. Appendix to Captain Parry's journal of a second voyage [microform] : for the discovery of a north-west passage from the Atlantic to the Pacific, performed in His Majesty's ships Fury and Hecla in the years 1821-22-23. Science; Botany; Sciences; Botanique. 168 MR. FISHER ON ATMOSPHERICAL REFRACTION. azimuth circle; and also, from the great contraction of the spirit within the principal level, that the extremities of the bubble would not be visible. The expedient in this case that suggested itself to me. so that the principle of repetition in the instrument would still apply, was by repeating
. Appendix to Captain Parry's journal of a second voyage [microform] : for the discovery of a north-west passage from the Atlantic to the Pacific, performed in His Majesty's ships Fury and Hecla in the years 1821-22-23. Science; Botany; Sciences; Botanique. 168 MR. FISHER ON ATMOSPHERICAL REFRACTION. azimuth circle; and also, from the great contraction of the spirit within the principal level, that the extremities of the bubble would not be visible. The expedient in this case that suggested itself to me. so that the principle of repetition in the instrument would still apply, was by repeating in altitude by reflection from the surface of mercury, making the horizontal wire of the back telescope to bisect the illumined horizontal row of holes upon the meridian mark, m order to ensurtl from the instrument, as well as the necessity of continually changing its situation as the star moved in azimuth. The I next employed was by observing the difference of the North Polar distances of high and h)W stars as they passed the meridian, keeping the horizontal wire of the back telescope (which was clamped to the circle) in contact with the meridian mark as before; this methcxl, although probably sufficiently ;iccurate for very low altitudes, yet was subject to two sources of inaccuracy, which were. 1st. the uncertainty of a distant terrestrial objtx't being alike refracted during the interval of the meridianal passages of the two stars, and. 2nd. the principle of repetition was lost. Both these exj)edients were ultimately rendered useless, for as the winter advanced and the temperatures bocanu* very low, I found the difficulties in the use of this instrument in the way were not real, as the atilfiiess or difficulty in the azimuth motion arose more frt)m the gradual accumulation of ice in the centre-work than from the eflect (tf the cold uiKjn the metals; Ibr by using the precaution of expelling every particle of ice from the instrum»Mit before every observation,
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Keywords: ., bookcentury1800, booksubjectbotany, booksubjectscience, bookyear1
