165 
call in medical aid, but a much larger number were only 
slight cases. 
The evaporation in my room in a square glass dish of the 
same size as the tin out of doors, and placed on book shelves 
sufficiently open for air to pass behind, gave from about 
0*85 mm. to Til mm. per day, not varying much from day 
to day, though, as a rule, greatest when the temperature out 
of doors was coldest. Average temperature of room 55° F, 
to 61° F. Dr. Volland’s figures reduced to millimeters give 
a mean evaporation in the room from IT 4 mm. to 1T8 mm. 
per day for the months of December, January, and February. 
Comparison of Instruments : Dry and Wet Bulb. 
My screen was made solely for the evaporation experi- 
ments and was very open, being too open for regular 
meteorological observations, as thus there was a danger of 
snow being blown in laterally ; on the other hand, for the 
comparison of instruments, this openness was an advantage ; 
so when a Mason’s (August’s) wet and dry bulb hygrometer, 
made by Casella, was offered to be lent me by Mr. Steffen, 
as well as two hair hygrometers similar to those used by 
the Swiss meteorological stations, offered me by Dr. Ruedi 
and Mr. J. Addington Symonds,* I yielded to the temptation 
and hung them up in my box, at an 18 inches lower level 
than my ice tin, and by short-interval observations followed 
their action in various ways through the winter. 
As is well known, the difficulties in obtaining the amount 
of moisture correctly with the wet and dry bulb thermo- 
meters, when the temperature is below freezing, are very 
great. There are difficulties which occur about the time 
that the freezing point is passed which probably cannot be 
got over, but I think that when the temperature is steadily 
below freezing for a long time, precautions may be 
taken which will lead to a greater amount of correctness 
* Mr. Martini, the optician, also lent me instruments for temporary 
comparison. 
