A red drop at plus 42 degrees indicated that the
fluid had risen to that point or above it. At this period the daily
visits of the sun were very short and, owing to the obliquity of his
rays, afforded us little warmth or light. It is half-past eleven before
he peeps over a small ridge of hills opposite to the house, and he sinks
in the horizon at half-past two. On the 28th Mr. Hood, in order to attain
an approximation to the quantity of terrestrial refraction, observed the
sun's meridian altitude when the thermometer stood at 46 degrees below
zero, at the imminent hazard of having his fingers frozen.
He found the sextant had changed its error considerably, and that the
glasses had lost their parallelism from the contraction of the brass. In
measuring the error he perceived that the diameter of the sun's image was
considerably short of twice the semi-diameter, a proof of the uncertainty
of celestial observations made during these intense frosts. The results
of this and another similar observation are given in the footnote.*
(*Footnote. The observed meridian altitude of sun upper limb was 2
degrees 52 minutes 51 seconds. Temperature of the air minus 45.5 degrees.
By comparing this altitude, corrected by the mean refraction and parallax
with that deduced from the latitude which was observed in autumn, the
increase of refraction is found to be 6 minutes 50 seconds, the whole
refraction therefore for the altitude 2 degrees 52 minutes 51 seconds is
21 minutes 49 seconds.
