kettle of
fish! A hot room, for example, would expand the rod and lengthen it."

"And make the clock tick slower," put in Christopher eagerly.

"Precisely."

"Then the clock would go slower sometimes than others."

"Exactly that! The variation was not great, of course, and we now have
learned how to meet it by lengthening or shortening the pendulum by
means of a screw placed near the bob. Nevertheless the variation is
there. A common wire pendulum will vary approximately a minute a week; a
brass rod will, on the other hand, vary that same minute in five days
instead of seven. Wood, a material showing less change than metal, will
vary only a minute in three weeks.

"All this we have learned to make allowance for. But the poor old
clockmakers had to gather these facts by long and tiresome experiment.
At length brass pendulums which, they discovered, made the most trouble,
were replaced by those of iron or lead which, being of softer material,
expanded and contracted more readily. In our day you will sometimes see
a very finely adjusted astronomical clock whose pendulum terminates in a
hollow glass or iron receptacle filled with mercury, instead of the
usual metal bob."

"There are two of them at the store."

"To be sure there are! For the moment I had forgotten that."

"And all this time while clockmakers were fussing round about bobs and
pendulums, did the people have to keep on running to the cathedral or
the public square to find out what time it was?"

"No, indeed! By 1600 you could buy for a moderate sum a clock to use at
home. Not that it was a very accurate timekeeper. Nevertheless it gave a
fair idea of the hour, which was all that was demanded of it," laughed
McPhearson, busying himself with his screwdriver.

"What sort of clocks were the first ones?"

"They were not like ours, you must remember that. There was, for
instance, the bird-case clock, a small chased or perforated brass affair
from four to five inches square, and named because its shape suggested a
cage for birds. I spoke of it before. Then there was the lantern clock.
Both these varieties were made to hang on the wall and were wound by
pulling down the weights that dangled from them."

"They had no springs, pendulums or things?" questioned Christopher
wonderingly.

"That was before the days of springs. This particular type of clock,
however, had a pendulum; but it was only a pendulum driven by weights
showing the pendulum idea in its c