880 



KEPOllT 1887. 



on a plastic support is wrong: in principle. Hence in experiments on tke cnisblncr 

 of stone, and of Portland cement and concrete, he has adopted the plan of 

 preparing the faces on which the crushing pressure acts, with a thin layer of 

 plaster. This can easily be worked to smooth and parallel surfaces^ used to 

 receive the iron plates of the crushing shackles directly, if necessary ; but the 

 author generally interposes a sheet of millboard, which is a very hard and only 

 slio-htly compressible material. It seemed desirable to try what was the 

 ditference of the crushing strength of blocks supported in these two ways. Two 

 series of 4 in. cubes of Portland stone and Yorkshire grit were obtained of very 

 uniform quality. The results of the tests are given below. The author has been 

 surprised at the very great reduction of strength which occurs when a thin plate 

 of plastic material like lead is used on the faces to which tlie crushing pressure is 

 applied. It will he seen that the crushing pressure of blocks between lead plates 

 is in one case only three-fifths, and in another only three-sevenths of that of blocks 

 prepared with plaster and crushed between millboard. One block was cemented 

 carefully between two rigid iron plates with parallel surfaces, and this carried a 

 little more, but only a little more, load than the block prepared with plaster and 

 crushed between loose millboards. An examination of the mode of fracture of 

 the blocks shows why the lead has so dangerous an effect on the strength. The 

 blocks crushed between millboards sheared approximately at ■45'^ in the way well 

 understood, forming regular pyramids ; but the blocks crushed between lead broke 

 up into a number of vertical prisms with nearly vertical faces. The lead, flowing 

 inider the crushing pressure, produced by friction a tension in the block at right 

 angles to the crushing pressure, tearing the block in pieces and completely altering 

 the angle of fracture. The pressure of fluidity of lead is known to be from H to 

 3 tons per square inch, and these pressures were exceeded in the crushing- 

 experiments. 



The result seems important, because it is still a common practice to use lead, or 

 deal, or some other plastic or compressible material, in crushing experiments, and 

 it is not generally known that this has the eft'ect of diminishing the crushing 

 resistance. 



Crushing of Stone Blocks, 4 in. cubef {appro.iimctteh/). 



The lead plates were 0-085 in. thick. 



3. Ei-pamive Working in Direct-acting 'Pumping Engines. 

 By Henry D.wey, M.Inst.O.E. 



In the Report of the Swansea meeting of the British Associar vi is printed a 

 paper by the author on the above subject, dascribing a method by i\i" m of which 

 expansive working is secured without the use of a flywheel. 



