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 Rh either of the others, and it is therefore to be again taken as the model, and the operation repeated.

It will be observed that the process now described includes three parts, and consists in getting up the surfaces to one another in the following order:— 1st. Nos. 2 and 3 to No. 1. 2nd. Nos. 2 and 3 to each other. 3rd. No. 1 to Nos. 2 and 3. These parts compose an entire series, by repeating which a gradual approach is made to absolute truth, till further progress is prevented by inherent imperfection.

In the earlier stages, the operation may be greatly expedited by judicious management. It has been already remarked, but it cannot be too often repeated, that the general outline of the surface should be solely regarded in the first instance, and the filling up deferred till after general truth has been secured. By this method, the first course of the series will be short, and the progress made will be both more speedy and more sure, the minuter detail being gradually entered upon, without the risk, otherwise, incurred, of losing previous labour. As, however, the surfaces approach perfection, the utmost caution and vigilance will be necessary to prevent them from degenerating. This will inevitably Rh