. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . e. Even the colored glass shadeis not always necessary—as when the disc of the sun is just seen APPENDIX. 621 through thin clouds and is not too bright to be observed with thenaked eye. The colored glass shade may be merely a pieceof colored glass fitted over the eye-piece, or the glass may beset into a frame very similar to the object glass cover and readilytaken off and put on. In the latter case the glass must be
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools, and a hand-book for the use of engineers in field and office . e. Even the colored glass shadeis not always necessary—as when the disc of the sun is just seen APPENDIX. 621 through thin clouds and is not too bright to be observed with thenaked eye. The colored glass shade may be merely a pieceof colored glass fitted over the eye-piece, or the glass may beset into a frame very similar to the object glass cover and readilytaken off and put on. In the latter case the glass must be optically perfect/ , with the sides perfectly plane andparallel, so that there shall be no refraction of the image, orsuch glass as is used for the sun shade of a sextant. The second method (h) does not require any calculation of aformula; the true meridian is given directly but it requires theuse of a special instrument, whose adjustments must be madewith great care or the resulting azimuth will often be in error bya much larger amount than the error in the adjustment. Aproper appreciation of either method requires an understand-ing of certain astronomical relatione. Fig. 1. Fig. 1 represents the orthographic projection of the celestialsphere, projected on the plane of the meridian of the P Z E represents the meridian of the observer. Z = the = the polar axis of the = the plane of the equator. ^ = the position of the = the latitude of the observer = (f>. ZP = 9O°- = co0. h. SG = the true altitude of the sun SZ = 90°-h = coh. ST = the declination of the sun, north or south of the equator = = -90 ° — 5 = CO 5, also called p = polar distance. 6^2 RAILROAD dONSTRtJCTiON. The essential sign of 5 must be considered. If the sun issouth of the equator (as it is from about September 21 to March21), 5 is negative and if the declination is (say) S 20°, 5= —20°.Then CO 5 = 90°-5 = 90°-(-20°) = 110°. Z = the angle from the pos
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Keywords: ., bookauthorwebbwalt, bookcentury1900, bookdecade1920, bookyear1922
