460 TRANS. ST. LOUIS ACAD. SCIENCE. 



to be conclusively settled that this river valley does not have the marked 

 influence upon the magnetic needle which had been suggested in previous 

 reports. It is. of course, possible that the eft'ect here is masked by other 

 effects due to other causes. It is, perhaps, well to consider the question an 

 open one until other regions which now indicate the influence of variations 

 of contour are examined with more minuteness.* The later work of the 

 summer showed that the area of maximum declination which had been 

 detected lies to the east of Jefferson City, extending as far as Hermann. If 

 it is connected with the region of equal values to the west, it is by a nar- 

 raw isthmus immediately south of Jefferson City. 



Station 90 a. — Marion, Cole Co. Lat. 38° 42'; Ion. 92° 25. About 13 

 miles above Jefterson City. Station on the right bank of the Missouri river 

 about midway between the road and the water's edge, and about 1,000 ft. 

 below the village. The instrument was set under a large elm tree. Two 

 polar observations and two complete declinations. 



Station 90 b. — This station bears S. ico° 58' W. from station a. and is 

 distant about Scoft. The meridian at this station was deduced from obser- 

 vations at station a. by an obvious method explained in the previous report. 

 The magnetic meridian was determined as follows : — Before the declinome- 

 ter was dismounted at station a., the University magnetometer was set up, 

 and the two needles were read simultaneously at 5 p.m. The declinometer 

 was then dismounted and was set up at station b., and simultaneous read- 

 ings were again made, at 5:30 P.M. The magnetometer needle changed 

 I'. 4, and this correction was applied to the reading at station^. This 

 practically makes the result at this station as good as that at station a. 

 The result at station b. differed 3'.^ from the mean at station a., and the 

 two at a. differed 2'.g from each other. Instead of being more than at Jef- 

 ferson City, the declination was about 50' less. Hence from Jefferson City 

 the declination diminishes very rapidly to the N.W. 



Station 91 — Providence, Boone Co Lat. 38° 49'; Ion. 92° 28'. Near 

 the E. line of sec. 20, tp. 47, r. 13 W. The station of observation was on 

 the northern edge of the village, near the Columbia road. Starting at 

 the N.E. corner of the residence lot owned by J. T. McBain, the station is 

 reached by going in an easterly direction, at right angles to the front fence, 

 a distance of 8 ft., and then northerly but parallel to the front fence a dis- 

 tance of 165 ft. The Columbia road passes in front of McBain's house, 

 and then turns westward winding up the steep bluft". Polaris observa- 

 tion and morning elongation of the needle. The declination was practi- 

 cally the same as at Marion. From this point the declination evidently 

 increases to the N.W., although not rapidly, the change between Provi- 

 dence and Arrow Rock being only 16'. 



* The observed declinations in the Magnetic Survey of Fiance and Belgium, by Rev. 

 S. J. Perry, published in the Trans, of the Royal Soc. of London, 1872, indicate a similar 

 tendency of the needle to set at right right angles to contour lines. The stations are not 

 sufficiently numerous, however, to be conclusive. 



