=76. Special Instruments Employed in Taking Samples.=—In general a sharp spade or post-hole auger is quite sufficient for all ordinary sampling but for certain special purposes other apparatus may be used. The instrument which is used by King[60] consists of a thin metal tube of a size and length suited to the special object in view, provided with a point which enables it to cut a core of soil smaller than the internal bore of the tube and at the same time make a hole in the ground larger than its outside diameter. Its construction is shown in figure 11, in which A B represent a soil tube intended to take samples down to a depth of four feet. A′ is a cross-section of the cutting end of the tube, which is made by soldering a heavy tin collar, about three inches wide, to the outside of a large tube allowing its lower end to project about one-half an inch. Into this collar a second one is soldered with one edge projecting about one-quarter of an inch and the other abutting directly against the end of the soil tube. Still inside of this collar is a third about one-half an inch wide which projects beyond the second and forms the cutting edge of the instrument. [Illustration: FIGURE 11. ] The construction of the head of the tube is shown at B′. It is formed by turning a flange on the upper end of the tube and then wrapping it closely with thick wire for a distance of about three inches, the wire being securely fixed by soldering. The soil tube should be of as light weight as possible not to buckle when being forced into the ground, and the cutting edge thin. The brass tubing used by gas fitters in covering their pipes has been found very satisfactory for ordinary sampling. With a one inch soil tube four feet long it is possible to get a clear continuous sample of soil to that depth by simply forcing the tube into the ground with the hand and withdrawing it, or the sample may be taken in sections of any intermediate length. Later in the season when the soil becomes dryer it is necessary to use a heavy wooden mallet to force the tube, and this should be done with light blows. The closeness with which it is possible to duplicate the samples in weight by this method will be seen below, where from each of four localities three samples were taken from the surface to a depth of four feet. SHOWING VARIATIONS IN THE DRY WEIGHT OF TRIPLICATE SAMPLES OF SOIL. ─────────────────────────┬──────────┬──────────┬────────── │ A. │ B. │ C. ─────────────────────────┼──────────┼──────────┼────────── I. Surface to four feet│716.6 gms.│715.5 gms.│710.3 gms. II. Surface to four feet│715.4 gms.│687.1 gms.│731.2 gms. III. Surface to four feet│654.0 gms.│688.3 gms.│709.0 gms. IV. Surface to four feet│714.0 gms.│687.8 gms.│719.3 gms.