270 PROCEEDINGS OF SECTION A. 



climate, from the snows of Kosciusko to the buniin<^ plains of the 

 interior and the humid heat of l*ort Darwin, we can obtain nearly 

 all that man requires. Our marvellous growth in the j)a^t is only 

 a foretaste of the future, and under such sunny skies we should 

 be, as I trust we are, in spite of the clouds of depression which 

 occasionally hang over us — with, however, silver linings not far 

 away — a happy and contented people. 'J'he lines have fallen to us 

 in pleasant places, and tridy we have a goodly heritage. 



4.— SOME OF THE DIFFICULTIES OF OBTAINING 

 EXACT MEASUREMENTS IN ASTRONOMY, 



By TF. E. COOKE, M.A., Adelaide Observatory. 

 [Abstract.] 



These refer to — 1st, naHir point; 2nd, effect of temperature 

 upon a micrometer; 3rd, determination of co-latitude; 4th, instru- 

 mental flexure; and 5th, refraction. 



1st. The old forms of mercury trough (non-amalgamated) fail to 

 give satisfactory images of the wires, but this difficulty has been 

 partially overcome by amalgamating the sides and bottom of the 

 trough. The nadir point is subject (at Adelaide) to two kinds of 

 variations, viz., a change in short periods of time, or during a 

 night's observing, evidently depending upon local variations of 

 temjDerature ; and an irregular, but, on the whole, progressive change 

 from day to day, especially noticeable during the summer months. 



'2nd. The nadir point reading changes whilst the observer is 

 simply standing near the micrometer, and if the observer stand on 

 the same side of the instrument the change is always in the same 

 direction. During an hour's observing of stars, all of which are on 

 the same side of the zenith, the nadir point alters by as much 

 (sometimes) as 2" 5", and it invariably increases Avhilst observing 

 stars on one side of the zenith and decreases for stars on the other. 

 It seems necessary, therefore, to take at least two nadir point 

 readings during each evening's work. 



3rd. The latitude obtained for the same place with the same 

 instrument varies from year to year, and this sometimes to an 

 extent which cannot be explained by Chandler's periodical "varia- 

 tion in latitude." I cannot suggest any direct remedy for this, but 

 perhaps close attention to the other points raised may indicate a 

 method of obtaining this important constant with greater exactness. 



4th. The flexure of the transit circle varies from time to time, 

 and its law does not seem to be satisfactorily determined. A 

 correction of the form/sm. Z.D. gives ap))arently the best results, 

 and the quantity f (horizontal flexure) ought to be determined 

 once a week at least, or probably with greater frequency. The 



