196 Mr, Powell's experimental inquiry into the nature of 
(15.) In these experiments it will be evident upon inspec- 
tion, that the ratio of the effects produced on the white and 
black bulbs, is in every instance considerably greater when 
they were affected only by that part of the total heating in- 
fluence which is transmitted through a transparent screen, 
than when they were exposed to the whole. This then would 
indicate, that on the removal of the screen some new heating 
power was brought into action which affected the ratio by the 
addition to each of its terms, of quantities in a ratio expressed 
by that of the difference of the exposed and screened results 
above given. This ratio is evidently one differing a little 
from equality, and agreeing nearly with that of the diameters 
of the bulbs inversely. 
(16.) The experiments now detailed will probably be con- 
sidered sufficient to substantiate the conclusion ; but in re- 
searches of this kind, where great numerical precision is 
unattainable; it seemed desirable to give the experiments 
that confirmation which they wanted in point of intrinsic 
accuracy, by frequent repetition and variation. With this 
view I made a great number of trials with a large differential 
thermometer ; the bulbs were about one inch in diameter 
and nearly three inches apart. The bore of the tube was 
about of an inch. Many of the experiments made with 
this instrument I shall not mention, as, although all agree- 
ing to confirm my former conclusions, they were complicated 
by several unnecessary conditions. 
(1 7.) In order to obtain results in the most simple manner, 
it was desirable to get rid of any action on one of the bulbs, 
and to expose only the other ; the instrument thus acting 
simply as an air thermometer. The effects on each bulb, one 
