If there are two or more shafts sunk on the workings, it will be an easy matter to ascertain if the needle can be depended on for laying out any further surface work, as the underground survey connecting the shafts can be laid down on the surface, or the direct bearing and distance calculated, when its correctness is tested by the terminating point of the survey. 2. =TO FIND DEPTH OF SHAFT AT ANY POINT, TO CUT A VEIN WHOSE DIP IS KNOWN.= RULE:--Multiply natural tangent of angle of dip C (Fig. 50.) by the distance from outcrop to proposed shaft AC. The result is the depth required, AB. =_By Protractor and Scale._=--Rule on paper a line AC of the required distance, then at C set off the angle of dip and draw AB at right angles to AC. Then scale off AB = depth of shafts. [Illustration: FIG. 50.] 3. =GIVEN DEPTH OF SHAFT AND ANGLE OF DIP TO FIND WHERE IT OUTCROPS.= --Then AC = AB × natural tangent of angle ABC. Or by scale and protractor by inspection. 4. =GIVEN DEPTH OF SHAFT AB AND DIP OF VEIN ANGLE CB TO FIND DISTANCE BC BETWEEN BOTTOM OF SHAFT AND OUTCROP.=--BC = AC × natural sine ACB. Or by scale and protractor by inspection. RAINFALL. One inch of rain = 22,680 gallons, or 102·35 tons of water per acre. NOTES ON BELTING. _Co-efficient_ of friction between ordinary leather belting and cast-iron pulleys or drums = ·423. Ultimate strength of ordinary leather belting = 3086 lb. per square inch. Belts vary from ³/₁₆ in. to ¼ in. thick, average ⁷/₃₂ in. _Power of Single Leather Belts._ To calculate the power of single leather belts, the following formula may be used: Let HP = actual horse-power. W = width of belt. F = driving force. T = working tension from 70 to 150 lb. V = velocity of belt in feet per minute. Then F = (W × T)/2. HP = V × F/33,000. W = 33,000 × HP/F × V. =EXAMPLE:=--A 10-inch belt running 2500 feet per minute, what horse-power will it transmit? Assuming the working tension to be 100 lb., F = (10 × 100)/2 = 500. HP = (25,000 × 500)/33,000 = 378 horse-power. Nystrom gives this rule:--HP = (V × F)/550. V = velocity of belt in feet per second. F = force in pounds transmitted by belt. The first rule gives good practical results where there is no great inequality in the diameter of the pulleys. _Double Belts_ transmit 1½ times as much as single belts. _Splicing Belts._ +----+-----+-----+-------+--------+------------+ Width of Belts. | 1 | 2 | 3 | 3 to 6| 6 to 8 | above 8 in.| Lap in inches . | 2 | 4½ | 5½ | 6 | 8 | 10 | +----+-----+-----+-------+--------+------------+ _Rules for Double Leather Belting._ A = covered area of driven pulley in square inches. V = speed of belt in feet per minute. H = indicated horse-power. W = width in inches. H = AV/66,000 A = (66,000 H)/V. W = A/L, where L = length of belt on driven pulley in inches. Another authority simply says H = {70 to 80} × (WV)/33,000