If a person is desirous of ascertaining heights by a mountain barometer, let him begin by adjusting the instrument in his own study; and having made the upper contact, let him write down the reading of the vernier, and then let him derange the UPPER adjustment ONLY, re-adjust, and repeat the reading.

When he is satisfied about the limits within which he can make that adjustment, let him do the same repeatedly with the lower; but let him not, until he knows his own errors in reading and adjusting, pronounce upon those of the instrument.

In the case of a barometer, he must also be assured, that the temperature of the mercury does not change during the interval.
A friend once brought to me a beautifully constructed piece of mechanism, for marking minute portions of time; the three-hundredth parts of a second were indicated by it.

It was a kind of watch, with a pin for stopping one of the hands.

I proposed that we should each endeavour to stop it twenty times in succession, at the same point.