When the rock to be sunk through is unstratified, or if stratified, when of great thickness, recourse must be had to the action of explosive agents. The explosives most frequently used for this purpose are guncotton, dynamite, lithofracteur, and gunpowder. Lithofracteur is now often employed, and always with considerable success, as its power is similar to that of dynamite, but, what is particularly important in vertical bore-holes, its action is intensely local; it is, moreover, safe, does not generate fumes more harmful than ordinary gunpowder, requires smaller holes, and but little tamping. The dangerous character of guncotton has hitherto prevented its adoption for ordinary operations, while the comparatively safe character and convenient form of gunpowder have commended it to the confidence of workmen, and hence for sinking operations this explosive is generally employed. We shall therefore, in treating of blasting for well sinking, consider these operations as carried out by the aid of gunpowder alone. The system of blasting employed in well sinking is that known as the small-shot system, which consists in boring holes from 1 to 3 inches diameter in the rock to be disrupted to receive the charge. The position of these holes is a matter of the highest importance from the point of view of producing the greatest effects with the available means, and to determine them properly requires a complete knowledge of the nature of the forces developed by an explosive agent. This knowledge is rarely possessed by sinkers. Indeed, such is the ignorance of this subject displayed by quarrymen generally, that when the proportioning and placing the charges are left to their judgment, a large expenditure of labour and material will produce very inadequate results. In all cases it is far more economical to entrust these duties to one who thoroughly understands the subject. The following principles should govern all operations of this nature.