ments. 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. We
were both equally unpractised, and our first endeavours showed that we
could not be confident of the twentieth part of a second. In fact, both
the time occupied in causing the extremities of the fingers to obey the
volition, as well as the time employed in compressing the flesh before
the fingers acted on the stop, appeared to influence the accuracy of our
observations. From some few experiments I made, I thought I perceived
that the rapidity of the transmission of the effects of the will,
depended on the state of fatigue or health of the body. If any one were
to make experiments on this subject, it might be interesting, to compare
the rapidity of the transmission of volition in different persons,
with the time occupied in obliterating an impression made on one of the
senses of the same persons. For example, by having a mechanism to make
a piece of ignited charcoal revolve with different degrees of velocity,
some persons will perceive a continuous circle of light before others,
whose retina does not retain so long impressions that are made upon it.

SECTION 3. ON THE FRAUDS OF OBSERVERS.

Scientific inquiries are more exposed than most others to the inroads of
pretenders; and I feel that I shall deserve the thanks of all who really
value truth, by stating some of the methods of deceiving practised by
unworthy claimants for its honours, whilst the mere circumstance of
their arts being known may deter future offenders.