Tide Mills

Tide mills, as their name imports, are such as employ for their first mover the flowing and ebbing tide, either in the sea or a river.

Mills of this kind have not often, we-believe, been erected in England, though several of our rivers, and particularly the Thames, the Humber, and the Severn, in which the tide rises to a great height, furnish a very powerful mover to drive any kind of machinery, and would allow of tide-mills being very advantageously constructed upon their banks. The erection of such mills is not to be recommended universally, as they are attended with a considerable original expense; beside that, some of their parts will require frequent repairs: but in some places, where coal is very dear, they may, on the whole, be found less expensive than steam engines to perform the same work, and may, on that account, be preferred even to them.

We have not been able to ascertain who was the first contriver of a tide-mill in this country, nor at what time one was first erected. The French have not been so negligent respecting the origin of this important invention, as to let it drop into obscurity; but have taken care to inform us that such mills were used in France early in the last century.

Belidor mentions the name of the inventor, at the same time that he states some peculiar advantages of this species of machine. "L'on en attribue," says he, " la premiere invention à un nommé Perse, maître charpentier de Dunkerque, que mérite assurément beaucoup d'éloge, n'y ayant point de gloire plus digne d'un bon citoyen, que celle de produire quelqu'invention utile à la société. En effet, combien n'y a-t'-il point de choses essentielles à la vie, dont ne connoît le prix que quand on en est privé : les ffloulms en général sont dans ce cas-là. On doit sçavoir bon grê à ceux qui nous ont mis en état d'en construire partout : jar exemple, à Calais, comme il n'y serpente point de rivières, on n'y a point fait jusqu'ici de moulins à eau, et ceux qui vont par le vent chômant une partie de l'année, il y a des tfms où cette ville se troxive sans farine, et j'ai vu la garnison en 1730, obligé de faire venir du pain de Saint-Omer, M beu qu'en se servant du flux et reflux tie la mer, on pour- fait construire autant de moulins à eau que l'on voudrait : il J a d'autres villes dans le voisinage de la mer sujettes au même inconvénient, parcequ'apparemment elles ignorent le moyen d'y remédier.

Mills to be worked by the rising and falling of the tide, admit of great variety in the essential parts of their construction ; but this variety may perhaps be reduced to four general heads, according to the manner of action of the water-wheel.

1. The water-wheel may turn one way when the tide rises, and the contrary when it falls.
2. The water- wheel may be made to turn always in one direction.
3. The water-wheel may fall and rise as the tide ebbs and flows.
4. The axle of the water-wheel may be so fixed as that it shall neither rise nor fall, though the rotatory motion shall be given to the wheel, while at one time it is only partly, at another completely, immersed in the fluid.

In the mills we have examined, says Dr. Gregory, the first and third of these divisions have been usually exemplified in one machine ; and the second and fourth may readily be united in another : we shall, therefore, speak of them under two divisions only.

1. When the water-wheel rises and falls, and turns one way with the rising tide, and the contrary when it ebbs.

2. Tide-mills, in which the axle of the water-wheel neither rises nor falls, and in which that wheel is made always to revolve in the same direction.

In selecting a site for the erection of a mill, the engineer mast be careful not to make choice of a spot that is liable to be flooded. When the water in the mill-tail will not run off freely, but stands pent up in the wheel-race, so that the wheel must work or row in it, the wheel is said to be tailed, or to be in back-water or tail-water ; which greatly impedes the velocity of the wheel, and, if the flood be great, completely stops it.

Every mill that is well and properly constructed, will clear itself of a considerable depth of tail-water, provided there is, at the time, an increase in the height of the water in the mill-dam or head, and an unlimited quantity of water to draw upon the wheel. Common breast-mills will bear two feet of tail-water, when there is an increase of head, and plenty of water to be drawn upon the wheel, without prejudice to their performance ; and nulls that are well constructed, with slow moving wheels, will bear three and even four feet and upwards of tail-water.

Mr. Smeaton mentions having often an instance of six feet ; and it is a common thing in level countries, where tail-water is most annoying, to lay the wheel from six to twelve inches below the water's level if the pond below, in order to increase the fall of water ; and, if judiciously applied, is attended with good effect, as it increases the diameter of the wheel, and though it must always work in that depth of tail-water, it will perform full as well, because the water ought to run off from the bottom of the wheel, in the same direction as the wheel turns.