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DRILLS. 101

considerably less than the smallest cavity the dentist
ever attempts to fill—that is about one thirty-second
of an inch in diameter, and the largest about one-fifth

of an inch. Inclusive of these extremes, there should
be about ten sizes of each particular form. These
instruments are used for opening cavities. With them
a more regular and perfect orifice is made in small
and medium-sized cavities, than by any other method.
They are also used to some extent for forming the
cavities, and even sometimes, in large cavities, for
making retaining-points for a filling.
Within comparatively a recent period a new form

of drill has been introduced by Dr. Scranton ; and
hence denominated u Scranton s Drill."
Its form is spherical, and in the manufacture the
bulb is formed as for the ordinary bur drill ; but in-
stead of having cut upon it numerous serrations, thus
forming a series of sharp edges : a cutting is made
upon two opposite sides with a small round file. The
instrument then presents two concave and two convex
sides, with four sharp longitudinal edges ; these may

be so inclined as to cut, only when rotated in one
direction ; or to operate alike well when rotated either
to the right or the left.
This instrument has two or three advantages over
the ordinary bur ; it can be kept sharp with the oil
stone till it is almost entirely used up ; and will con-