An attempt is here made to furnish some infor mation Of an elementary character on a special branch Of technical education which has been seriously neg lected in this country. But the reader will understand that it is only an elementary treatise. Its publication in serial form, where each article must, more or less, be complete in itself, has to a large extent determined the method of handling the subject, and I am fully Cog nisant of the drawbacks Of the work in that respect.