324 PROCEEDINGS OP THE AMERICAN ACADEMY. 



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 apparatus closely resembling the first. While planning a new 

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

 enough known, that difference of magnetic attraction on 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. Next 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 milligram in twelve hundred grams, it would have been detected, but 

 no such difference existed. We found by trial how much a weight of a 

 hundred grams 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 Rumford Committee of the American Academy of Arts and Sci- 

 ences having made a grant in aid of this experiment, we carried out our 

 original plan of making a steel structure so rigid as to permit easy and 

 satisfactory observation. In this new apparatus all the optical parts are 

 carried by a steel frame built of plate and angle-iron, somewhat like a 

 bridge girder. A cubical steel box, fourteen inches on each edge, con- 

 stitutes the centre of the structure, which is in the form of a cross. To 

 each of the four sides of this cube are firmly attached arms, each about 

 six feet and a half in length. Each arm is made of steel plates, three 

 eighths of an inch thick, eighteen inches wide at one end, and six inches 

 wide at the other, standing on edge, and kept fourteen inches apart by 

 suitable plates, angle-irons, and other braces ; thus are formed hollow 

 beams of great rigidity, especially in a vertical direction. This frame- 

 work is shown in Plate 1, from which it is seen that the structure is in 

 effect two rigid beams, each fourteen feet long, crossing at right angles, 

 and symmetrical as regards strength and rigidity. 



On two ends of the cross, S and T, Figure 1, are two upright cast-iron 

 frames, fastened by bolts, each of which carries four mirrors, marked 2, 

 4, 6, and 8. Against the corners of each of these frames rest four pine 

 rods, three quarters of au inch in diameter and fourteen feet long. 

 Each rod is supported throughout its length by a brass tube an inch in 





