﻿136 Transactions. — Miscellaneous. 



the cliannel. In tlie greater part of its length it is a narrow ridge stretching 

 three-quarters of a mile in a direction E.N.E. from Pahiki ; thence it curves 

 backwaixls W. by N. for about three-eighths of a mile, assuming a hook-shape. 

 On a flattish table bank in tbe bend of this hook, the lighthouse has been 

 erected, the depth of water at low springs being fi ve feet, and at high water 

 sixteen feet. 



At the request of the late Colonial Marine Engineer, the stnicture of the 

 sandspit was ascertained — about two years ago — by Captain Burgess, Chief 

 Harbour-master, Auckland, from boi-ings made on the site of the proposed 

 works. It was found to consist of loose sand and shells, with shingle, over- 

 lying at a depth of seven to fourteen feet a hard crust, the exact thickness of 

 which was undetermined. 



Below this was a homogeneous sub-stratum of soft muddy and sandy clay ; 

 a very hard, but not rocky bottom was reached at a depth of thirty-one feet 

 fi'om the surface of the sand. 



At the time of Mr. Balfour's death, he vas understood to have made some 

 progress with a design for this lighthouse ; however, no sketch or trace of 

 such was found, and the writer was intrusted by his Honour the Superin- 

 tendent with the design and erection of the work. 



In determining the nature of the foundations, the qiiestion was a narrov/ one. 

 Screw piles offer by far the most advantageous method of supporting heavy 

 weights on sub-marine sandbanks. But the details of form of screw, area, and 

 pitch of thread, etc., have to be adapted, as well as can be, to the nature of 

 the material in each case ; not only with a view to the efficient support of the 

 structure, but also to the screwing, and the amount of torsion necessary to be 

 borne by a long pile (in this case forty feet) working down through stiff 

 material. It is unnecessary to describe in full the calculations leading to the 

 form and elements of the screw pile adopted ; it may be sufficient to remark, 

 that for the sake of permanency, cast iron was used for the complete pile 

 (excepting the joint bolts) ; the stem is ten inches oiitside and eight inches 

 inside diameter ; diameter of screw flange, three feet three inches ; pitch, six 

 inches. The form of screw is conical, the thread commencing at nothing, near 

 the pointed end of the pile, and attaining its full width of fourteen and a half 

 inches in one and a half tu.rns, it then made one turn more at the full diameter. 

 The conical shape was found of great service in penetrating the hard crust 

 existing a little way below the surface. Imbedded timber was the only con- 

 tingency to be feared, and its actual existence at the bottom was proved by one 

 of the borings made by Captain Burgess. Several tests were made before 

 screwing down, to guard against this contingency, and the nature of the ground 

 may in a measure be judged from the fact that little more than the weight of 

 the rods was necessary to send them from the surface to the hard. This 



