_Another Method._--Stick a pin in each corner of a square board (Fig. 41), and look diagonally across them, first in the direction of the line to which you wish to run at right angles, and then for the new line sight across the other two pins. A SIMPLE LEVELLING INSTRUMENT. Fasten a common carpenter’s square in a slit to the top of a stake by means of a screw, and then tie a plumb-line at the angle so that it may hang along the short arm, when the plumb-line hangs vertically and sights may be taken over it. A carpenter’s spirit-level set on an adjustable stand will do as well. The other arm will then be a level (Fig. 42). [Illustration: FIG. 42. FIG. 43. LEVELLING INSTRUMENTS.] Another very simple, but effective, device for finding a level line is by means of a triangle of wood made of half-inch boards from 9 to 12 ft. long (Fig. 43). To make the legs level, set the triangle up on fairly level ground, suspend a plummet from the top and mark on the cross-piece where the line touches it. Then reverse the triangle, end for end, exactly, and mark the new line the plumb-line makes. Now make a new mark exactly half way between the two, and when the plumb-line coincides with this, the two legs are standing on level ground. For short water races this is a very handy method of laying out a level line. TO MEASURE THE HEIGHT OF A STANDING TREE. Take a stake about your own height, and walking from the butt of the tree to what you judge to be the height of the timber portion you want, drive your stake into the ground till the top is level with your eyes; now lie straight out on your back, placing your feet against the stake, and sight a point on the tree (see Fig. 44). AB equals BC. If BC is, say, 40 ft., that will be the height of your “stick of timber.” Thus, much labour may be saved in felling trees the timber portion of which may afterwards be found to be too short for your purpose. [Illustration: FIG. 44. Measuring Height of a Standing Tree.] LEVELLING BY ANEROID BAROMETER (Fig. 45). This should be used more for ascertaining relatively large differences in altitudes than for purposes where any great nicety is required. For hills under 2000 ft., the following rule will give a very close approximation, and is easily remembered, because 55°, the assumed temperature, agrees with 55°, the significant figures in the 55,000 factor, while the fractional correction contains _two fours_. Observe the altitudes and also the temperatures on the Fahrenheit thermometer at top and bottom respectively, of the hill, and take the mean between them. Let _B_ represent the mean altitude and _b_ the mean temperature. Then 55000 × (B-b / B + b) = height of the hill in feet for the temperature of 55°. Add 1/440 of this result for every degree the mean temperature exceeds 55°; or subtract as much for every degree below 55°. [Illustration: FIG. 45. ANEROID BAROMETER.] TO DETERMINE HEIGHTS OF OBJECTS. _By Shadows._