National Geographic : 1890 Apr
National Geographic Magazine. ing the difference of the equations to be 0s.26. On May 1, one set gave 0 s.3 2 , and another 01.29. On May 2, only one set was made giving 0.36, a variation of 0s.07 in two days. In June 1884, one year later, another series of observations of the same character was made at the Naval Observatory in Washington, and on the same nights the personal equation machine invented by Prof. Eastman, was used as a comparison. This is an instrument in which an artificial star is made to record its own transit over the wires of a reticle, while the observer records the same with a chro nograph key. The difference is manifestly the personal error of the observer. This gives the absolute equation of the observers, and their difference is the relative equation, and should accord with that found by the method of alternate stars. Some of the results were as follows :-On June 4, the difference by machine of their personal errors was 0s. 1 6 and by star observations O s .24, on the 15th of June the machine gave 0s.10 and the stars 0s.24, on the 16th, machine 01.14, stars, 0 s.13, a very close agreement, on the 17th, ma chine gave 0".07 and stars 01.18. The observer N. combined with another, C., who had not had as much experience in observing, gave still more discordant results. On June 20, the machine gave as their relative equation, 0 s.0 8 , while star observations gave 0s.27, on June 23, machine 0.13, stars 0".51, and on June 28, machine, 0.20, stars Os.35. In the case of the first two observers a mean of the determinations amounting to about 0s .20 might have been applied to the measurements made by them, but as these were made under all conditions of climate, in latitudes varying from 30° N. to 36° S. and in different states of health and bodily com fort, it was concluded not to introduce any correction at all rather than one that might be considerably in error. In all of the work it has been the custom as far as possible to place the observers alternately east and west of each other, so that the result of personal error in one measurement is neutralized to a greater or less extent in the next. Of course the method of exchanging stations and making two measurements of each meridian distance would afford the best solution of this problem, but except in certain favorable conditions, this is precluded by considerations of time and expense. In the measurement between Galveston and Vera Cruz mentioned above, it had been the intention to exchange stations, but by the time the first measurement was finished the season was rather far advanced, there was danger of yellow fever in Vera Cruz and an observer going there at that.