350 
PHYSICS: C. BARUS 
results for three different apparatus, observations on the same vertical 
being made at the same time. The individual observations were taken 
thirty minutes apart Three or four means were deduced for the day. 
Apparatus I on the N.S. face of the pier fronting East has already been 
referred to in connection with figure I. Apparatus II was placed in 
a niche on the EW. wall of the pier fronting north, surrounding on all 
sides within 1 meter by the interior brick walls of the building. It 
thus receives secondary radiation only, and the graph in its details, 
departs utterly from curve I, particularly on clear days. If curves I 
and II were smoothed, however, they would show some resemblance. 
Curve III are the results for the metal case (on the E.W. wall fronting 
south) with the needle kept in the partial vacuum, pressure ^, marked 
on the curve. On the morning of September 12 and 13 at 39 and 36 
cm. the two bodies M and m repelled each other. Even at lower pres- 
sures {p = 8, 5, 3, etc.) the results seemed fluctuating. Hence after 
September 14, I observed for p < 1.5 cm., only (numeral omitted), 
there being a sHght leak in the apparatus so that a higher vacuum 
could not be held for a half hour. The astonishing feature of these 
high vacuum (III) results is that they agree very closely with the 
observations (I) made in a plenum; whereas if III had also been observed 
in a plenum, the results would necessarily be in total opposition to I, 
as the repulsions at the beginning of curve III indicate. 
Now it may be shown by direct tests {Science, I.e.) that the radiant 
forces of a hot body, M are repulsions for ^ < 4 cm. and attraction for 
higher pressures in the case. The exact pressure of radiant equilibrium 
resulting from this inversion is for incidental reasons difficult to specify; 
but one may estimate that in high vacua, y varies about 5 mm. per 
cm. of pressure p. 
It follows from this that in the plenum apparatus (I) with eastern 
exposure, the attracting body M must be relatively warm in the morn- 
ing and cold in the afternoon; while in the vacuum apparatus (III) 
with southern exposure, the body M is relatively cold in the morning 
and warm in the afternoon; for in such a case the radiant forces have 
the same sign. Hence the agreement in kind of the plenum graph, I, 
and the vacuum graph, III, in figure 2, after September 14, is a demon- 
stration in a dark room, at practically constant temperature, of the 
rotation of the sun. 
1 Advance note from a Report to the Carnegie Institute of Washington, D. C. 
