564 KEPORT — 1887. 



any given length of time. The only thing that interfered at all with the 

 perfect regularity of the experiment, as checked every five minutes, was 

 a very slight variation in the speed of the engine ; and an increase of 

 speed of one revolution per minute on 180 revolutions per minute could 

 at once be detected, and was followed after a few minutes by a percept- 

 able rise or fall in the temperature of the outflowing water, as the 

 quantity passing was always almost exactly the same. 



The diagrams of the speed of dynamometer, weight lifted, and of the 

 temperature and weight of water heated show what these very slight 

 fluctuations were ; and when they were contrasted with the large volume 

 of water heated (viz., about a gallon per minute, twenty degrees) it will 

 be seen how slight they were ; and further, as no loss of power on the 

 one hand, or loss of heat on the other, was sustained, it was of minor 

 importance, if indeed of any importance, that the fluctuation should be 

 sometimes slightly above and sometimes slightly below the given point, 

 as the total power was actually registered as well as the total heat pro- 

 duced. The result showed a ' mechanical equivalent of heat ' ^ 769 feet, 

 that is to say, that one pound of water raised 1° Fahr. was equal to one 

 pound lifted 760 feet, and it will be remembered that Professor Joule 

 made it 772 feet. 



It is not to be wondered at that the ' equivalent ' obtained was slightly 

 lower than that obtained by Professor Joule in his last experiments, as all 

 losses of heat were prevented, and no losses had to be calculated ; nor did 

 the specific heat of the apparatus enter into the calculation, as the apparatus 

 was practically kept in a normal state throughout the experiment, and in 

 fact for days together. The authors are aware that the experiments 

 described are by no means complete, and objections may on that account 

 be justly taken to them ; but they are anxious to bring the work so far 

 as it has gone before the British Association, in order to benefit by the 

 suggestions and criticisms which discussion would not fail to produce. 

 They intend to resume the experiment at no distant date, and feel 

 sanguine that absolutely triistworthy results will eventually be arrived at. 



A small improvement will be made in the machine before prosecuting 

 further experiments, viz., certain precautions to prevent the possibility of 

 any heat being taken up from the surrounding water by any parts of the 

 dynamometer that may be slightly below its general temperature close to 

 the point where the cold water enters. 



Note. — Since the above paper was read, the authors have heard of the experi- 

 ment conducted by Professor Marks in 1885 in the United States, with the same 

 object, with the ' Tatham Dynamometer,' belonging to the Franklin Institute, and 

 in which experiment the equivalent of heat was calculated as equal to 77281 foot- 

 pounds for one degree Fahrenheit. See Journal of the Franklin Institute, volume 

 for 1885, p. 453. 



On an Electric Current Meter. 

 By Professor G. Forbes, M.A., F.R.S. L. & E. 



[A communication ordered by the General Committee to be printed in, extenso 

 among the Reports.] 



At the present moment the mind of electrical engineers is much directed 

 to the successful means of distributing electricity to a large district from 

 central stations by means of that class of induction apparatus which has 



