XX
INTRODUCTION
Both of these had been commenced in the summer of 1881.
In order to push on the three-fold task to his satisfaction he
devoted to it the greater part of the autumn vacation. Thus
the investigation on electric inertia was soon finished; on the
other hand the evaporation problems took up much more time
without giving much satisfaction.
15th October 1881.
I am now devoting myself entirely to the research on evapora-
tion, which I began thinking of in the spring, and of which I have
now some hope.
10th March 1882.
The present research is going on anything but satisfactorily.
Fresh experiments have shown me that much, if not all, of my
labour has been misapplied; that sources of error were present
which could scarcely have been foreseen, so that the beautiful
positive result which I thought I had obtained turns out to be
nothing but a negative one. At first I was quite upset, but have
plucked up courage again; I feel as fit as ever now, only I do
regret the valuable time which cannot be recovered.
13th June 1882.
I am writing out my paper on evaporation, i.e. as much of the
work as turns out to be correct; I am far from being pleased with
it, and feel rather glad that I am not obliged to work it out com-
pletely, as originally intended.
In the midst of this period of strenuous exertion comes
the slight refreshing episode of the invention of the hygro-
meter (VII. in this volume). In sending a charming de-
scription of this little instrument, "so simple that there is
scarcely anything in it," Hertz explains to his parents how
the air in a dwelling-room should be kept moist in winter.
There can be no harm in reproducing the explanation here.
2nd February 1882.
On an
I may here give a little calculation which will show father how
the air in the morning-room should be kept moist.
average the atmosphere contains half as much water-vapour as is
required to saturate it; in other words, the average relative
humidity is 50 per cent. Assume then that this proportion is
suitable for men, that it is the happy-or healthy-mean. In a
cubic metre of air there should then be definite quantities of water,