There are numerous types and various sizes of batch mixers. A one-bag batch machine is most suitable for general work, though there are smaller mixers that may prove handy. Some of the smallest sizes are operated by hand, but the medium and large sizes are power operated. Mixers can be had with or without the power plant attached and may be stationary or on wheels, which facilitate moving to different sites. Engines used for sawing wood, the larger ones used for pumping water, and tractors furnish sufficient power to operate an average mixer. Figure 7 shows a homemade mixer built of discarded farm implement parts and operated by the farm engine. Directions for operating a mixer are generally furnished with the machine. The tendency is to use too much water in mixing concrete in a machine. The consistency of the mixture should be as described under the heading "Consistency" on page 8. The mixing should be continued for at least a minute after the drum has been charged, but a better mixture is secured if two minutes are allowed. At the end of each day's work the machine should be thoroughly washed, and when not in use it should be well greased and covered. HAND MIXING. Hand mixing is the more economical on the farm unless a large amount of work is to be done at one time. Few tools need be purchased, and, as a rule, only farm help need be employed. The following tools will be needed in mixing and placing plain concrete: Two or more square-end short-handled shovels, 1 heavy garden rake, 1 sprinkling can or bucket (if a hose is not available), 1 52-gallon barrel, 2 wheelbarrows with metal trays, 1 sand screen (Fig. 8), 1 tamper (Fig. 9), 1 wood float or trowel (Fig. 10), measuring boxes (Fig. 11), mixing board (Fig. 12), 1 spader (Fig. 13). The number of shovels and wheelbarrows needed will depend upon the size of the batch, number of men mixing, and the layout of the work. Long-handled pointed shovels will be found more convenient at the sand and gravel piles. A bottomless box is necessary for convenient and accurate measurement of the sand and gravel. Where wheelbarrow measurement of materials is practiced, as in charging a mixer, the capacity of the wheelbarrow should be determined by use of a measuring box. The box may be made as illustrated in Figure 11, from boards 12 inches wide. The dimensions in Table 2 are of boxes for use in measuring quantities for mixtures of various proportions, assuming that one bag of cement is used in a batch. If two bags are used in a batch the boxes should be filled twice. [Illustration: Fig. 8.--Sand screen.] [Illustration: Fig. 9.--Tampers.] [Illustration: Fig. 10.--Wooden float.] [Illustration: Fig. 11.--Measuring box.] [Illustration: Fig. 12.--Mixing Board.] [Illustration: Fig. 13.--Spading tool.] Table 2.--_Inside dimensions of measuring boxes for various proportions._ [1-bag batch, box 12 inches deep.]