1871.] W. E. Ayrton— Quant. Meth. of Ted. Telg. Earth. 177 



The jaw is semilunar, narrow, thin, concentrically very finely, 

 and radiately distantly and indistinctly, striated, the anterior con- 

 cave edge is nearly perfectly entire. 



The radula is long, moderately narrow, consisting of about 80 

 transverse, slightly angular series of teeth, there being 53 teeth in 

 each series. The centre tooth is smaller than the adjoining, with a 

 simple, inflected and pointed tip ; it is contracted towards the base. 

 The 14 inner laterals are longer and stronger than the 12 outer 

 laterals. They are all tri-cusped ; at first the median cusp is by far 

 the largest, gradually, the lateral increase in size, while at the 

 same time the median cusp decreases, until on the outermost lateral 

 teeth the three cusps are almost equal. On the whole the form of 

 the teeth agrees better with that of the Helicid^e than with the 

 Zonitid^:. The dental formula is 12 -f 14 — 1 — 14 -h 12. 



On a quantitative method of testing a " Telegraph Earth," — 

 by W. E. Ayrton, Esq. 



[Received and read 6th April, 1871.] 



The method that has been used up to the present time for testing 

 a telegraph " earth" has been a qualitative method only, that is 

 to say, although it may in a rough way have answered the question, 

 is an " earth" good or bad, it was quite unable to give any answer 

 to the question, how good or how bad. 



In Europe the ordinary way to make an " earth" is to use the 

 iron gas, or water pipes, but in most places in India such pipes do 

 not exist, so that some large piece of metal has to be buried for 

 this purpose. A coil of iron wire, a piece of an iron post, or a 

 copper plate have been used at different times. Now as the nature 

 of the ground in the immediate neighbourhood of this buried piece 

 of metal greatly affects its electrical utility, it becomes a question 

 of great practical importance to determine in absolute units the 

 resistance of the " earth" used in each particular case. 



The following method devised by Mr. Schwendler is at present 

 in use in the Indian Telegraph Department. 



