Note 22 — CHAPTER 8.

The observed meridian altitude of sun upper limb was 2° 52′ 51″. Temperature of the air minus 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 shows 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 minus 45.5°. So that in the parallel 68° 42′ where, if there is no refraction, the sun would 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, 1821. Observed meridian altitude of sun lower limb 2° 35′ 20″, sun apparent diameter 29° 20′. For apparent altitude 2° 35′ 20″ the mean refraction 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 minus 41°. Wind north, a light breeze, a large halo visible about the sun. January 15th, 1821. Observed an apparent meridian altitude sun lower limb 4° 24′ 57″. Sun 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 minus 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 calculating 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.—Mr. Hood’s Journal.