166 OGBURN— LATITUDE OBSERVATIONS. [April 20, 



The pier is composed of concrete, the top face being 27 inches 

 square, and rises from a foundation of the same material in the 

 form of a cube whose dimensions are 5 feet, placed wholly beneath 

 the surface of the ground. 



The zenith telescope is from the work-shop of Warner and 

 Swasey, the optical parts by Brashear. The clear aperture of the 

 object glass is 4^ inches, its focal length 48 inches and is furnished 

 with electric illumination. The base and vertical axis are of mas- 

 sive construction. It maintains its position in adjustment with little 

 variation. A magnifying power of 144 has been employed through- 

 out. No record of errors in azimuth, collimation and level has 

 been kept. The sum of these corrections for any star of the ob- 

 serving list during the period embraced by these observations was 

 always less than one second of time. 



The observations were made with a view of determining a value 

 for the Constant of Aberration according to the plan suggested by 

 Dr. Kiistner of Berlin. This entails the grouping of the greater 

 part of the observations around four periods in the year which in- 

 clude about eight months altogether, but their distribution is such 

 that the results are equally available for investigating the varia- 

 tion of latitude. The method of conducting the observations has 

 been identical with that followed at Flower Observatory. Since 

 the two stations are practically on the same meridian and the decli- 

 nations of the stars are eliminated, the two series are directly 

 comparable. 



At Bethlehem the list of stars comprise 4 groups of 10 pairs 

 each and the usual adjustment of each observed latitude to the mean 

 latitude which would result from the average of the 80 stars decli- 

 nations have been made. Observations for investigating the effect 

 of temperature on the micrometer value were made by two methods 

 and the final results for the temperature coefficient for the two 

 years are in such close agreement as to inspire some confidence in 

 their trustworthiness. These values for the two years, in units of 

 the sixth decimal place are 146 and 138. The individual determi- 

 nations are so discordant, however, that I have not considered their 

 introduction into the reductions advisable, since the average correc- 

 tion for the micrometer for any group of stars is less than one 



