TRANSACTIONS OF SECTION J. 439 



portional to the pull on the test-bar. The end of this pointer 

 in its motion always touches a piece of smoked glass, to which 

 is given a travel in its own plane proportional to the extension 

 of the test-piece. In this way the diagram is drawn. After 

 the test the glass is varnished to fix the black, and the neces- 

 sary particulars about the test are written on it with a scriber. 

 The glass is then used as a negative, and copies produced by 

 photography. 



Professor Unwin's autographic stress-strain apparatus con- 

 sists of a revolving drum, whose angular displacement is pro- 

 portional to the position of the poise- weight which denotes the 

 load on the specimen. The extension produced by the load is 

 recorded by means of a wire passing over pulleys and connected 

 with the test-piece ; a pencil attached to this wire draws the 

 diagram. 



Mr. Aspinall and Professor Hele Shaw have each devised 

 apparatus on the same principle as that adopted by Professor 

 Unwin. In Mr. Wickstead's apparatus the stress is measured 

 by the compression of a spiral spring, acted upon by a small 

 auxiliary ram in a cylinder, in which the pressure is the same 

 as that in the main hydraulic cylinder. This auxiliary ram is 

 kept in constant rotation, to reduce the friction of the cup- 

 leather to an exceedingly small amount. A pencil attached to 

 a rod moved by the auxiliary ram draws a line on a drum 

 whose angular displacement is proportional to the stretching 

 of the test-piece. 



The limit of elasticity, ultimate strength, and total extension 

 are perfectly recorded by the apparatus above described ; but 

 the portion of the diagram from a to b and from c to d on the 

 diagrams recorded by Professor Kennedy's apparatus, as shown 

 in previous woodcut, are much more perfectly shown than in 

 any of the others. 



Mr. J. A. McDonald, M.Inst.C.E. and M.Am.Soc.C.E., 

 and the author devised an apparatus which was made at the 

 University, and used in drawing the stress-strain diagrams 

 shown in the author's pamphlet on " New South Wales 

 Timbers of Commercial Value." These diagrams are for com- 

 pression and cross-breaking, but an arrangement has since 

 been added which enables diagrams to be drawn for tension- 

 tests of iron and steel. The apparatus can be best understood 

 by inspecting it at work in the testing-machine. It has 

 been in constant use for the last four years, and has proved 

 to be very convenient. A few of the diagrams obtahied from 

 it are shown in Plates VIII., IX., and X. The following 

 tables, Nos. I. to V., illustrate the method of recording the 

 results of testing at the University of Sydney for ordinary 

 commercial tests. More complete information on the timber- 

 tests may be obtained from the author's pamphlets on this 

 subject. 



