TRANSACTIONS OF THE SECTIONS. 103 



roadway is not subject to a wave of as many inches. The weight of the roadway has 

 not been increased by more than five or six per cent. 



Remarks on the Connexion which exists between Improvements in Pitwork 

 and the Duty of Steam-engines in Cornwall. By J. S. Enys. 



After adverting to the admission of the truth of progressive increase of duty, it was 

 shown that considerable changes have been made in the course of seventy years, in the 

 methods by which water is Hfted out of the mines in Cornwall; and that in compa- 

 ring the duty of earlier periods, an allowance of the difference of the Imperial and 

 Winchester bushel of coal ought to be made. The distinction between horse-power 

 and duty, pointed out by Mr Parkes, was alluded to : one excludes, the other includes, 

 the friction of the pitwork ; and the remarks attached to each in Lean's report, show 

 the necessity of adverting to the different conditions of the pitwork, in an attempt to 

 estimate with accuracy the relative merit of different engines separate from the pit- 

 work. In an endeavour, some time ago, to trace the causes of the great variation of the 

 duty, a small amount of expansion was observed in engines remarkable for a low duty, 

 and the reasons assigned were, either weak pitwork — flat rods — heavy load per square 

 inch on the piston, and old boilers — and often the joint action of the above causes. 

 The strength of the pitwork, or of the boilers, in different cases, seems to become the 

 limit of expansion in the engines. In reference to deficiency of water from pumps, in 

 proportion to the calculated quantities, on which duty is founded, two causes have 

 operated in inducing a strong belief that it is less than at any former period : — 1 . Greater 

 attention to the pitwork by the managers of the mines, under whose care it is placed, 

 to the exclusion of the engineers of the steam-engine by which it is worked. 2. The 

 general employment of the plunger-pump, — the latter instantly shows the slightest 

 defect of the packing, and allows of an easy remedy ; while the bucket-pump, on the 

 contrary, does not show the defects in the packing ; and the operation of tightening 

 it is attended with great difficulty, — so much so, as often to cause the repacking to 

 be delayed to the last moment that the pump will lift water. The first cause, though 

 it has a tendency to. decrease duty in proportion to improved water delivery, has in a 

 still greater degree the tendency to reduce the friction of the pitwork on a given load : 

 yet it is not easy to assign the exact values : on the whole, a reduction of total re- 

 sistances probably occurs in shafts of equal depth ; but, on the other hand, the great 

 increased depth of many shafts obviously produces a greater proportional friction on 

 a given load. Under these circumstances, it becomes the fairer method to select the 

 duty of engines working the deepest shafts, for a comparison with the duty of the ear- 

 lier periods, when engines were worked so differently as regards the steam. Mr. J. 

 W. Henwood (Huel Towan) estimates the deficiency of water delivery at 7 or 8 per 

 cent. ; Mr. T. Wicksteed (Holmbush) 10 per cent, water from three lifts measured 

 and weighed ; Mr. Enys (Eldon's engine. United Mines) 4 per cent., four strokes of 

 the engine with one plunger-lift having been measured. The absence of attention in 

 earlier times can only be assumed from the known habits of the miner, and the ab- 

 surd stories prevalent of particular instances of neglect. Another great, but almost 

 inappreciable change, has occurred in the increase of weight in the rods for a given 

 load, due to deeper shafts and more expansion ; but the circumstance of the greater 

 weight of rods admitted of the reciprocal action of a still greater amount of expansion 

 in Watt's engines : in heavy pitwork, the accumulated force stored up at the com- 

 mencement is restored at the end of the stroke. The only decrease of duty is occasioned 

 by a greater amount of friction in the gudgeons of the balance beams, arising from the 

 weights required to balance the rods ; but, on the other hand, a direct gain is obviously 

 due to the smaller quantity of water required to be expended as steam, to produce, 

 by means of greater expansion, the same mean power. The present form of rod, 

 with lifts alternately on each side, where the shaft admits of this method, was pro- 

 bably due to Watt or Murdoch. Smeaton, at the Chacewater Atmospheric Engine, in 

 1775, seems to have effected the introduction of one rod for a portion of the shaft, 

 and dispensed with the older practice of bringing up to the arc of the beam a separate 

 rod for each lift. The plunger-pump seems to have effected another change of some im- 

 portance, in the velocity with which the larger portion of water is raised. The engines 

 arc usually made to go, out-of-doors, at rather more than half the velocity of the in- 

 door stroke, the piston moving in-doors from 240 to 260 feet, and out-of-doors from 



