162 
Captain Kater's description of 
next me to expand ; a supposition which appears to be in 
some degree corroborated by the micrometer giving an in- 
creased reading on my return, after having been absent for 
some time from the Observatory. Whatever may be the 
cause, it constantly operates in one direction, and seems to 
be the principal source of the errors which are observable. 
I now wished to ascertain whether by encreasing the length 
of the float, or by adding to its weight, the length being the 
same, I should attain greater accurary. I therefore procured 
two other cast iron floats, the one twelve inches long, four 
wide, and a quarter of an inch thick, and the other of the 
same dimensions as that before described, except that its 
thickness was half an inch, and its weight 4 lb. 8oz. troy. 
Iron pins were fixed in the sides of these floats in place of 
the grooves, and grooves to receive the pins were attached 
to the sides of the box. The box in which both floats were 
used was fourteen inches long and six inches wide. 
Before I made trial of the new floats, I browned that used 
in the preceding experiments by rusting it with nitric acid, 
and then rubbing it with oil ; imagining that I might thus 
diminish any small affinity which the iron might have for 
the mercury. With the float thus browned, the following 
experiments were made. 
