PRITCHETT ON THE TRANSIT OF MERCURY. 613 



As the values used in the Berlin Jahrbuch depend upon meas- 

 urements with double-image micrometers, it is to be expected 

 that they should be smaller than those derived from the filar mi- 

 crometer measures. But the magnitude of the diffei-ences shows 

 that the values in use in the American Ephemerls at present must 

 be considerably in error, and, while it may be preferable to retain 

 in the Ephemeris values dependent on filar micrometer measure- 

 ments, there is needed a new set of determinations dependent 

 on recent measurements with an equatorial of moderate size and 

 good definitim. 



Mr. O. B. Wheeler, of the Missouri River Commission, has 

 been kind enough to communicate the result of his observations, 

 which are given herewith. 



OBSERVATIONS BY MR. O. B. WHEELER. 

 Observer, O. B. Wheeler; Recorder, G. A. Mark. 



Observations of the Transit af JMercitry^ May 9, IS91. 



Station — Government Geodetic Point. "Water Tower." 



St. Louis, Mo. 

 Latitude. 38'^ 40' 12"; Longitude, 90° 12' 32". 

 Height of station above ground, 160 feet. 



Instruments. — Telescope a new refractor of three inches clear aperture, 

 marked A. S. Aloe & Co., and generously loaned by this firm; mount- 

 ing, altitude, and azimuth ; magnifying power used, no. Chronom- 

 eter (mean time), John Hutton, No. 353. 



Atmospheric Canditions. — Sky perfectly clear, except somewhat smoky 

 near the horizon; atmosphere steady; a strong wind from the south, 

 and station not protected from it. h. m. s. 



The Results. — ist contact (Greenw. M.T.) 11 5405.3 (strong indenture). 

 2d '• " •' II 57 22.9 (geometric contact). 



2d •• •' '• 115744,2 (true contact). 



The Observations. — 



//. m. s. 

 1st contact, 5 52 47.5 (strong indenture when lirst seen). 

 2d •' 5 56 05.0 (geometrical contact, estimated). 



2d " 5 56 26.3 (light breaks across). 



Only one "tick" was given for each observation, and the times are 

 as noted by the recorder from the face of the chronometer. 



