Compressed Air Work And Diving 1909

THA WING OUT EXPLOSIVES. 103 country for the handling and storage of explosives, and it is essential that only methods sanctionecl by the authorities should be used. The reason why nitro- glycerine requires thawing is that when frozen or con- gealed it becomes, under certain conditions, less sensitive to shock, and therefore more difficult to cletonate, and liable to miss fire. On the other hånd it is also known, owing, it is thought, to a certain molecular change which takes place, that in this conclition it sometimes becomes more sensitive, and certain accidents which have happened have been attributed to its frozen conclition. Usually dynamite becomes less sensitive to shock and biasting gelatine more so when frozen. It will therefore be seen that thawing out is an essential preliminary to use of nitro-glycerine when frozen or congealed, and when the explosive is in this condition care should be taken not to fracture or drop it. The nitro-glycerine may become congealed even though the cartridge still feels soft. Its conclition, therefore, must be juclged by the temperature and not by the feel of the cartridge. Although methods of calculating the quantity of explosive requirecl for charging boreholes have been formulated, this is so entirely a matter for practical experience and judgment that no definite rule can be given. The efficiency of the biast clepends a great cleal upon the skiil of the driller in “pointing” the drill. In caisson work the best plan is to start at the centre, making the hole at rather a flat angle, so that the rock will be lifteel upwards. As holes are clrilled in succession in the direction of the cutting edge, the direction of the drill will be more in a downward direction until, when the cutting edge is reached, they will be pointing clown at an angle of about 60 clegs. with the horizontal.