222 Smithsonian contributions to knowledge vol. 27 



inch beyond the end of the porcelain insulator. The terminal which passed 

 through the insulator was also extended for something like half an inch beyond 

 the porcelain and bent to a proper extent to co-act with a piece of platinum 

 wire inserted in the interior wall of the plug which formed the other terminal. 

 After making this improvement in the plugs practically no difficulty was experi- 

 enced from short-circuits caused by the soot. 



In making the tests of this experimental engine it was found practically 

 impossible to absorb the power by a Prony brake in a sufficiently uniform man- 

 ner on account of the fact that the engine was being run without a fly wheel. 

 The consequent variation in the torque and speed during each revolution caused 

 such great fluctuations in the reading of the scales which measured the pull of 

 the Prony brake that no confidence could be felt in the accuracy of the readings 

 and, therefore, no confidence could be placed in the determinations of the effect 

 which different changes in the engine produced. A water-absorption dynamom- 

 eter consisting of a number of flat, circular discs fastened to a shaft and ro- 

 tating between other parallel flat discs arranged in a circular drum which was 

 filled to any desired extent with water was Immediately planned, and the con- 

 struction of two of them was begun so that power could be taken from both 

 ends of the engine shaft, which, on account of its necessary lightness, was 

 apt to be injured by being twisted when all the power was taken from one 

 end of the shaft. In order to continue the tests on the engine while this 

 dynamometer was being made it was decided to employ one of the propellers 

 as a dynamometer. Although no accurate tests had been made to determine 

 just how much power was required to drive these propellers at various speeds, 

 yet the fundamental law was knowni that under the same conditions the power 

 required to drive any propeller would vary as the cube of the number of revo- 

 lutions, ami since the Prony brake tests had given an approximation as to the 

 amount of power which the engine developed at certain speeds, the law of the 

 propeller, and extrapolations from the data obtained in the tests of the smaller 

 propellers in L898, enabled further approximations to be made as to the amount 

 of extra power which the engine developed when certain changes enabled it to 

 drive the propeller at increased speeds. This method had also the greal ad- 

 vantage that, since the power required varies as the cube of the number of 

 revolutions, it is practically impossible for the engine to " run away " with the 

 propeller and cause serious damage through the possible excessive strains in- 

 troduced by high speed. This feature is also possessed by water-absorption 

 dynamometers of the type which were built and used in the later tests. 



The construction of water jackets for this engine proved an exceedingly 

 formidable task, it being impossible to braze the jackets directly to the walls 

 of the cylinders without risk of ruining them. Tt therefore became necessary 

 to attach them liv means of stuffing boxes, which, on account of their large size 



