300 



REPORT — 1851. 



The personal equation derivable from these experiments is as follows:— 



W. Mallet's observations. 



Exp. 1. 21-00 



2. 21-25 



3. 21-00 



4 . 21-25 

 4-)84.-50 



Average 21-125 



Divided by 5 revolutions =4<""225 

 = time of one revolution of large 

 dial. 



R. Mallet's observations. 

 Exp. 



21-80 

 21-50 



21-4.5 

 21-75 



4)86-50 

 Average 21-625 



Divided by 5 revolutions =4'"-325 

 = time of one revolution of large 

 dial. 



Then 



4.-325 

 4-225 

 0*100 total difference, 



0"-100 



and = 0"-05 = the error in defect in W. Mallet's observations and in 



2 

 excess in R. Mallet's. Then, as 0"-05=the error for one revolution of large 



dial, — — =0"-00417=error for one division of large dial, or for one revo- 



12 0"'00417 

 lution of small one, and — ^7; =0"-00014 error for one division of small 



dial. 



Everything being now prepared,'and a remarkably calm and fine day pre- 

 senting on the 3rd of October, 1850, the several workmen and gunners were 

 collected on the island, and the experiments actually commenced at about 

 10 o'clock A.M. were completed by 6 o'clock p.m., the accurate admeasurement 

 of the base line or range from the datum stone A, near the blasts, to the 

 observation station at seismoscopc, being left for a future day. 



Ten holes in all had been prepared for blasts, and five were charged and 

 tamped at once to avoid damping the powder by its remaining too long in the 

 holes. Each hole received from 18 to 20 lbs. of powder. They were fired 

 (the first five first, and then the last five) in the order or number of the expe- 

 riments in the following table. The letters attached in the second column 

 refer to the Map, and show the position of the blasts in the order in which 

 they were fired, it having been deemed expedient to fire in succession those 

 least likely to run into each other, and so, by the fractures produced, spoil 

 the eff'ect of the subsequent ones. 



There was one hole (G) a " wet one," i. e. a run of water into it ; but it 

 nevertheless fired well and gave a good result, having been stopped with clay 

 before charging. Only two of the holes " threw " much rock to any distance, 

 the whole force of the' charge being expended in all the others only in rend- 

 ing and shaking the rock. Not one gave a loud report ; nothing beyond a low, 

 hollow groan. In one case (D), a solid mass of rock, weighing by measure- 

 ment from 90 to 100 tons, was shifted horizontally from its bed, not less than 

 25 feet having been fractured from the parent rock on three sides. 



