AT NIGHT, FROM THE EARTH, ETC. 
125 
their stems passing through a piece of wood planted firmly in the ground, and 
whose thickness was two inches and breadth three inches, with the bulbs at least 
nine inches from the wood. 
Bodies in the state of powder were generally in quantity such that about thirty square 
inches of surface were exposed, and their thickness was about half an inch. The 
filamentous substances exposed a surface of about 100 square inches, and their thick- 
ness was about half an inch. With respect to the other substances, the circumstances 
under which the observations have been made are sufficiently explained in the sec- 
tion of observations. The thermometer was laid on long grass in such a manner 
that the bulb was not covered by a single blade; on short grass it rested on the fork 
of two Y’s, so that the bulb was sustained on the top of the grass ; on metallic plates 
it was so laid that the bulb occupied the centre of the plate ; on substances in powder 
it was so placed that the bulb occupied the centre of the mass, and just in contact 
with it, the stem being supported by pieces of wood ; on filamentous substances the 
bulb occupied the centre, and care was taken that it did not sink within, or was 
covered by any portion of the substance ; and the same plan was adopted throughout 
all the experiments. 
During a series of observations the thermometers were frequently removed from 
one substance to another ; those whose readings were the highest were interchanged 
with those which were the lowest, and so on : in the case of observations of the same 
kind of metals at different heights, or of different thicknesses, or indeed in any specific 
comparison, they were always interchanged among themselves ; it is possible that 
had the same instrument remained on the same substance, or in the same position, a 
more even set of readings in some cases might have been made : but this inter- 
change of instruments removed all doubt as to the cause of the differences in the 
readings being altogether due to the difference of position, or to the inherent quality 
of the body itself upon which the instrument was placed. Some of the other pre- 
cautions used in taking the observations were as follows. First, the observer ap- 
proached the instrument at that end which was the most distant from the bulb, and 
held his breath during the time ; and the readings were taken as quickly as possible, 
so that no heat should be imparted to them from the observer’s body. In con- 
sequence of their divisions being cut on glass, a difficulty was at first experienced in 
reading them without placing the reading-lamp too near to the instruments : habit 
after some time enabled the observer to place the lamp so that the divisions were in- 
stantly perceived. Another and a greater difficulty was that of readily seeing the 
top of the mercury, in consequence of the fineness of its column : after various ex- 
periments it was found that this became readily seen by slipping a piece of coloured 
card under the instrument at about the place where it was known the column termi- 
nated : the colours found to answer best were drab and yellow. I may mention here 
that attention was paid to every precaution that experience suggested, or reflection 
pointed out as desirable. 
