let it be so formed as to take off any acute angle which would be apt to break off in sand.

The arms should become larger as they approach the centre of the wheel, (see Emer

son, page 179) and the eye, E, should be sufficiently strong to resist the driving of the wedges, by means of which it is to be fixed on the shaft. This cannot be brought easily to calculation.

On the other hand, care must be taken not to make the eye so thick as to endanger unequal cooling.

It should be somewhat broader than the breadth of the teeth, in order that it may be the firmer on the shaft: this breadth must be greater in proportion as the wheel is large.

When the ring ae is about an inch thick, it is common to make the eye about an inch and quarter thickness, and about broader than the ring, when the wheel is about four feet diameter.

Small wheels have generally but four arms, but it being improper to have a great space of the ring unsupported, the number of arms should be increased in large wheels.

In order to strengthen the arms with little

increase of metal, it is not unusual to make them feathered, which is done by adding a thin plate to the metal at right angles to the arm, as represented by figure third. Fig. 4 is a section of Fig. 3, at A B.

The same rules apply to bevelled wheels; of the practical mode of laying down the working drawings of which we have already spoken. But it is proper to observe, that the eye of a bevelled wheel, should be placed more on that side which is furthest from the centre of the ideal cone of which the wheel forms a part.

When wheels are beyond a certain size, it becomes necessary to have patterns sometimes made for them, cast in parts, which are afterwards united by means of bolts.

A very good mode to prevent the bad effects of unequal contraction, is to have the arms curved, as in the second figure, the curved parts are commonly of the same radius as the wheel, and spring from the half length of the arms.

Materials of Patterns.

The patterns should be made of well seasoned wood. The most proper is clean mahogany, but that being now very expensive, white deal is most commonly used. Beech is very often used for the teeth, and being a close grained wood, it may be made very smooth.

It is almost superfluous to say, that the workmanship of wheel patterns should be such as to produce great accuracy, and a smooth surface, the former being essential to the good movement of the wheels, and the latter to make the patterns produce a good clean impression in the sand.

I am aware, that it is a common practice, in many places, to make teeth very large in the pattern, and after fixing the wheels on their shafts, to chip and file the teeth to the proper size.

I doubt, however, whether this practice be

really advantageous; for besides the great time which it occupies thus to dress the iron teeth, and the consequent expense, there is the loss of the outer skin (if I may use the expression), of the cast iron, which is by far its most smooth and durable part. In cotton mills, therefore, this method is now but seldom practised *.

As it may be of use to millwrights, I take the liberty of inserting the following tables from a respectable periodical publication.

* Messrs. Peel, Williams, and Co. have, after great time, trouble, and expense, made and arranged a very great number of patterns of wheels, so as to suit almost every case that can in practice occur. They have published a complete list of them, which they intend inserting also in the "Repertory of Arts." In my opinion, what they have done is a material national benefit; their expense, I am informed, for patterns, has not been less than four thousand pounds. There is, however, every reason to think, that it will be an excellent thing ultimately for themselves, as well as of great practical utility to the public.

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