COPY.
REPORT ON DR. WALKER'S NOTE ON
HYGROMETRIC METHODS.
2:237 1
432
RES
IR 26 APR 19
4
I am aware that the methods of measuring humidity
have been frequently investigated by physicists of first
class repute both in Europe and America.
I am also aware that the use of Blanford's tables
for deducting the humidity from readings of sling thermome-
-tera ie open to critcism. It was to remedy this defect
that the correspondence was opened.
As regards my statement that the Hong Kong practice
is known to be more accurate than the Indian method, in his
note of 17th. November, 1917, Dr. Walker states that "a
sufficiently sensitive thermometer swung round one end, in
the shade for two minutes, will give a true reading of the
air temperature; and that for a wet bulb thermometer the
result is alightly more accurate than that given in the
shed under existing conditions".
The situation appears to me to be this:-
The psychrometric factor A varies considerably with
the wind velocity until the latter reaches a certain value.
For unaspirated thermometers therefore a different formula
ie required for each velocity below this value. The labour
of such a procedure would, however, be prohibitive, and up
to the date of the St. Petersburg Conference the Interna-
-tional Meteorological Committee were unable to make
definite recommendations as regards hygrometric methods.
In 1906 however Hr. J.R. Sutton, of Kimberley,
suggested a solution which I recommend in my report of 14th.
April, 1918; namely, that the readings of unaapirated wet
bulb thermometers should be corrected for wind velocity.
I recommend the use of the sling thermometer for
first class Observatories because I have always considered
this