The continuity and regularity of machine work are also reflected in certain economies of measurement. The faculty of self-registering, which belongs potentially to all machinery, and which is more utilised every day, performs several services which may be summed up by saying that they enable us to know exactly what is going on. When to self-registration is applied the faculty of self-regulation, within certain limits a new economy of force and knowledge is added. But machinery can also register and regulate the expenditure of human power. Babbage well says:--"One of the most singular advantages we derive from machinery is in the check which it affords against the inattention, the idleness, or the knavery of human agents."[64] This control of the machine over man has certain results which belong to another aspect of machine economy.[65] These are the sources of all the improvements of economies imputed to machine-production. All improvements in machinery, as applied to industrial arts, take therefore one of the following forms:-- (1) Re-arrangement or improvement of machinery so as to utilise more fully the productive power of nature or man. Improvements enabling one man to tend more spindles, or enabling the same engine at the same boiler-pressure to turn more wheels, belong to this order of improvement. (2) Economies in the source of power. These will fall under four heads-- 1. Substitution of cheaper for dearer kinds of human power. Displacement of men's labour by women's or children's. 2. Substitution of mechanical power for human power. Most great improvements in the "labour-saving" character of machinery properly come under this head. 3. Economies in fuel or in steam. The most momentous illustration is the adoption of the hot blast and the substitution of raw coal for coke in the iron trade.[66] 4. The substitution of a new mechanical motor for an old one derived from the same or from different stores of energy--_e.g._, steam for water power, natural gas for steam.