As they topped the brow of the slope, the professor tightened the reins a little.

It wouldn't do to let the old mare overwork herself.

But, instead of slackening her pace, she sprang forward more swiftly than ever.
"That's odd!" murmured the old gentleman.

"Can any thing be the matter, I wonder ?" and he gave another steady pull on the reins.

The wagon was jerked forward with such a wrench as almost to throw him backward.