284 THE SCIENTIFIC PAPERS OF 



meter, the current of water is again contracted between conical 

 surfaces, for the same purpose as before, namely, to equalize its 

 flow. 



In calculating the quantity of water that will effect one complete 

 revolution of the screw-drums, it is necessary to compute the clear 

 net area between them and the external casing, supposing all the 

 surfaces to be covered with a film of stationary water (by adhesion) 

 -rvoth part of an inch in thickness, and to multiply the same by 

 the pitch of the screw. The correction for adhesion amounts to 

 an inappreciable quantity for large meters, but constitutes a con- 

 siderable percentage in the calculation for small meters, being 

 equally exact for both. 



The difficulties that have been encountered in the manufacture 

 of this meter apply principally to the spindles ; although relieved 

 from all constant pressure, they have nevertheless to maintain the 

 drums in their central position, and to resist a strain endways, 

 caused by the mere friction of the water in passing along the 

 vanes. They have in consequence to be made of hard metal ; 

 German-silver was chosen in the first instance, but could not be 

 depended upon for strength. Steel is the best for hardness, but is 

 soon corroded by the water, notwithstanding all attempts to 

 protect it by zinc, or by a casing of brass, through which only the 

 rounded point projected to receive the end strain at the bottom of 

 the bearing. Agate points or plates are rapidly ground away 

 when used in water. A hard bronze was found to be the most 

 suitable metal, and indeed answers well for meters of large size, 

 but it is difficult to produce the spindles for small meters of that 

 metal. 



The difficulty at first experienced of producing screw-drums of 

 correct shape and uniform size, without incurring a large amount 

 of workmanship, was successfully removed by casting them, and 

 many other parts, in metallic moulds. Gutta-percha was also tried 

 by the manufacturers, which, being slightly lighter than water, 

 was with its spindle exactly equal to the weight of water which it 

 displaced ; but it could not be made sufficiently correct and rigid 

 in the vanes. After some time the manufacturers succeeded in 

 casting drums for the larger meters of bronze, and in dry sand, 

 with great accuracy. There was considerable difficulty at first in 

 finding workmen who would fit the essential parts with the great 



