(a) The variability in velocity of the air circulating in the fan casing is pronounced. This condition is chiefly due to:- (i) the uneven distribution of the air through the fan; • (ii) the sudden increase of area into which the air enters immediately on emerging from the fan-wheel; • (iii) departures from the true spiral or volute in the manufacture of the fan casing. • • (b) The velocity of air effluent from the runner of a Sirocco fan is considerably higher than the peripheral speed of such a fan; at some points of the discharge the air velocity may exceed the fan speed by over 70 per cent. • • (c) The velocity of air re-entering the fan is higher than the peripheral speed of the fan. The general direction of the re-entrant flow coincides with that of normal flow. • • (d) The effect of slight departures from the true spiral or volute is appreciable. Makers should endeavour to adhere more closely to the true curve. • • (e) The transmutation of kinetic energy into pressure energy in fan casings of the type investigated, is most inefficient. The width of the casing in relation to the width of the fan must produce considerable loss of energy due to shock; it is analogous to a sudden enlargement where the head lost due to shock is ((V₁ - V₂) / 2g))². • • (f) Where no diffuser exists, it would seem preferable to design a casing to function solely as a collecting and guiding medium of the air discharged from the fan to the avasée, leaving the conversion. of energy to the latter adjutage. (Note: A casing designed in accordance with this principle is being installed in the Mining Laboratory. See Appendix F) • • (g) The shape of the casing at its commencement, i, e. , at the "beak" of the fan, is of great importance. The marked throttling effect below the "beak" of a Sirocco fan, demonstrated in both Sections B and D, is caused by the reduction in section at that point. A casing should increase in section progressively from its commencement. • • (h) An adjustable clearance between the fan runner and the commencement of the casing, such that the shape of the "beak" could be also adjusted to suit the clearance, would appear to be a distinct advantage. Although not specially related to any of the foregoing conclusions the following may be added as a general conclusion: - At the outset we endeavoured to emphasise the importance of effective and efficient ventilation in our deep and extensive mines, and especially in safety - lamp mines. Although our mining legislation makes no provision for a ventilating engineer at a colliery, the time seers opportune for the appointment of an official of such a character about a mine. While the manager must necessarily be capable of arranging a ventilation scheme which would be both practical and efficient, nevertheless his duties are so multifarious that he requires all the skilled assistance that can be given him. He is provided with a fully qualified surveyor, engineer and electrician, together with their respective staffs, but in the most essential factor towards the successful exploitation of the mine, namely, ventilation, he has no such expert aid. The duties of the under-manager, oversmen and deputies are also diversified to such an extent that ventilation matters are usually considered only in emergency, although the last named officials have perforce to carry out their regular statutory inspections in this connection. It would be quite impossible to assess the value of an efficient ventilating engineer at a mine. His sphere of activities would extend round the whole ventilating circuit. Underground, his regular and skilled inspection would lead to the removal of a 11 sources of unnecessary and avoidable leakage, and to the efficient distribution of the air. His frequent consultations with the deputies could not fail to raise the standard of district ventilation to a high plane of health and safety. At the surface, the fan plant would be reliably and regularly tested and any glaring causes of inefficiency (e.g. natural ventilation) remedied as far as possible. If the fan was provided with an adjustable clearance, as has been suggested, its regulation for maximum efficiency would be assured; again, any structural alteration of the évasée which would lead to increased economy would be readily manifest to such an engineer. In suggesting this appointment it is not intended that the ventilating engineer should be given authority to over-ride the manager in ventilation matters connected with the mine; the authority of the latter would remain, as now, supreme. Nevertheless, the criticisms and proposals made by such an engineer on all points pertaining to the ventilation of the mine would be invaluable to the manager. The whole of the activities of this engineer would be concentrated on the fulfilment of Section 29 (1) of the Coal Lines Act (1911) , and on assuring himself that the adequate amount of ventilation therein stipulated was being maintained with the maximum degree of efficiency possible.. The measure of his success must inevitably be reflected in the balance-sheet.