Dip

Dip. 1. The dip or inclination of the needle at any point on the earth's surface means the angle made with the horizontal by the earth's magnetic force at that point. If a straight piece of steel wire be supported by a horizontal axle passing through its mass-centre, it will practically assume any inclination that is given to it. But if it then be magnetised it tends to adjust its length in the direction of the earth's magnetic force; and when placed in the magnetic meridian so as to point to magnetic north and south we find it will assume a definite angle of inclination to the horizontal that is subject to very slight change from year to year. Such an action is due to the fact that the earth is a magnet, and just as in the case of a bar magnet a small needle is differently inclined to the two ends at different regions about the bar, so with the earth a dipping needle shows a varying inclination at different points on the earth's surface. In the region of the earth's magnetic poles, which are not far removed from the geographical poles, dipping-needles point vertically downwards, and the inclination has its maximum value of 90°. Along a somewhat irregular line passing round the earth in the equatorial regions the inclination is 0°, and the needle therefore horizontal. This line is called the magnetic equator. Lines of equal dip are called isoclinic lines, just as those of equal declination are called isogonic lines. The magnetic dip in London at present is about 67° 20'. It has been steadily diminishing since 1720, when it attained a maximum value of 74° 42'. These observations are made with a dip-circle, which is a needle carefully supported at its mass-centre and capable of rotation in a vertical plane about a horizontal axis. It admits of accurate adjustment in the magnetic meridian, and possesses a graduated vertical scale for the measurement of the required inclination of the needle.

2. The inclination of a stratum of rock, or of a fault, to the horizon, the stratum in question being said to dip at such and such an angle towards such and such a point of the compass. Thus the angle is the amount of dip; the point of the compass the direction. Horizontal beds have no dip, vertical ones a dip of 90°. Dip is measured by a clinometer or plummet with a graduated semicircle. A line at right angles to the direction of dip is called the strike, being the general direction of the outcrop or exposed edge of the stratum. Thus beds dipping east or west will have a north and south strike. The Secondary rocks of England have a general direction from south-west to north-east in the southern half of the country and due north and south in the northern half, their general dip changing similarly from south-east to east. Whatever their inclination, beds appear horizontal along the line of strike, and it is only sections or exposures at right angles to this line of strike that exhibit the true dip. Other sections show apparent dips. The lower the angle of dip the wider will be the outcrop (q.v.), on level ground that of vertical beds being equal to their thickness. If, therefore, we know the angle of its dip and the width of its outcrop, we can readily calculate the thickness of any bed and can similarly estimate its depth below the surface at a moderate distance from any part of its outcrop. Where strata dip in two opposite directions from a line of elevation we have an anticlinal (q.v.) or saddle-bach; where in all directions from a centre, a perielinal, dome, or qnaquaversal dip; where from opposite directions towards a depressed axis, a synclinal (q.v.), or trough; where from all directions towards a centre, a centroelinal or basin.