ve over above the reversing
dogs to the full width between the housings.

The parabolic form of housing is elegant in appearance, but
theoretically right only when of uniform cross section. In some of the
counterfeit sort the designers seem to have seen the original Sellers,
remembering the form just well enough to have got the curve wrong end
up, and knowing nothing of the principle, have succeeded in building a
housing that is absolutely weak and absolutely ugly, with just enough
of the original left to show from where it was stolen. If the housing
is constructed on the brace plan, should not the braces be straight,
as in the old Bement, and the center line of strain pass through the
center line of the brace? If the housing is to take the form of a
curve, the section should be practically uniform, and the curve drawn
by an artist. Many times housings are quite rigid enough in the
direction of the travel of the table, but weak against side pressure.
The hollow box section, with secure attachment to the bed and a deep
cross beam at the top, are the remedies.

Raising and lowering cross heads, large and small, by two screws is a
slow and laborious job, and slow when done by power. Counterweights
just balancing the cross head, with metal straps rather than chains or
ropes, large wheels with small anti-friction journals, and the cross
head guarded by one post only, changes a slow to a quick arrangement,
and a task to a comfort. Housings of the hollow box section furnish an
excellent place for the counterweights.

The moving head, which is not expected to move while under pressure,
seems to have settled into one form, and when hooked over a square
ledge at the top, a pretty satisfactory form, too. But in other
machines built in the form of planing machines, in which the head is
traversed while cutting, as is the case with the profiling machine,
the planer head form is not right. Both the propelling screw, or
whatever gives the side motion, should be as low down as possible, as
should also be the guide.

There is a principle underlying the Sellers method of driving a planer
table that may be utilized in many ways. The endurance goes far beyond
any man's original expectations, and the explanation, very likely,
lies in the fact that the point of contact is always changing. To
apply the same principle to a common worm gear it is only necessary to
use a worm in a plain spur gear, with the teeth cut at an angle the
wrong way