TRANSACTIONS OF THE SECTIONS. 209 



On the Effects of Screw Propellers when moved at different Velocities and 

 Depths. By G. Rennie, F.R.S. 



From experiments which had been made under his observation, it was desirable 

 that the screws of vessels should be of small dimensions, light, and of rapid motion, 

 and that their effect should be increased by their being as deeply immersed as pos- 

 sible. He also recommended the disc-engine for driving small screws at high rates of 

 velocity. ^^______ 



On the Blasting and Quarrying of Rochs. 

 By William Sim, of Furnace Granite Quarries, near Inverary. 



This paper gives the results of the author's experience in the blasting of a very hard 

 description of granite by means of a system of mines, charged with gunpowder, con- 

 fined in boxes, the charges varying from 1500 to 6720 lbs. 



Mr. Sim considered the subject under the following heads : — 



1. As to the best position in a quarry for a large blast. 



2. The mode of placing and forming the mines. 



3. The formula adopted for calculating the quantity of gunpowder required. 



4. The placing of the gunpowder, electric wires, and safety fuse, and the stemming 

 of the mines. 



5. General results, with estimate of cost, and the applicability of the system to 

 various descriptions of rocks. 



The paper has been published in full in the ' Civil Engineer and Architect's 

 Journal,' and in the Glasgow ' Practical Mechanics' Journal' for October 1855. 



On the Transmission of Time Signals, By Professor C. PiAzzi Smyth, 

 Astronomer Royal of Scotland. (See Section A.) 



An Account of Experiments on Combustion in Furnaces, ivith a view to the 

 Prevention of Smoke. By Dr. Taylok. 



On the Friction Break Dynamometer. 

 By James Thomson, A.M., C.E., Belfast. 



In this paper Mr. Thomson explained the nature and principles of the Friction 

 Break Dynamometer, characterizing it as a highly valuable apparatus for the measure- 

 ment of mechanical power. He turned special attention to matters having important 

 bearings on its successful employment in practice, and to the consideration of which 

 he had been led by experience in its use on the large scale. 



The chief difficulties to be contended with, he stated as follov.'s : — 



1. The heat generated in the consumption of the mechanical power. 



2. Vibration or even entire instability of the arm of the break due to ovalness or 

 other imperfections of the friction drum. 



3. Tremor of the driving shaft occasioned by alternate sticking and slipping of the 

 drum in its friction blocks, instead of steady slipping. 



In regard to these matters he made statements and explanations to the following 

 effect : — Unless the drum be very large with reference to the power to be consumed 

 by it, the heat generated by the friction usually requires to be carried off by streams 

 of water carefully distributed over the drum. On the proper distribution of the water, 

 and its regularity of supply, much of the success of the apparatus depends, since great 

 irregularities in the friction are liable to result from imperfect arrangements for the 

 water supply. 



In the practical employment of the friction break dynamometer it is often necessary 

 to form the drum in two halves, in order that it may be got into its place on the 

 driving shaft. This arrangement, however, is often a source of great detriment to 

 the working of the apparatus, on account of the difficulty or impossibility of bringing 

 the two halves of the divided drum so correctly together as to form a sufficiently 

 perfect cylindrical surface for producing a uniform friction. In cases therefore in 



1855. 14 



