The principal feature of the system (and the secret of its great success) is the very high pressure at which the oil is injected into the cylinder. This pressure is kept up at about 4,000 lb. the square inch, so that the oil fuel, when it enters the cylinder and encounters the hot compressed air therein, is in the form of a very finely atomized mist, a conjunction of circumstances most favourable for ignition. As in all great inventions, the simplicity of the arrangement is not the least of its merits. This somewhat rough, yet brief, outline will suffice to explain the astonishing success of the Vickers heavy oil engine, but, if the whole history of these (and other) noteworthy experiments could be written, a highly interesting story would be produced, showing indomitable perseverance in the face of discouragement, difficulty, and very heavy expense. I have avoided touching upon the ordinary kerosene engines, for I imagine they are too well known to need more than passing reference here; nor have I gone into the details concerning the advent of the ordinary Diesel engine, which was a German invention. I have preferred rather to deal with a British invention which is already revolutionizing oil engine construction generally, and which, obviously, has limitless fields open to it. CHAPTER IX PETROLEUM IN ENGLAND There will be no chapter in this little treatise which will be more carefully perused than the present one, for the subject is of direct interest to every reader, whether actually associated with the search for oil or not. To-day, as I have already mentioned in another chapter, this country is dependent for practically the whole of its petroleum requirements upon foreign oil-producing countries, and though ample evidence is forthcoming to suggest that there are possibilities of obtaining liquid oil in England--in fact, many years ago this was actually obtained in not inconsiderable quantities--it is very strange that only recently have serious efforts been made in the direction of systematic search for the valuable liquid. That large quantities of petroleum can be produced in this country is agreed by all who have given the subject more than passing thought; the question is, by what means shall this production be brought about. While it is problematical as to the amount of commercial success which will attend the present search for liquid oil, though those who are most competent to judge believe that large stores of liquid oil will be found, it is already certain that there are vast possibilities in England for the production of petroleum from the treatment of the bituminous shales which freely abound in many parts.