[26] Miss Short, who has erected and who superintends an observatory on the Calton hill, Edinburgh, is the descendant of a brother of Mr. Short. She is in possession of a large Gregorian reflector, about 12 feet long, made by Mr. Short, and mounted on an Equatorial axis. It was originally placed in a small observatory erected on the Calton hill, about the year 1776, but for many years past it has been little used.
[27] A particular account of the Earl of Rosse’s fifty-feet Reflector, which is now finished, is given in the Appendix.
[28] Philosophical Transactions for 1800, Vol. XC. p. 80, &c.
[29] In using telescopes within doors, care should generally be taken, that there be no fires in the apartment where they are placed for observation, and that the air within be nearly of the same temperature as the air of the surrounding atmosphere; for if the room be filled with heated air, when the windows are opened, there will be a current of cold air rushing in, and of heated air rushing out, which will produce such an undulation and tremulous motion, as will prevent any celestial object from being distinctly seen.
[30] The above directions and remarks are abridged with some alterations from Dr. Pearson’s “Introduction to Practical Astronomy.”—Vol. II.
[31] Pearson’s “Practical Astronomy.”—Vol. II.
[32] The mother-of-pearl dynameter may be purchased for about twelve shillings. See fig. 57, a, b, c, p. 260.
[33] Reid’s Enquiry into the Human Mind, chap. iv.
[34] The distance of Saturn from the sun is 906,000,000 of miles; it is sometimes nearer to and at other times farther from the earth, according as it is near the point of its opposition to, or conjunction with the sun. If this number be divided by 200, the supposed magnifying power of the telescope, the quotient is 4,530,000, which expresses the distance in miles at which it enables us to contemplate this planet. If this number be subtracted from 906,000,000, the remainder is 901,470,000, which expresses the number of miles from the earth at which we are supposed to view Saturn with such an instrument.
[35] Irish Transactions, Vol. X. and Nicholson’s Philosophical Journal, Vol. XVI.