Wild Sports or the West.
L'Union Medicale—name withheld by request of the gentleman.
London Lancet.
TO FIND THE TIME OF TWILIGHT.
To the Editor of the Scientific American:
Given latitude N. 40° 51', declination N. 20° 25', sun 18° below the horizon. To find the time of twilight at that place. In the accompanying diagram, E Q = equinoctial, D D = parallel of declination, Z S N a vertical circle, H O = the horizon, P = North pole, Z = zenith, and S = the sun, 18° below the horizon, H O, measured on a vertical circle. It is seen that we have here given us the three sides of a spherical triangle, viz., the co-latitude 49° 9', the co declination 69° 35', and the zenith distance 108°, with which to compute the angle Z P S. This angle is found to be 139° 16' 5.6". Dividing this by 15 we have 9 h. 16 m. 24.4 s., from noon to the beginning or termination of twilight. Now, in the given latitude and declination, the sun's center coincides with the horizon at sunset (allowance being made for refraction), at 7 h. 18 m. 29.3 s. from apparent noon. Then if we subtract 7 h. 18 m. 29.3 s. from 9 h. 16 m. 24.4 s., we shall have 1 h. 57 m. 55.1 s. as the duration of twilight. But the real time of sunset must be computed when the sun has descended about 50' below the horizon, at which point the sun's upper limb coincides with the line, H O, of the horizon. This takes place 7 h. 16 m. 30.8 s. mean time. It is hoped the above will be a sufficient answer to L.N. (See SCIENTIFIC AMERICAN of Dec. 1, 1883, p. 346.)
B.W H.