282 REPORT — 1851. 



Hence the quantity of powder was found to be so large that I determined 

 to abandon my original intention of experimenting at a mile range, and to 

 limit myself to half a mile, and for this chose the best half of the range, viz. 

 the northern half mile, which was quite level and free from any error as to 

 measurement, being all to the north of Shanganah River. 



I also concluded that less than (4)^ X 2 lbs.=32 lbs. of powder would pro- 

 bably do, as the impulse given to the seismoscope by the 2 lbs. at a furlong 

 range was more powerful than requisite for distinct observations. Subse- 

 quent trial on the 29th of October, 1849, proved this to be correct, and that 

 25 lbs. of powder gave an impulse distinctly visible at half-mile range*. 



* It is of some importance to refer to the effects in concussion of the earth and air that 

 have been occasionally observed to result from the explosion of known quantities of powder 

 at determinate distances, because we thus get some measure (though a very inadequate one) 

 of the amount of explosive or elastic force that must be concerned in the production of 

 such concussive movements of the ground and of such subterranean noises, or sounds con- 

 veyed thence through the air, as have been actually noted by earthquake narrators. 



The slight shocks frequently observed at Dublin by Professor Lloyd, and only capable of 

 observation through the refined instruments of the magnetic observatory, as well as those so 

 long and so continuously experienced at places such as Comrie in Perthshire, East Haddam in 

 Connecticut, and Bale in Switzerland, are probably not due to true explosive efforts beneath, 

 but rather to successive fracturing of beds of rock under the slow and gradual process of 

 elevation or of depression, taking place either immediately beneath or perhaps at a considerable 

 distance away ; but our present experiments show how comparatively insignificant an explo- 

 sive origin may be sufficient to transmit an instrumentally appreciable shock to a considerable 

 distance. 



I have referred in the Transactions of the Royal Irish Academy, vol. xxi., to the undula- 

 tion of the ground, felt for several miles, when the large powder magazine was blown up in 

 Sir John Moore's retreat to Corunna. Sir Christopher Wren demolished the great centre 

 tower of old St. Paul's by a small mine of only 18 pounds weight of powder buried beneath 

 one of the angles. The explosion, he informs us, lifted bodily through some inches above 

 3000 tons of masonry, and " the fall of so great a weight from a height of 200 feet gave a 

 concussion to the ground that the inhabitants round about took for an earthquake." — 

 JVren's Parentalia, pp. 283, 285. 



When the great chalk cliff at Seaford was thrown down by the explosion of nearly 26,000 

 lbs. of powder (Sept. 1850), it is stated, " The rumbling noise was probably not heard half a 

 mile off. Those who were in boats a little way out from the cliff state that they felt a slight 

 shock. It was much stronger on the top of the cliff ; persons standing there felt staggered 

 by the shaking of the ground, and one of the batteries (galvanic) was thrown down by it." 



" In Seaford, three quarters of a mile off, glasses upon the table were shaken, and one 

 chimney fell. At New Haven, a distance of three miles, the shock was sensibly felt ; about 

 300,000 tons of chalk was said to be an approximation to the weight moved." — Times, Sept. 

 1850. 



When the late fatal explosion occurred at Hounslow Powder Mills, the following circum- 

 stances are narrated. 



" A correspondent at Brighton informs us that the late fatal explosion at Hounslow was 

 distinctly felt and heard at Sussex, a distance from Hounslow of between 50 and 60 -miles. 

 At Petworth, upwards of 40 miles off, three reports were distinctly heard, and a corre- 

 sponding number of shocks were felt, so that the inhabitants ran out of their houses, 

 imagining that they had felt shocks of an earthquake. Mr. Heslop, a teacher of languages, 

 while walking near the mansion of Sir Isaac Lyon Goldsmid, at Hove, near Brighton, heard 

 a noise as of distant thunder, but as the sky was clear he imagined that he must have heard 

 a roar of artillerj'. It was then 25 minutes to 4 by his watch. Mr. Smith, a gardener and 

 nurseryman, at the north part of Brighton, also distinctly heard the noise. The most con- 

 clusive evidence, however, that the explosion was felt between 50 and 60 miles from the spot 

 is furnished by the following paragraph, copied from the Sussex Advertiser of Tuesday, a 

 paper which was printed and in town before the news of the explosion reached Lewes : — 

 ' Accounts have reached this office from several persons, living in different parts of Lewes, who 

 depose to having experienced a slight shock, as if of an earthquake, at about 20 minutes to 

 4 o'clock on Monday afternoon. There was no train passing through the tunnel at that 

 hour. Moreover, the shock was felt in parts of the town not affected by the passage of 

 trains. All the persons alluded to experienced the shock at the same period of time.' From 

 various other places there are similar accounts, all agreeing as to time, but varying a little in 



