The supreme advantage claimed by the advocates of continuous driving, is equality of tension throughout.

The installation at the grain elevators of The Manchester Ship Canal does not appear to lend countenance to this assertion, from the fact that well defined irregularities are observable when the machinery is in motion, by the trailing span gradually slackening down as each lap completes the circuit, when it tightens up again and repeats the operation so long as movement continues.

Part though not the whole of this system has been supplanted by multiple ropes with, we are informed, a considerable saving in friction.

Another and more definite example is that of a vertical drive at the Belfast Linen Mill. Here the tension pulley occupied a central position as shown by Figure 15. But the turning moment could never be relied upon because of inequalites which were clearly discernible on the driving pulley, for the farthest lap proved to be very tight while each of the others gradually slackened until the far side fell entirely away from its groove.

FIGURE 16. — VERTICAL DRIVING.

This drive has since been converted to multiple ropes and the power is now most effectually conveyed from the basement to the fourth story of the mill, after the manner shown above although the pulleys are much nearer equal in size.

After all, is equality of tension an absolute necessity to efficient power transmission? If the principle of positive movement be admitted then the answer must be in the negative, and for this we can scarcely present a better argument than that offered to a certain consultant, who had drawn a chalk mark across the ropes in the expectation that they would maintain. the same relative position after repeatedly traversing the circuit.

Suppose, said the disputant, you can imagine each set of grooves to be tooth wheels as on a bicycle and each spliced rope to be a chain, would you consider it an evidence of slip if one chain hung below another, because it contains more links?

If the sheaves were loose as in a set of blocks then we could understand how easily tension regulations could be effected.

On the other hand this tension does not appear to be so much dependent upon the jockey pulley as upon the degree of tightness at which the ropes are originally fixed, if it does, then an amount of creep or slip must be accounted for.

Again there is the replacement difficulty. In multiple driving more than one rope seldom gives way at a time, and that generally at the splicing which draws out until the rope slackens far below its fellows. The offender may then be hung aside to await a favorable opportunity, leaving the remainder to carry on the work.

In a continuous drive dependence must be placed upon one splicing which may be considered to be almost always under strain, and as sometimes happens, it may unravel, be carried round and lashed about until the entire range is denuded. To be renewed only at the end of a lengthy process during which the machinery is standing.

Is it not also true that the addition of a single unnecessary pulley, particularly if that be weighted in a vain attempt to regulate the tension, adds considerably to that friction which we labor almost incessantly to minimize.

CASING OF PULLEYS.

An important consideration often omitted when arranging for

rope driving, is the casing of pulleys with sheet metal or boards without which the deplacement of air by the arms of large high speed pulleys adds materially to the profitless load.

In the absence of data from which to ascertain the starting point of economy by such arrangements, it may be as well to submit the opinion of three well-known engineers who have had considerable experience in rope driving.

One firm recommends the casing of all pulleys small or large on the grounds that by so doing a saving of 50 horse power in 1,000 may be effected.

Another advocates casing pulleys 7 feet diameter and over, running at a peripheral velocity of not less than 4,000 feet per minute.

While still another firm advises casing all sizes, but declares economy to be noticeable on large high speed pulleys.

MINIMUM DIAMETER OF SMALLEST PULLEY.

The item which appears most naturally to offer itself first for consideration in our specification, is the bending and consequent

FIGURE 17. DETRIMENTAL EFFECT OF BENDING OVER SMALL CIRCUMFERENCES.

gripping capability of the rope itself. Soon after cotton came into use for main driving ropes it was discovered that a three

strand rope of this material would cling firmly to a pulley thirty times its own diameter, but would gradually relax its hold as the relative diameter decreases from this standard. Subsequent practice places the validity of this conclusion beyond the possibility of doubt.

That the vitality of the rope also suffers from the undue strain by the use of abnormally small pulleys may be best demonstrated by the diagram which in order to emphasize what actually takes place, exhibits a rope passed round a circle only fourteen times its own diameter. Lines drawn to an angle of 45 degrees through the ogee curves representing the instertices of the strands, are carried forward along the horizontal lengths until one of them intersects the vertical centre line. This extended, determines the radius of the inner circle to which lines drawn through the bent portion of the rope are tangent, and these well display the inner contraction at the expense of an extended outer periphery.

PROPORTIONATE DURABILITY OF ROPES ON LARGE AND SMALL

CIRCUMFERENCES.

Too much importance can scarcely be attached to the relative proportions of pulleys and ropes for it is absolutely certain that both driving force and durability suffer most materially from the non-observance of the thirty diameter regulation and that nothing, not even space restriction, has gone so much against the successful application of ropes to the driving of dynamos as a persistent effort to get below this natural limitation.

Therefore, instead of adding to the burden of the drive by employing an abnormal quantity of thick ropes to make up for grip depreciation when large pulleys prove impracticable, it is far better to divide the total bulk nominally required into the necessary number of smaller ropes.

Any advance upon thirty diameters is of course favorable to the life of the ropes.

Pulley diameters should also bear some relation to velocities. In very slow drives a little latitude may in necessary cases be