﻿Thomson. — On Economising the Current of Large Rivers. 143 



ness of the stream would prevent tlie screw being applicable, and where there 

 was large floatage capacity, at any rate, required to support the mill and its 

 contents ; but the great size, cost, and weight of the paddle form of wheel 

 create a most important objection to its general use in the Clutha, where only 

 the temporary works of gold mining for the most part are engaged in. On the 

 contrary, the great depth of the stream is particularly applicable to the screw, 

 working as it does so much below the surface, and when its smallness and 

 lightness make it so easily handled, and its cheapness and simplicity render it 

 of easy construction and repair. 



The floating paddle wheel in a seven mile current requires forty square feet 

 of floatboard to give a power of twelve horses, and of floatboards there require 

 to be eleven in niimber ; while one screw of eight feet diameter would give the 

 same power, and this, instead of requiring to be floated like the paddle wheel 

 high out of the water by sufficiently strong barges or punts, can be immersed 

 and attached not only to barges and punts, but to a buoy, wire rope, boom, 

 bridge pier, piles, or other fixtures suitable to the various situations. 



The Otago sluice head, by the Goldfields Regulations, being equal to 

 ninety-five cubic feet of water delivered per minute, machines of the above 

 dimensions will raise one sluice head to an elevation of seventy feet, or seven 

 heads ten feet, and this without intermission day and night. 



The advantage of the screw, when made of timber (as I would support) is 

 in its easy construction and repair — this fact should be particiilarly noted ; 

 besides, the screw is the only portion of the apparatus, whether for mills or 

 pumping gear, that need be subject to accidents from floods. Accidents from 

 floods can also be avoided by drawing the screw into the banks till the danger 

 from drifts is over. On the Clutha, with its great body of water, the con- 

 struction of the screw need not now be scientifically correct, but may be of the 

 rudest description, the fault in form being amply compensated for by the 

 superabundance of power. Thus, the screw may be easily made by a common 

 carpenter and blacksmith — in fact, out of an old gin case and a piece of 

 scantling I would engage to make a very efiective two horse-power machine. 

 The contrivance then, I am sanguine in stating, supplies that which is wanted 

 by the sluicing, dredging, pumping, and other enterprises on the banks of the 

 Clutha, viz., — an inexpensive and simple machine for economising the power 

 of the current. The nature of the contrivance is simple. A model is now on 

 the table for inspection. I will be happy to show it at woi'k in the stream of 

 the Water of Leith, at any time the members of this Society may appoint, when 

 they could judge of 'its effectiveness themselves. 



The model will be seen to be made of wood, in the make-shift fashion much 

 had recourse to on the diggings, the only portion of it executed by skilled labour 

 being the brass force-pump and india-rubber tubing. The screw (or more 



