TRANSACTIONS OF SECTION A. 34.7 
to many others) that instructive experiments might be made hy observing the 
temperatures of two equal and similar thermometers, placed beside each other on 
a wooden table (or on two similar tables of the same material), or on a cushion or 
layer of very fine cotton-wool ; each thermometer between the folds of a doubled 
sheet ; one of white cloth and the other of black; both exposed in the open air 
under sunlight, or under the light of a more or less cloudy sky, or under moon- 
light or starlight, or in the darkest attainable cellar. 
7. Not being able at the time to undertake any experimental work, the 
author asked Dr. Glazebrook if he could conveniently allow some such experi- 
ments to be made under the auspices of the National Physical Laboratory. He 
kindly consented, and asked Dr. Chree to commence an investigation of the kind. 
On July 28 the author received from Dr. Chree the appended description of his 
work and statement of results. 
8. It is very interesting to see in Dr. Chree’s results how large are the differ- 
ences in the temperatures of the thermometers under black and under white cloth, 
ranging from 0°5° to 0:6° C., even at times when the sky is covered with dark 
clouds; and how comparatively moderate are the differences, ranging from 1-]° to 
3:6° C., at times of exposure to direct sunshine. 
9. Returning to §4, with one of the globes black over its whole surface and 
the other white: Suppose the two to be taken to 1,000 times the earth’s distance 
from the sun, and suppose, all at about the same distance (for simplicity of 
calculation), 999 stars, each equal to our own sun, to be scattered through space 
round the place of our ideal experiment. The total of radiational energy coming 
from all these suns to the place of observation will be about one-thousandth of 
the amount, per unit of time, coming from our own sun in the case of §§ 1, 2, 3; 
and the difference of steady temperature between the white globe and the black 
globe may be about one-thousandth of that which it would be in §§ 1, 2,3. This 
last supposition would be somewhat similar to an exposure to starlight on a cloud- 
less night, at the top of a high mountain of our earth, with a large number of large 
screens between the tested globes and the mountain top. It does not, however, 
seem probable that any differences of temperature will be perceptible on the two 
thermometers exposed only to stellar radiation from the sky. Even less of 
difference may be expected when the two thermometers are the darkest attainable 
cellar. The bolometric method would, of course, be much more sensitive than 
the comparison of two ordinary thermometers, even of the most extreme 
sensibility, and it will, the author thinks, be worth while to try, in cases in which 
the thermometric method fails, or almost fails, to show any difference between 
the two temperatures. 
3. On Temperatures of Thermometers wnder Black Cloth and under 
White Cloth. By Dr. Caartes Curer, F.2.S. 
Experiments with two ordinary thermometers, Nos. 1184 and 1207, Thermo- 
meters placed horizontally on small stands fastened to the outside window-sill 
of the wooden room on the Observatory roof, some 40 feet above the general 
level of the ground, The arrangement was as shown diagramatically, 
i North 
SS SS SP 
The north side of this upper room is entirely in the shade until well on in the 
afternoon. The cloths were wrapped several times round the bulbs and a small 
adjacent part of the stems, The thermometers were interchanged and the cloths 
were interchanged to eliminate difference between positions and between ther- 
mometers. Readings were taken at intervals throughout two days, 
