(377) Having thus described the essential portions of a ring spinning machine, some of the difficulties and principles of the mechanism may be dealt with. It is quite certain that the full theoretical reasons for the successful accomplishment of this work are not now forthcoming, but an approximation to them is possible. The actual spinning process is merely a twisting together of the fibres of any material by the rapid revolution of a flyer or spindle while the fibre is being delivered at a definite rate. In this case the twist is put in by the rotation of the traveller, which, as shown, is actuated from the spindle. As it has never yet been accomplished to take off the yarn from the spindle at the same rate as it is being wound on, it is necessary that the twisted fibre should be collected on the spindle or on a bobbin super-imposed on it. In order to do this, as was shown in Chapter X., it is essential that the eye or guide through which it is delivered to the spindle should travel either faster or slower than any fixed imaginary point on the spindle. The latter is the invariable rule with the ring frame, and it will be seen that as the bobbin is revolving at a quicker rate than the flyer eye, or in this case the traveller, it will take up the yarn and gradually wind it on to itself. Of course, in the case of a mule this does not happen, the winding arrangement being there altogether different. The amount of “lead” which the bobbin has should correspond approximately to the number of inches of yarn delivered by the rollers. That is to say, if the yarn is receiving ten twists to the inch, the bobbin should take up approximately, during ten revolutions, one inch of yarn. Now it is quite clear that if the velocity of the bobbin varies, the speed at which the traveller is pulled round the ring will vary also, but that, owing to the resistance caused by its weight and frictional contact with the ring, it will always tend to lag behind the bobbin. A little examination will show that the weight of the traveller is really the determining, or, at any rate, the most important, element in the case. As has been explained, the rotation of the traveller is caused by the pull exercised on it by the yarn. Now the velocity at which ring spindles are revolved is very great, averaging in ordinary cases at least 8,000 per minute. It is quite clear that a traveller rotating at that speed will tend by centrifugal force to fly outwards, and thus cause its inner lip or edge to press against the inside of the ring. Although it is quite true that this contact is only a slight one it exists, and constitutes one of the elements in the case. But it is also evident that the greater the weight of the traveller the greater will be the force that is exerted against the inside of the ring. While the author does not wish to do more than express his own opinion in this matter, there seems to be substantial ground for belief that the tangential pull on the yarn between the traveller and the point at which it reaches the bobbin will to a great extent counterbalance the tendency to fly outwards. It therefore seems probable that the reasonable explanation of the drag of the traveller is to be found in the resistance set up by its weight rather than by its frictional contact with the ring. This is the principle upon which travellers are made, their weight being carefully graded in order to suit various counts of yarn and velocities of spindles. There is another feature in which the weight of the traveller is evidently of importance, and that is its relation to what is known as ballooning.