24 Mr. F. J. Bigger on Alexander Mitchell 



set off secretly, with the new instrument packed up and hidden 

 away. They arrived at Dunbar's lock and hired a boat, but dis- 

 missed the boatman. John rowed his father over to a sandbank, 

 and there they screwed down a model screw pile, leaving the post 

 standing above water, and taking the bearings of its position. 



Very early next morning, before the working world was astir, 

 they rowed out again, examined the pile, and found it was firmly 

 fixed where they had placed it. That must have been a moment 

 of great satisfaction to both. In fact that night's experiment laid 

 the foundation of Mitchell's success as an engineer. 



The following is an extract of the evidence of Alexander 

 Mitchell, taken from the report of the Select Committee of 

 Parliament on lighthouses :— 



14th July, 1845. 



" I am a civil engineer, and reside in Belfast. I have carried 

 on my profession for about twelve years. I have been employed 

 m erecting lighthouses on the coast of Ireland, upon the screw 

 pile principle, of which I am the inventor and patentee. When 

 erecting the lighthouse on Maplin Sands I experimented on the 

 very ground where the lighthouse now stands. We always think 

 it necessary everywhere, before undertaking any work of the kind, 

 to ascertain as well as we can the nature of the ground, in order 

 to proportion the size of the screws to the nature of the ground, 

 and to the depth to which we are likely to penetrate. 



" I prepare a rod in the nature of a boring rod, of about 30ft, 

 and one and a quarter inch diameter, and put a small screw at its 

 lower extremity, of the same form as the screws we use in forming 

 foundations, the screw having a flat, broad flange of six inches 

 diameter. In a very short time, say 15 or 20 minutes, by means 

 of four cross levers upon the boring rod, we screw it down to the 

 depth of 27 feet ; so far as we found the ground penetrable. 



" Then the next thing we wished to ascertain was, what weight 

 that would support without sinking. Upon the cross levers which 

 we used in turning it down we placed a few boards, to form a 

 platform, upon which we placed 12 men, whose weight might be a 

 ton, and we placed another rod at a short distance, merely sticking 



