assistant at the valve; so that the column of water is kept at about the
same place, and the pressure thus rendered very nearly constant. The
result was satisfactory, though not in the degree anticipated. It was
possible to keep the mirror at a constant speed for three or four seconds
at a time, and this was sufficient for an observation. Still it would have
been more convenient to keep it so for a longer time.

I am inclined to think that the variations were due to changes in the
friction of the pivots rather than to changes of pressure of the blast of
air.

It may be mentioned that the test of uniformity was very delicate, as a
change of speed of one or two hundredths of a turn per second could easily
be detected.



Method Followed in Experiment.


It was found that the only time during the day when the atmosphere was
sufficiently quiet to get a distinct image was during the hour after
sunrise, or during the hour before sunset. At other times the image was
"boiling" so as not to be recognizable. In one experiment the electric
light was used at night, but the image was no more distinct than at
sunset, and the light was not steady.

The method followed in experiment was as follows: The fire was started
half an hour before, and by the time everything was ready the gauge would
show 40 or 50 pounds of steam. The mirror was adjusted by signals, as