therein and cut off to exact level. There is no more difficult measurement to make correctly than one with a long steel tape between two distant points without intermediate supports; because, in the first place, the double measurement on shore and in correct position is a slow and tedious one to make, involving as it does the use of the level to obtain the sag, which must be exactly alike in both cases, and, in the second place, the conditions of wind and temperature are likely to vary to such an extent as to cause errors that are very difficult to correct.

All base-line measurements should be made in cloudy weather, or just after sunset, or even at night; and the temperature should be noted for each fifty feet measured, as all lengths must be reduced to those for an assumed standard shop temperature of seventy degrees Fahrenheit. Even slight variations of temperature will cause errors of importance in the length of an ordinary base-line, the change in length per degree of temperature and per unit of length being about 0.0000066. For a base-line of one thousand feet and a variation of one degree the change in length would be eight one-hundredths of an inch. This, it is true, is no great amount, but there is always a liability of there being a difference of as much as ten degrees between the average temperatures for measurements made on two different days, and as much as two or three degrees in a single measurement of a base-line.

In using a steel tape it is better to start from the one-foot mark rather than from the end, unless the ring be placed back of the zero-point.

The author's method of measuring a base-line on comparatively level ground is to run in a line of stakes of at least three inches by one inch section and from two feet upward in length, spaced at intervals of about ten feet and put in to exact line and level, with a large flat-headed tack driven to line on each stake, and the true base-line scratched with a knife along the top of each tack. The line is measured by stretching the tape with a uniform pull of six pounds over the line of stakes, keeping the one-foot mark or the zero-mark, as the case  may  be,  over the centre that is cut on the hub at the