoothing of the marking compound, the operator should carefully observe whether any bright portions of the plate can be seen, through being entirely deprived of tange with too much rubbing, and if such portions are noticed, the operator will avoid applying his work to the bright parts; but if the standard surface is too small to admit such a selection, more tange should be rubbed on to the bright places, and again smoothed until of a proper thickness.

Although the plane surfaces of the small pillar-table being made, have but a few inches of area, it is necessary for the learner to observe the same order of reduction as if the surfaces were large, the middles being scraped previous to the surrounding parts, according to the directions in page 157.

During the final adjusting of the planes, to make them parallel to each other, great care is necessary; and this parallelism can only be obtained by a proper use of a gap-gauge or calliper. A tightly riveted calliper is the preferable tool, which must be delicately adjusted with a gentle hammering, so that the points of the calliper's feet shall only very tightly touch the two plane surfaces while measurement is being performed. But such gentle measurement is of no avail unless one end of the cylindrical block is a plane at the time of applying the callipers. If one extremity is a little hollow, and the opposite extremity a little prominent, the measurement with callipers will cause both surfaces to appear flat, if the calliper only is used to ascertain the condition of the work. To avoid such an error, one plane surface must first be produced by careful scraping, and with repeated rubbings to the standard plate. When one plane is thus finished, it becomes a sort of base or primary plane to which the opposite end of the block is to be made parallel. Such a finished plane is therefore not altered during the final scraping in the course of adjustment by means of the callipers, all the scraping to make the extremities parallel to each other being performed upon the opposite end, which of course is the roughest. The plane surface first finished or the primary plane, here referred to, corresponds to, and resembles in some respects, the plane surface which is in contact with the surface-plate shown in Fig. 621, because the boundary of the hidden plane in the upper part of the block which is being marked with the scriber, could not be easily scribed without first smoothing the lower end sufiicient to prevent it rocking on the table.