noticed when it was twelve, by the
clock in the house, and he found that, when it was twelve by the clock,
the shadow of the post came exactly to the line indicated by the
direction of the compass needle; and so he knew that that was a correct
meridian line.

JONAS'S DIAL.

That evening, Rollo told his father about his hour-glass, and also about
Jonas's noon line. His father said it was very difficult to draw a
meridian line.

"O no, father," said Rollo; "Jonas has drawn one, and he told me how,
and it was a very easy way."

"Yes," said his father, "it is easy to draw something which you can call
a noon mark; but it is a very difficult and delicate operation to do it
with any considerable degree of exactness."

"I think that Jonas's is exact," said Rollo.

"It probably may be as exact as he could make it with his means and
instruments; but there are a great many sources of error which he could
not possibly have avoided."

"What?" asked Rollo.

"Why, in the first place, the clock is not exact. It is near enough to
answer all the purposes of a family; but it may often be a minute or
more out of the way. Then besides, while Jonas is going from the clock
out to the barn, the shadow is slowly moving on, all the time; so that
he cannot tell exactly where the shadow was, when it was precisely
twelve by the clock.

"Then again, it is not always exactly noon when the shadow comes to the
north and south line. It varies a little at different seasons of the
year, though it is so near that we say, in general terms, that at noon
all shadows of upright objects point to the north. Still, it is not
_precisely_ true, except on a very few days in the year. Then, again,
the post of the barn door is not exactly upright."

"I thought they always made door posts exactly upright," said Rollo.

"They do make them as nearly upright as they can, with the common
carpenters' instruments; but they are not _exact_. To set a post of any
kind, with great precision, perpendicular to the horizon, would require
very expensive mathematical instruments, and very laborious and nice
observations. Then, again, if the clock had been exact, and the post
perfectly upright, Jonas could not have marked the place of the shadow
exactly. The shadow has not an exact and well-defined edge; and then,
even while he was marking at one end, the shadow would be moving along
at the other end, and so his noon mark would not be exactly straight."

"Why,