ASTRONOMY. 103 



land Abbe has directed attention to a neglected correction in the use 

 of refraction tables, which appears as a function of the latitude. 

 Thirty inches of mercury in the barometer at the equator indicate a 

 less density of the atmosphere than 30 inches at the ]ioles, consequently 

 the barometer readings should be corrected for differences of lati- 

 tude. This is accomplished by simply adding to the formula one more 

 factor for gravity. Professor Abbe shows that the difference of latitude 

 between Pulkowa and Washington makes a difference of 0". 1 in the 

 refraction at 45° zenith distance, and increases with the zenith distance. 

 We have here a ])artial explanation, at least, of systematic differences 

 in declination shown by different catalogue-. . 



Correction for differential ref roof ion in declination. — Professor McNeill, 

 of Princeton, has devised (Astron. Nachr., 114::385) a method of correct- 

 ing micrometer observations for refraction, ai)plicable to the diagonal- 

 square micrometer, the ring micrometer, and others of the same class. 

 The correction to the observed difference of declination is not deter- 

 mined by a special separate computation, but the true difference is 

 directly determined, the corrections being ai)plicd to tlie logarithms in 

 the course of the computation. Tables are given which will be found 

 very useful to observers. 



In a "Zusatz" to this communication, Dr. Krueger gives a resume of 

 differential refraction formuhe for ring and filar micrometers. 



M. Railau suggests (Bull, astron., 3 : 373) that Professor McIsTeill's prin- 

 cipal table may be replaced by a simple graphical table which will give 

 at a glance the correction sought. 



The diurnal nutation of the earths axis. — M. Folie, about three years 

 ago, submitted to the Paris Academy a theory of the diurnal nutation 

 of the earth's axis, based upon the assumption that the earth has a 

 fluid nucleus; and he has recently given (Compt. Rend., Dec. 13, 1886) 

 some practical illustrations of his formulfe. These formuJiie contain 

 two constants to be determined by observation : the constant of diurnal 

 nutation itself, and the longitude, referred to an initial meridian. Very 

 accordant results are obtained from the rather meager observational 

 material available, the value of the diurnal constant being about 0".2. 

 The new correction applied to a series of observations of Polaris made 

 at Pulkowa, smooths out the discordant observations in a most surpris- 

 ing manner. Further investigation of this subject seems highly desir- 

 able. 



An abstract of the paper read by Prof. J. C. Adams at the Phila- 

 delphia meeting of the American Association, September 11, 1884, " On 

 the general values of the obliquity of the ecliptic, and of the precession 

 and inclination of the equator to the invariable ])lane, taking into ac- 

 count terms of the second order," has appeared in the Observatory for 

 April, 1880, vol. 9, p. 150-154. 



