work around me. Similar shots were fired when the engine was at the centre of the 4th shaft (which could be seen from above). The times of these signals were taken by an assistant. During this time I was at the telescope, and noticed the second when any peculiar phase of disturbance appeared. The computation of the distance of the engine from the eye at a given time is very simple. From the known distance of the south end of the tunnel and the 4th shaft from the eye, we know the times taken by the sound of the gun to reach the observatory. The temperature was during the whole series so near 32° that the velocity of sound for that temperature, 363-13 yards, may be used without sensible error. The effect of wind must also have been insensible. Hence the signal from the south entrance was 1.77 too late, that from the shaft 1.84. Correcting the times and dividing by their difference the distance of the shaft from the entrance, 1162 yards, we have the velocity of the train (which, however, I have given in miles per hour, as affording a more familiar measure of the disturbing power). Then the difference of the time of phase and corrected time of entrance gives the place of the engine on the line, and the perpendicular is given. In the following record of the observations, the first column contains the number, the second the times, the third the facts observed, and the fourth gives the distance, then follow occasional remarks. In the disturbances, I specially recorded as most definite the cross (4), and the arrangement of bars of parallel stars (5). The slighter disturbances which precede or follow the former were seldom entered, though quite sensible. 58 o Cross very strong 478 58 10 Parallel lines (5) very 7 57 16 Image much agitated... 1077 Velocity 30'9 miles; train's weight 57 30 Cross strong 374 58 16 (5) still stronger... 329 38 58 25 (5) ten lines, quite cover 302 58 45 Cross very strong 431 59 14 Cross strong 803 1045 |