to detect the FitzGerald-Lorentz Effect. (>83 



by excessive displacement of fringes owing to temperature 

 changes and the like. But patience is a possession without 

 which no one is likely to begin observations of this kind. 

 Rons of twenty and thirty turns, involving 320 or 480 

 readings, were not uncommon. A run of thirty turns meant 

 that the observer, who could sometimes make a turn of sixteen 

 readings in 65 or 75 seconds, walked half a mile while making 

 the severe effort involved in keeping his eye at the moving 

 eyepiece without the least interruption for half an hour. The 

 work is, of course, somewhat exhausting. 



Observation with this apparatus could not begin till the 

 month of August, and we had to stop without having accom- 

 plished as much as was desirable. During the busy season 

 of the school year, observation is impossible. We had there- 

 fore expected to resume our work in June. But we then 

 found that our pine apparatus had so much suffered from the 

 dry air of the building, that we could not maintain the 

 adjustment of our fringes. We could not, in the time, build 

 another apparatus of timber which had not been dried all 

 winter : nor was it thought well to construct another appa- 

 ratus closely resembling the first. While planniug a new 

 apparatus, we made a couple of experiments to show, what 

 was well enough known, that difference of magnetic attraction 

 <>n the iron parts of our apparatus could not disturb our 

 observations. We suspended two massive pieces of iron at the 

 ends of one arm, so that one should be in the lines of magnetic 

 force of the earth's field, and the other transverse to them, 

 these relations being reversed on reversing the position of the 

 apparatus. But observations with this load of iron gave the 

 same result as before. Xext we placed an analytical balance 

 on one arm, with which to weigh a bar of iron at the extremity 

 of that arm. It was so placed that at one azimuth the bar was 

 nearly in the lines of force, and at another was transverse 

 to them. If there were a difference of half a milligramme 

 in twelve hundred grammes, it would have been detected; 

 bot no such difference existed. We found by trial how much 

 a weight of a hundred grammes displaced our fringes, and so 

 learned. as was known before, that the influence of the earth's 

 magnetism could not be a disturbing factor. 



The Rum ford Committee of the American Academy now 

 came to our aid, and we carried out our original plan of 

 making a steel structure which should permit easy and satis- 

 factory observation. In this apparatus, all weights are carried 

 by two steel girders which intersect in a cross. With steel, 

 we could have perfect symmetry in the two arms, which is 

 impossible with wood. On the steel framework, two sets of 



