4. It has all the advantages of falling moulds without the trouble of getting them out, and it can always be ascertained what any part of the falling line is out of the centre of the plank.
5. It takes the minimum thickness of stuff, as it can always be seen exactly what thickness will be required.
6. The system of bevelled joints does away with the short ramps, and thereby saves both labour and material; besides there is only one joint instead of three.
It will be seen that in no case in this book is extra thickness of stuff required.
PLATE 12.
PLATE XII.
ON OBLIQUE PLANES AND THEIR TRACES.
If the surface of a solid is neither horizontal or perpendicular it is oblique.
Thus, if we place a box on the table, the top of the table represents the horizontal plane and the side of the box the vertical plane, and the intersection of the box and the table is the ground line, or X Y. Draw a line out square from the box on the table, marked N N, Fig. 1. Hold the end of a book on this line, with its edge against the side of the box in an inclined position; mark a line on the side of the box: this line is the vertical trace, because the oblique plane has cut the vertical plane on this line. Before moving the book mark a line on the table: this is the horizontal trace, for the same reason that the oblique plane has cut the horizontal plane on this line.