OBSERVATIONS WITH THE ZENITH TELESCOPE. 



807 



Giving equal weights to each of the 37 equations, we find in units of the third 

 decimal place 



x = -\- 2.15 ±2.69 

 y = — 0.367 ± 0.262 



Therefore 



B = 50". 50215 — .000367 (T— 45°) 



Tins is the value of R employed in the present series of Latitude Observations. 



Progressive and Periodic Errors. 



Two series of measurements of the screw have been made by means of the 

 measuring engine of the U. S. Naval Observatory, designed by Prof. Harkness for inves- 

 tigating the photographs of the transit of Venus. This was done in February, 1891. 



The correction for progressive error was adjusted by a graphic process, no assump- 

 tion being made as to the law involved. The result is as follows. The first column 

 gives the micrometer reading, the second the correction : 



8 



+ .0202 



i) 



.0182 



10 



.0102 



11 



.0141 



12 



.0121 



13 



.0100 



14 



.0080 



15 



.0040 



16 



.0019 



17 



.0000 



18 



.0000 



19 



.0001 



20 



.0002 



2.1 



+ .0002 



22 



.0011 



23 



.0031 



24 



.0052 



25 



.0007 



26 



.0081 



27 



.0096 



28 



.0111 



29 



.0126 



30 



.0140 



31 



.0155 



32 



.0170 



If we derive tins correction from the transits of the circumpolar stars observed for 

 determining the value of one revolution of the screw, assuming the error to be uniformly 

 progressive we have the following values: 



8 



and 32 



+ 



.0208 



9 



31 





.0174 



10 



30 





.0144 



11 



29 





.0117 



12 



2S 





.0092 



13 



27 





.0071 



14 and 



26 



+ 



.0052 



15 



25 





.0030 



16 



24 





.0023 



17 



23 





.0013 



18 



22 





.0000 



19 



21 





.0001 



The corrections deduced by means of the measuring engine were employed m this 

 reduction. 



