_Dosing Devices._—Dosing devices referred to in Chapter III are usually purchased directly from the manufacturer, and while their cost varies a little, depending upon the cost of freight, an ordinary single automatic siphon may be estimated at $15, the difference in price varying a little with the different makes of siphon. If an alternate discharge is required, then two siphons must be installed, by means of which alternate intermittency is secured, the variation, however, being only from one to the other and back again to the first. If a plural alternate discharge is to be used, the cost may be estimated roughly for a 6–inch siphon at from $50 to $75 for each unit, this price including the necessary piping but not the cost of setting. _Filling Material._—Artificial sand filters require a sand of uniform size and one free from dirt. These two requirements add very materially to the cost of sand, since it is almost impossible to find a natural sand which fulfils the necessary requirements. A few sections of the country are fortunate in having sand in the vicinity suitable for filtration purposes without any washing or screening. Such parts of the country, however, are limited to those where sand has been deposited by glacial action, and is essentially silicious in character. It is hopeless to expect to find suitable sand in the centre of New York State, for example, and even with washing and screening, sand in this locality is far from being desirable. It will be found, further, that after this undesirable sand is washed and screened the cost of the final product is so great that it is usually cheaper to use broken stone either as a filter or as a contact bed.