274 REPORT 1851. 



were acceded to, and the efficient assistance that was rendered us by the 

 extremely intelligent men both of the Artillery and Coast Guard services 

 whose cooperation we obtained. 



I now proceed to describe the operations and instruments used in the ex- 

 periments as to the 



Bate of Wave-transit in the Killiney Bay Sand. 



The site for this class of experiments was chosen on the shore of Killiney 

 Bay, where a straight and almost perfectly level range of strand, consisting 

 wholly of deep and dead sand, of particles, chiefly of quartz, from the size of 

 small pins' heads to large mustard seed, resting on a deep bed of hard clay 

 and gravel, is found stretching along more than five miles of shore, free from 

 all rock in situ, and with but very few small-sized granite boulders at one or 

 two spots imbedded in the sand. 



The sand contains from 80 to 90 per cent, of white and yellow quartz, 

 about 6 or 8 per cent, of gray and green slate, 3 or 4 per cent, of black ar- 

 gillaceous limestone, effervescing with acids, all in grains, and occasional 

 fragments of felspar, schorl, and mica. The specific gravity of the whole 

 taken in water is 2*481 • A cubic foot of its solid materials would therefore 

 weigh 1 55*06 lbs. avoir. A cubic foot of the moist sand was ascertained by 

 experiment to weigh, when compressed to about the same extent as it is found 

 on the strand, 1 1 1*05 lbs. The degree of porosity, or the amount of intersti- 

 tial space in the sand to its solid material in a given bulk, is in the ratio of 

 44'*01 : 111*05, or very nearly as 1 : 2*5. 



Measuring the Base. — On the 18th of July, 1849, a range of a statute 

 mile =5280 feet, was roughly marked out, staked and chained over by myself, 

 William Mallet and Prof. Downing, T.C.D., who kindly assisted us in the 

 measurement. Two accurately made fir-wood measuring-rods were prepared, 

 each of 2| inches square in the middle, lessening to 1^ inch at each end, and 

 of 35 feet in length, shod with brass at both ends, which tapered to \ inch 

 square (see Diag. 1.). These were accurately adjusted by a brass two-foot 



Terminations of Rod. 



^U> 



Diag. 1. 



Standard, made by Troughton, in the possession of Mr. George Yeates of 

 Dublin, mathematical instrument maker, and said to be a copy of the old 

 Exchequer standard, which he lent for the purpose. The final adjustments 

 were made by a micrometer, dividing on the varnished face of the wood and 

 on the straight side, both the rods lying parallel on a bench prepared to sup- 

 port them level and straight. The adjustment was compared on the day of 

 measurement and found correct. The wood was straight-grained yellow 

 pine-fir, quite dry and seasoned. 



The mile to be measured accurately, having been now exactly ranged out 

 by the theodolite and pegged in line, a heavy oak stake of 6 inches square was 

 driven down 4 feet into the sand, at the northern extremity of the range 

 (just under the battery. No. 7, Killiney Bay), and a brass composition nail 

 was driven into the centre of the head of the stake when level with the sur- 

 face of the sand. 



A cord of about 250 yards in length was now stretched by hand from this 



