Mr. Henwood on the Cornish Piwijmig- Engines. 489 



that place downward they extended about 300 feet, and were 

 12 inches square. They were kept in their places by thirteen 

 setsofguides, which exposedasui'faceofabout53"5 square feet*. 



From the surface to 396 feet deep in Binner Downs, the 

 rods were 14 inches square; and from thence downward, 

 there were about 258 feet of I'l inch rods; these were also 

 retained by thirteen sets of stays, having an area of about 

 35-6 feet. 



The rods, from the surface to 470 feet deep in East Crinnis, 

 were 15 inches square, and thence about 200 feet deeper they 

 were ] 2 inches : eleven sets of stays retained them in their 

 places, and exposed a surface of about 38*8 feet. 



Where the rods touch the stays they are protected by thin 

 planks of some hard wood, which are always well covered 

 with grease ; they seldom fit very accurately. 



Table IV. 



Duration of the experiments, number of strokes made, materials 

 consumed, 8fc. 



The engines were taken without any previous preparation, 

 and they were worked without intermission, at a speed just 

 sufficient to keep the mines clear from water; but without 

 permitting the pumps to draw air {go in fork). The workmen 

 exercised their own discretion in the mode of working; for 1 

 purposely abstained from any other interference with them 

 tiian was sufficient to satisfy mysielf tl)at every thing was ex- 

 posed to my notice, and fairly and honestly performed. 



• Tlie lengths of tlie lifts and of the i-ods do not coinciclc, hcciiuse the 

 former overlap each otiicr in every ease, in order tiiat the lii^hcr punijis 

 may draw out of the same cisterns into which tlic lower empty ; and be- 

 cause the rods which take the different lifts are also doubled at the stts-oj}'. 



