48 REPORT— 1869. 



of electrification retained at any given interval is proportional to the quantity of 

 chart^e remaining at that time. The longer battery-contact is maintained, the 

 slower will a core or cable lose its charge, and conversely. In a cable which has 

 been charged by contact with a battery for one minute, and afterwards held free 

 for one minute, the electrification will be the same as if, instead of being held free, it 

 had been left connected to a battery having the last tension, thus : — If the discharge 

 after one minute's contact and one minute's insulation be 180, and the immediate 

 discharge 200, the duration of contact being also one minute, the total eflfect for 

 electrification at the end of the minute's insulation will be 95 per cent, of what it 

 would have been if connected to the same battery for two minutes. By taking 

 these considerations into account the formula of Professor Fleeming Jenkin, 



may be rendered strictly applicable for deducing from the loss of static charge in 

 time i, the i-esistance for the same period of contact in absolute measure, or in 

 terms of that system which makes K and K functions of each other ; and we may 

 expect that the capacity K can be eliminated from this formula when R is known, 

 if we can determine the constant for electrification for the interval of time during 

 •which the core is held free. 



In this formula if the test is performed in the mamier here indicated t wiU be 60, 

 and the value obtained for R will be the resistance at the end of the second minute 



Q 



more nearly as — approaches 1. This resistance has then to be divided by a 



number which expresses the ratio between the first and second minutes' contact. 

 Approximately, and on short lengths of core, this may be obtained as follows : — 

 Recharge the core after being kept to earth for some liours, maintaining contact 

 with the batteiy for two minutes before noting the loss ; then by dividing the 

 percentage of loss in the first experiment by the percentage of loss given in the 

 second experiment, we shall obtain a number by which, if R be divided, the re- 

 sistance coiTesponding to one minute's contact may be found. 



The following ratio expresses the rate of increase on prolonged contact : — Let D 

 be the deflection at the end of the first period of contact and d the deflection at 

 the end of the wth period, then D : d : : d : deflection at the end of ii- minutes. 



I have to acknowledge my obligation to Mr. Hooper for placing at my disposal 

 the necessary instruments and cores for the subject of this paper. 



Instruments. 



On a New Anemometer for Measuring the Speed of Air in Flues and Chimneys. 



By A. E. Fletcher, F.C.S. 



This paper is a continuation of one read at the Dundee Meeting in 1867. 

 _ Not able at that time to give a mathematical interpretation of the working of 

 his instrument, the author, resting on experiment, drew up an empirical table of the 

 speed of air indicated by it. Now independent calculations are given, and the 

 results corroborated by experiment. 



The problem to be solved may be briefly stated thus : — 



The lower end of a vertical straight tiibe, open at both ends, dips into a liquid. 

 To what heig-lit wiU the liquid be raised in the tube by the action of a cun-ent of 

 air passing with a given velocity across its upper end ? 



On consideration, it will appear probable that the height of the column is but a 

 measure of the impact force of the air in motion ; experiment proves this to be the 

 case ; it shows that the liquid is drawn up to the same height it would have reached 

 had the stream of air been directed against the surface of the liquid in the cistern. 

 The problem is now exchanged for one easier of solution. 



