ia.[7]

[Footnote 6: The following is a diagram of the first principal meridian,
and of the base line running across southern Michigan. A B is the
principal meridian; C D is the base line. The figures on the base line
mark the range lines; the figures on the principal meridian mark the
township lines. E is township 4 north in range 5 east; F is township 5
south in range 4 west; G is township 3 north in range 3 west.
[Illustration] As the intervals between meridians diminish as we go
northward, it is sometimes necessary to introduce a correction line, the
nature of which will be seen from the following diagram:--
[Illustration: DIAGRAM OF CORRECTION LINE.]]

[Footnote 7: In Philadelphia the streets for the most part cross each
other at right angles and at equal distances, so that the city is laid
out like a checkerboard. The parallel streets running in one direction
have names, often taken from trees. Market Street is the central
street from which the others are reckoned in both directions according
to the couplet

"Market, Arch, Race, and Vine,
Chestnut, Walnut, Spruce, and Pine," etc.

The cross streets are not named but numbered, as First, Second, etc.
The houses on one side of the street have odd numbers and on the other
side even numbers, as is the general custom in the United States. With
each new block a new century of numbers begins, although there are
seldom more than forty real numbers in a block. For example, the
corner house on Market Street, just above Fifteenth, is 1501 Market
Street. At somewhere about 1535 or 1539 you come to Sixteenth Street;
then there is a break in the numbering, and the next corner house is
1601. So in going along a numbered street, say Fifteenth, from Market,
the first number will be 1; after passing Arch, 101; after passing
Race, 201, etc. With this system a very slight familiarity with the
city enables one to find his way to any house, and to estimate the
length of time needful for reaching it. St. Louis and some other large
cities have adopted the Philadelphia plan, the convenience of which
is as great as its monotony. In Washington the streets running in
one direction are lettered A, B, C, etc., and the cross streets are
numbered; and upon the checkerboard plan is superposed another plan in
which broad avenues radiate in various directions from the Capitol,
and a few other centres. These avenues cut through the square system
of streets in all directions, so that instead