Art X. — Observed Variations in the Dip of the Horizon. 



[Abstract of papei- i-ead before the Eoyal Society of Victoria, 

 on 13th June, 189o.] 



By Thomas Walker Fowler, M.C.E., F.R.G.S., F.G.S. 



The observations recorded in the table herewith were made 

 with a 12 inch Theodolite, by Troughton & Simms, reading by 

 verniers to 10". The elevation of the horizontal axis was 132 

 feet above mean sea level as determined by spirit levelling, and a 

 few days tide gauging the range of tide observed varying from 

 6' 6" to 10'. The station was (approximately) in latitude 38' 

 22' S. Longitude 144° 46' E., and the observations were taken 

 in part by the writer and in part by engineering students of the 

 Melbourne University. From the instrument station good views 

 could be obtained to the south over part of Bass Strait (average 

 depth about 35 fathoms), and to the north over part of Port 

 Phillip Bay (average depth about 5 fathoms) as well as to Arthur's 

 Seat, distant 9 nautical miles to the eastward, and elevated 996 

 feet above mean water level of Port Phillip Bay as determined by 

 spirit levelling. The angles of elevation were taken to a point 

 on the look-out tower 20 feet above the summit, or 1016 feet 

 above mean water level of Port Philip Bay. 



The observations were taken during very hot weather, when 

 the difference between air and sea temperatures was large. The 

 sea temperature to the south (in Bass Strait) was about 67°, that 

 to the north (in Port Phillip Bay) was not observed, Ijut, no 

 doubt was higher. 



The normal dip under average temperature and pressure would 

 be 0° ir 20" as against the maximum and minimum observed of 

 0° 21' 35" and 0° 5' 5" respectively. Reference may be made to 

 " Raper's Navigation," pages 61 and 194, for further observations 

 on this point. 



The altitude of point sighted to on Arthur's Seat, according to 

 the smallest observed angle, is 1003 feet; according to the largest 

 observed angle 1043 feet ; and according to the mean angle 1031 



