of rotation, and this movement is indicated upon a dial."

I nodded.

He cleared his throat again. "Of course, that is unsatisfactory."

"Damnably!" said I, but this reply did not check him.

"It works tolerably well at high speeds; at low speeds it is useless;
and then again there is a very rapid fluctuation, and the instrument is
of only approximate precision."

"Not it!" said I to encourage him.

"There is one exception," he continued, "to this principle, and that is
a speedometer which depends upon the introduction of resistance into a
current generated by a small magneto. The faster the magneto turns the
stronger the current generated, and the change is indicated upon a
dial."

"Yes," said I sadly, "as in the former case so in this; the change of
speed is indicated upon a dial." And I sighed.

"But this method also," he went on tenaciously, "has its defects."

"You may lay to that," I interrupted.

"It has the defect that at high speeds its readings are not quite
correct, and at very low speeds still less so. Moreover, it is said that
it slightly deteriorates with the passage of time."