its bottom was at least 42° above zero. This fact was ascertained by
a spirit thermometer; in which, probably, from some irregularity in
the tube, a small portion of the coloured liquor usually remained at
42° when the column was made to descend rapidly. In the present
instance the thermometer standing at 47° below zero, with no portion
of the fluid in the upper part of the tube, was let down slowly into
the water, but drawn cautiously and rapidly up again, when a red
drop at + 42° 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 was half past eleven before he peeped over a small ridge of
bills opposite to the house, and he sank 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° below zero, at the
imminent hazard of having his fingers frozen.
H e 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 at the bottom of the page*.
* i( The observed meridian altitude of © upper limb was 2° 52' 51". Temperature
of the air — 45° 5'. 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' 50", the whole refraction, therefore, for the altitude 2° 52' 51"
is 21' 49". Admitting that the refraction increases in the same ratio as that of the atmosphere
at a mean state of temperature, the horizontal refraction will be 47' 22". But the
diameter of the sun measured immediately after the observation, was only 27' 7", which
shews an increase of refraction at the lower limb of 3' 29,/. The horizontal refraction calculated
with this difference, and the above-mentioned ratio, is 56' 3", at the temperature
— 45° 5'. So that in the parallel 68° 42', where, if there was no refraction, the sun would
The aurora appeared with more or less brilliancy on twenty-eight nights in this month, and we were also gratified by the resplendent
beauty of thé moon, which for many days together performed its
circle round the heavens, shining with undiminished lustre, and
scarcely disappearing below the horizon during the twenty-four
hours.
During many nights there was a halo round the moon, although
the stars shone brightly, and the atmosphere appeared otherwise
clear. The same phenomenon was observed round the candles, even
in our bed-rooms ; the diameter of the halo increasing as the observer
receded from the light. These haloes, both round the moon and
candles, occasionally exhibited faintly some of the prismatic colours.
As it may be interesting to the reader to know how we passed our
be invisible for thirty-four days, his upper limb, with the refraction 56 3 , is, in fact, above
the horizon at every noon.
The wind was from the westward a moderate breeze, and the air perfectly clear. January
1st, 1281. Observed meridian altitude of © lower limb 2° 35' 20". © apparent
diameter 29° 20'. For apparent altitude 2° 35' 20", the mean refract^' n is 16' 5' (Mackay’s
Tables,) and the true, found as detailed above, is 20' 8" : which, increasing in the same
ratio as that of the atmosphere, at a mean state of temperature, is 41' 19" at the horizon.
But the difference of refraction at the upper and lower limbs, increasing also in that ratio,
gives 55' 16" for the horizontal refraction. Temperature of the air — 41°. Wind north,
a-light breeze, a large halo visible about the sun. January 15th, 1821.—Observed an
apparent meridian latitude © lower limb 4° 24' 57". © apparent diameter 31 5 . For
apparent altitude 4° 24' 57", the mean refraction is 10' 58" (Mackay’s Tables,) and the
true, found as detailed above, is 14' 39", which increasing in the same ratio as that of the
atmosphere at a mean state of temperature, is 43' 57" at the horizon. But the difference
of refraction between the upper and lower limbs, increasing also in that ratio, gives 48' 30"
for the horizontal refraction.
Temperature of the air - 35°, a light air from the westward, very clear.
The extreme coldness of the weather rendered these operations difficult and dangerous ;
yet I think the observations may be depended upon within 30", as will appear by their
approximate results in oajculating the horizontal refraction; for it must be considered
that an error of 30", in the refraction in altitude, would make a difference of several
minutes in the horizontal refraction.”—M r . H o o d ’s Journal. 2 L