gives when acting on a closed space, and which approaches the theoretical limit in proportion to the perfection of the ventilator.

Fifth, That this depression gradually varies and lowers according to conditions.

A little further explanation of these theories seems desirable for the benefit of those whose time has been more given to the distribution of air in mines, than to its economic production.

The equivalent orifice depends upon well known laws of the flow of fluids:

1st. That the speed of flow is the velocity due to the height of fall or height of column of the flowing air, which is represented by the formula for gravity—

2nd. That the quantity which passes through an orifice in a thin plate is two-thirds or 0.65 that of the quantity due to the area of the full orifice, and the formula then becomes, for what is known as the "vena contracta,"

where Q

=

Q = 8 ma√h,

quantity, m = 0·65, a = area, and h the height which must be reduced to air column, or as M. Murgue puts it

where 8, =

1000, and 8 = 1.2000, or the relative densities of water and air. Simplifying this for water gauge as usually taken in inches—

and if we assume these as normal densities we may remove the fraction and obtain

In this case V = volume in cubic feet per second; but as we generally speak of cubic feet per minute we may say

taking V = 1000 cubic feet per minute, and h = inches of water gauge.

The great value of this fiction is that it enables us to grasp at once the conditions under which a fan is working.

We hear of one machine giving 100,000 cubic feet per minute under a water gauge of 3 inches, and of another where 75,000 cubic feet is got with a water gauge of 4 inches; but this is not a tangible statement. If, however, we are told that the equivalent orifice of the first mine is equal to 22 feet, whilst that of the second is only 14, the difference is clearly demonstrated.

Then as to the Orifice of Passage, as M. Murgue says, the fan suited to blow a cupola may give a depression as great as the largest Guibal, but it would be useless to ventilate a mine-its orifice of passage is insufficient.

To obtain this in any case we must refer to his theory of initial and effective depressions (p. 4)—

where h, represents the deficiency or difference between the initial and effective water gauge; and we see a little further

the loss of effective depression increases.

To calculate the area of this orifice of passage we must refer to its value in the equation

That is to say, after finding for any ventilation the value of the initial and effective depression, the loss or useless depression is equal to a function of the volume which depends upon the orifice of passage, and from the above equation

reducing of course h, to the density of air in feet. We are now enabled to understand what is termed "the characteristic curve of the ventilator."

In order to compare two machines they are regulated to the same speed of periphery, or their results may be easily reduced to equal speeds since the volume varies as the revolutions and the depressions as the squares of the speeds.

The mine is altered, to say five different conditions: first by obstructing the passages; then in the normal state; and afterwards by opening some of the doors.

With the equivalent orifices of these five different mines, or conditions of mine, plotted as abscissæ, and the volumes as ordinates, we get a curve which shows clearly the effectiveness of each fan, and is called its "characteristic curve."

We are next shown how the effective depression varies and

diminishes by a function of the volume depending upon the orifice of passage.

The experiments upon the four different ventilators tested by the Commission of Gard are tabulated, and from them an equation for each is obtained, giving the initial depression and its coefficient of diminution as the volume increases. Taking, say Nos. 1 and 2 experiments on the Créal fan, with the square of the volume and depression observed, we getxy (89,283) = 1·0598,

xy (195,766) = 1.0183,

and from these constants are found.

The average of the various experiments should be taken. Little more need be added, but a short reference to the remarks on the effect of the natural ventilation of the mine upon such investigations, given in the report of the Commission of Gard, of which M. Murgue was a leading member, will be useful.*

The effect of natural ventilation is much less than is commonly supposed, since it is not the yields which are to be added, but the depressions, and the volume to be obtained therefore, is like the hypothenuse of the rectangular triangle, thus say in cubic metres = 20 + 5 = 20·615, and as x all conditions additive or subtractive are covered by the equivalent orifice, it may be entirely neglected.

If then, as is clearly demonstrated, the covered Guibal with évasée chimney most nearly meets theoretical requirements, how can its acknowledged defects in point of size, structural weakness, and heavy cost, be best met, so as to produce a machine perfect in every respect? The answer is to be found in a ventilator I have recently designed, and which will be at work at Pagebank Colliery by the time this issues from

* See Bulletin de la Société de l'Industrie Minérale,' 1878, p. 495.

b

the press. It is 20 feet diameter, built with, and from, a central wrought-iron diaphragm, like the Rammel and Schiele fans. It has the vanes in shape according to M. Murgue's demonstrations, made of iron and riveted to the diaphragm with angle irons, the sheet-iron cover, évasée chimney, and sliding shutter of Guibal type, with air admitted on both sides. The fan is driven by ropes, and will run about 110 revolutions, and much more if needed.

A. L. STEAVENSON.

DURHAM, 3rd February, 1883.