976 



The Shrawardine Tractor Trials, 1921. [Feb., 



the engine of the tractor was capable of producing. In nearly 

 every case submitted to the draw-bar test, the draw-bar pull 

 was limited by adhesion. This may have been due in part to 

 the hard state of the ground, but even making allowance for this, 

 the results must be considered disappointing. It is clear that 

 the problem of adhesion needs careful and prolonged study and 

 that this is a field of research which — like many others — should be 

 taken up as part of a programme for investigations in agricultural 

 machinery which may be framed either by the Ministry as part 

 of its research scheme or by manufacturers, or by both working 

 in conjunction. Trials cannot do more in this direction than 

 focus attention upon a common feature or defect of machines of 

 a wide range of types. 



It has already been said that except in some points of detail 

 the organisation of the trials was excellent, and it will not be 

 regarded as adverse criticism if reference is made to a point 

 which has been noticed elsewhere. In some instances advan- 

 tage was not taken of the dynamometer self-registering depth 

 gauge, and the determination of the average depth, load and 

 speed was in effect a matter for the unaided judgment of the ob- 

 server. This, however, was a failure of the human instrument. 



In the case of implements the scheme of test did not appear 

 to permit of measuring the actual disturbance of the soil (which 

 is the essential factor) nor of pursuing the test to a comparison 

 of the resulting yield of crop with a control plot. 



The Eeport. — The Report has been prepared on much the 

 same basis as the Report on the 1919 trials, although care has 

 been taken to give additional details where these seem likely to 

 be of service. A weakness appears to lie in the assumption made 

 for calculating " comparative acres " ploughed per hour and 

 other figures based on this unit. The assumption is that the 

 resistance per square inch of furrow section varies directly as 

 the depth of the furrow. This assumption is but tentatively 

 made and Mr. Watson does not profess that the figures are 

 strictly accurate : experiments recently carried out on behalf 

 of the Ministry confirm Mr. Watson's experience that .more 

 energy is absorbed in displacing a greater depth of soil than a 

 greater width. It would seem best, therefore, in the present 

 state of our knowledge to avoid such an assumption as was made, 

 unless indeed the factors to be employed in reducing all data 

 to a common denominator are determined by ad hoc experiment. 

 Unfortunately also the draw-bar horse-power results are open 

 to question, since in the great majority of cases, as the Report 



