MECHANICS AND USEFUL ARTS. 93 



tice great skill can be attained in judging of distances, and the 

 relative position of masses of troops ; while more minute details 

 could be subsequently obtained at leisure by field-glasses as to the 

 position of mountain gorges, passes, limits of woods, and the 

 course of streams. The trials hitherto made have been chiefly 

 carried on by professional aeronauts with hired balloons ; and it 

 is believed that the British Government have at the present time 

 no war balloons in store. 



The result of the observations of Captain Brackenbury and 

 Captain Noble, sent out from Woolwich on behalf of the English 

 Government to the respective seats of war, together with trials 

 and other sources of information, will, it is believed, result in 

 war balloons being manufactured in the Royal Arsenal, and that 

 officers of royal engineers, from generals downwards, will be 

 trained in their use. 



THE GREAT SUBMARINE BLAST IN THE HARBOR OF SAN 



FRANCISCO. 



It is well known that extended preparations have been making 

 for several months, under the direction of Col. Van Schmidt, C.E., 

 to remove what is known as Blossom Rock, the most dangerous 

 obstruction in the magnificent harbor of San Francisco. 



That so important an engineering work should have been 

 brought to so successful a conclusion reflects great credit upon 

 the skill of Col. Van Schmidt, and the particulars of this feat of 

 submarine blasting will be interesting to our readers. 



A careful survey of the rock was made at the outset in order to 

 ascertain the irregularities of its surface. This being done, and 

 plans being drawn, a coffer-darn was constructed around a por- 

 tion of the rock to be removed, and moored by means of a scow 

 loaded clown with heavy stones. 



The water was stopped from entering the dam by sand-bags 

 placed about its base, and an iron turret was erected within it. 

 This turret was then sunk 3 feet into the rock and cemented fast. 

 A platform 56 feet long and 20 feet wide was then erected, and an 

 engine and hoisting apparatus placed thereon, together with a 

 building in which to lodge the workmen and prepare their 

 meals. 



The plan of excavation was to scoop out the interior of the 

 rock, leaving an external shell of sufficient thickness to resist the 

 pressure of the surrounding and superincumbent water, and 

 finally to shatter and scatter the shell by blasting. The thickness 

 of the shell was about 6 feet. 



Pillars of rock were left- to sustain the shell, and when these 

 were removed they were replaced by timber supports. The 

 space excavated measured in the clear 140 by 50 feet, and it is 

 estimated that about 40,000 cubic feet of stone were taken out. 

 The shape of the surface of the rock was nearly oval, but for a 

 distance of about 120 feet it sloped very little. The height of the 

 highest pillar inside was 29 feet, and the lowest 4 feet. The 



