178 Prof. A. M. Mayer on the Magnetic Elongations 



national Coast Survey its renowned precision, and in rendering 

 accurate the comparisons and constructions of our Office of 

 Weights and Measures. A detailed description, with drawings, 

 of this instrument will be found in the " Report on the Construc- 

 tion and Distribution of Weights and Measures, Washington, 

 1857," written by Dr. A. D. Bache, the late illustrious Presi- 

 dent of our National Academy*. 



The measuring -instrument. 

 I will now describe the actual adaptation of this instrument 

 to our research. The drawings (figs. 1 and 2) give respectively % 

 an elevation and a plan of the apparatus. A beam of Georgia 

 pine, well seasoned, dried and then soaked with shellac varnish, 

 formed the base on which the instrument was lined and firmly 

 attached. This beam is 7 feet long, 8J inches deep, and 5f 

 inches wide. It rested on slips of hard wood at i, i, placed at 



Fig. 1. 



Fig. 2. 



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distances from the ends of the beam equal to one fourth of its 

 length. At a and b are two Vs of brass which supported the 

 terminal brass caps of the rods experimented on. These rods 

 were all 60*1 inches long, *5 inch in diameter; and each rod 

 weighed on the average 1520 grammes. While the ends of a 

 rod rested on the Vs, 1100 grms. of its weight was supported 

 by the two springs s, s, which took hold of the rod at distances 

 from its ends equal to one fourth of its length. The flexure of 

 the rod was thus in great part avoided ; and it could therefore 



* To my friend and colleague Dr. J. E. Hilgard, of the United-States 

 Coast Survey Office, I am indebted for the loan of the comparator used in 

 these researches. 



