276 



REPORT 1851. 



same manner, and so on to the end of the statute mile, marking each quarter- 

 mile as before, and also the extremity of the mile, by oak 6-ineh stakes and 

 brass nails. 



Transverse cord. 

 Diag. 3. 



PERMANENT 



STAKE rOR 



E3- 



RANCINC CORO 



QUARTER MILE 



This work was done on the 18th and 19th of July, 184-9. The only diffi- 

 culty experienced was in crossing the Shanganah River at its mouth ; here the 

 S5-foot rods had to be carried over the water on stakes temporarily driven in, 

 the several parties operating wading in the water, and a possible error of 

 perhaps two inches may have occurred here ; but from the checking of 

 this operation over the rest of the line with the steel chain, and with the 

 same operation repeated over parts of the line with the rods, I believe the 

 whole mile to be measured correctly within an error of 4- or 5 inches at most. 

 The first half-mile, going from north to south, is almost perfectly level ; in 

 the next quarter-mile the surface of the sand descends with almost perfect 

 uniformity from about the level of high water to that of low water of ordi- 

 nary tides, say nearly 10 feet, and again as gradually rises up to the extre- 

 mity of the mile at the south. This arose from the base line being a chord 

 to the curved line of beach of the bay. As the measurement was made along 

 the surface of the sand all the way, except crossing Shanganagh River, an 

 allowance must be made for this dip and rise again. 



Section of Mile measured. 



T 



This, when calculated, gives a difference between the line measured along 

 the surface of the sand and the true straight line carried through from end 

 to end, of rather less than \\ inch. 



Subsequent remarks will show that this part of the base line became wholly 

 unimportant to the experiments. 



The preparation of the several instruments and materials, with some ine- 

 vitable delays, occupied the time between this and the 25th of October, ISiO, 

 when, all being complete and the powder, with the galvanic batteries for 

 firing deposited at the tower No. 9 and battery No. 7, a set of preliminary 

 experiments was made to ascertain what quantity of powder it would be ne- 

 cessary to explode in order to render the pulse perceptible by the seismoscope 

 at the distance of a mile. 



To this end twelve canisters of tin plate were made, each of which con- 

 tained two pounds of powder, with room for the priming powder and wires, , 

 &c. These canisters were cylinders of about 8 inches long by 4- inches dia- 

 meter, with an opening of 1^ inch diameter at each end, prepared with a neck I 

 to be closed by a bung-cork. The primings were made of M'ood; a copper 



