/ No. 1008 DESCRIPTION AND INSTRUCTIONS FOR USE OF WELDON RANGE FINDER (ONE PLATE) REVISED MARCH 1, 1906 REVISED FEBRUARY 1, 1909 REVISED SEPTEMBER 2, 1911 WASHINGTON GOVERNMENT PRINTING OFFICE 1917 A- J. G'G'S oo CM War Department, Office of the Chief of Ordnance, Washington, September 2, 1911. This manual is published for the information and government of the Regular Army and Organized Militia of the United States. By order of the Secretary of War : William Crozier, Brigadier General, Chief of Ordnance. ( 3 ) 104720—17 . TABLE OF CONTENTS. Page. Description- Construction. Use r ____ Observations Instructions- in range finding 10 Range finding 12 ( 5 ) CD 00 00 , . ■ THE WELDON RANGE FiNDER 1. The Weldon range finder is a small hand instrument used in measuring ranges. The range finder consists of a metal case apd a cover. 2. Inside the case are mounted three prisms, a needle, and a level. (Fig. 5.) Between the first and second prisms, and between the sec- ond and third prisms, assembled one above the other, a window slot is made. By this arrangement, when the instrument is opened out and the protecting cover used as a handle, the three prisms can be brought into position before the eye of the observer, and any of them can be used without necessity for the observer taking his eye off the object he is observing. 3. Screwed into a seat in the case is a magnetic needle, which per- mits the instrument to be used as a compass. The metal cover, which forms the handle, has four notches cut in it. These serve to support a pencil in such position that when the needle points due north the pencil will point E. and W., NE. and SW., or NW. and SE., as the pencil is laid either transversely or diagonally in the notches. (Fig. 5.) The level, fixed below the third prism, is used to assist the observer in keeping the instrument upright when finding a range. Pasted on the inside of the cover is a sheet of memoranda giving the multiples to be used with the different prisms. 4. With each Weldon range finder a steel 60-foot tape is issued to each company of infantry and engineers, and a 100-foot tape to mobile artillery. The cases of tapes and instruments are finished with olive-drab enamel. A leather case and strap is issued with each range finder, in which it should habitually be kept when not in use. • 5. The range finder should be handled with due care. When necessary to clean the prisms, use a soft cloth to remove all dirt. Dust and grit should be removed by blowing or careful wiping, so as not to scratch the prisms. No moisture other than that deposited from the breath will be needed. The prisms should be kept per- fectly clean for observing. Under no circumstances should the prisms be removed from their seats except by a skilled workman at shops equipped for instrument work. ( 7 ) 8 6. The steel tape is graduated on one side in feet, tenths of feet, and hundredths of feet, and on the other side in yards. It is an accurate and sensitive instrument, and must be handled with skill and care. Avoid jerking, bending, or twisting it. If sand, grit, or dirt gets on it, clean it before winding. Also in wet weather, or in damp localities, keep the entire length of the tape well oiled to prevent rusting. I. CONSTRUCTION. 7. The instrument consists of three prisms securely fixed in metal frames. Each prism reflects at a different angle. These angles are 90\88° 51' 15", and 74° 53' 15". 1 8. For convenience of reference, the three prisms are called, re- spectively, first, second, and third. The first is 90° ; The second is 88° 51' 15"; The third is 74° 53' 15" ; and this is the order in which they will most frequently be used. The angles of the prisms are so calculated that when the first and second prisms are used together in the manner explained below the range is equal to 50 times the base; that when the second and third prisms are used together the range is equal to 4 times the base ; and that when the second and third prisms and then the first and second prisms are used together (see Problem 2) the range is equal to 200 times the base; that when the second prism is used alone the range is 25 times the base. II. USE. 9. In using the range finder, the instrument is opened out. There are two positions in which it can be held : (a) As shown in figure 5, with the apex of the prism toward the eye of the observer. The reflection of an object to the right of the observer is to be sought for in that side of the prism which is farthest from the object, and in the section of that side which is farthest from the apex angle. Thus, to reflect 0 , the observer would look for it in the prism at (9', figure 5. This method is preferable, and should be used when the target is to right of observer. (&) It can be held as shown in figure 6, with the apex of the prism toward the object. The reflection of 0 is in this case to be sought in the Corner nearest the apex of the prism. This method of holding will be used when the target is to the left of observer. , 1 The first and second angles, viz, 90° and 88° 51' 15", are the angles at the base of a right-angled triangle of which the base is to the perpendicular as 1 : 50. The comple- ment of the second, viz, 91° 8' 45", and the third are the angles of an obtuse-angled triangle, of which the base is to the side subtending the angle 74° 53' 15" as 1 : 4. 9 OBSERVATIONS. 10. In the first or right-angled prism the three corners are avail- able for observation, and therefore the whole surface is open. In a prism reflecting at an angle not a right angle, one corner is not avail- able for observation ; in the second and third prisms, these corners are covered or cut off. 11. To obtain a general idea of the method of using the range finder, the observer stands so that his right shoulder points to the target — the distance of which he is about to find. Holding the instrument by the cover, he looks with his right eye straight in the top prism of the three, and in that quarter: of it which is nearest to the figure 1 on the frame, and therein the object sought will be seen reflected. A little practice may at first be required to readily discern in the prism the object sought, but by holding the instrument close to the eye a large field of view is obtained, and a very slight movement of the head to the right or left is all that is required to bring a fresh field into view. In observing, several images may be seen. Rays of light forming the desired image suffer two internal reflections. If the range finder be held in a vertical position and rotated about a vertical axis, the desired image will remain stationary, whereas the other images rotate with the range finder. The image of the target must now be made to coincide with some object immediately in front of the observer^ known as the aiming point, and to effect this he must look into and also over the prism at one and the same time. Alignment can be effected by the observer moving to his right, or left, or backward, or forward. Having made the required coincidence of the image of the target, with the aiming point, the observer has formed a right angle at the prism between the line to the aiming point and the line to the target. 12. The observer now looks in the second prism, and sees that the image of the target and the aiming point do not coincide, for the target is seen to the right of the aiming point in the second prism. To obtain the range of the target, the observer must make the image and object again coincide with the second prism, and to effect this, he marks the spot upon which he stands, and 'moves backward — keeping the aiming point and his first position in alignment. Having gone some distance, he again looks in the second prism, and if the image of the target still falls to the right of the aiming point, it shows that he has not gone far enough. If, however, the target falls to the left of the aiming point, it proves that he has gone too far, and he must retrace his steps to a position where the images coincide. The range of the target is equal to the distance from this second position 10 (found by the second prism) to the first position (found by the first prism) multiplied by 50, i. e., if 20 yards separate the two positions, the range is 1,000 yards ; if 50 yards, the range is 2,500 yards. 13. To measure an angle correctly, the image of the target should be kept upright, and the image of the horizon as level as it is natu- rally ; for if the instrument be held so that the image of the ground appears to slope when it is really level, or made to appear level when it slopes, the required angle will not be measured. In observing the bubble of the small spirit level should be first centered, and the range finder rotated away from or toward the eye, always keeping the bubble centered, 'until the image of the horizon appears at it is naturally, or until vertical objects appear vertical. 14. Figure 7 (using prism No. 1) shows how alignment or coinci- dence of the target is made with the aiming point. Thus, if an object, (9, figure 7, appears to the observer’s left of an object, Z>, in front of him, O can be brought immediately under D by moving straight forward if D is distant, or to the right or diagonally to the front if D is nearer. If (9, figure 7, appears to the observer’s right of an object, say G in front, he must step back, or more to the left, until alignment has been effected. 15. Any glare from the sun on the instrument can be avoided by shading the instrument with the unoccupied hand. III. INSTRUCTION IN RANGE FINDING. 16. For instruction in range finding, a pole O (figure 1), should be set up at a known distance^ say 300 yards, from the point X. 0 is called the target. In a line at right angles to the target line XO , but at a greater distance than target O , let another pole Z>, be set up. D is called the aiming point. The point X , where the target O forms a true right angle with the aiming point Z>, should be permanently marked by a pole or picket k not higher than 4J feet from the ground to suit observers of different heights, so that the instrument can be conveniently rested on it and the appearance of a correct angle noted. One-fiftieth of the distance of O from X (in this case, 6 yards) should be measured from J on the line DX prolonged, and the point A so found should be marked by a short line 1 cut in the ground across the line of sight. At this point the reflection of O in the second prism will coincide with the aiming point D. 17. The learner having been taught how to hold the instrument, and to observe different objects to the right or left of him, should be 1 The extremities of this short line might be permanently marked by two small pegs, so that the observer may have nothing but the instrument to guide him in finding the point A. 11 taken to the point X , figure 1, and shown how the pole at the target coincides with the pole at the aiming point when the instrument is held properly, and what the effect is when the object is not reflected upright. He will then be provided with a picket and told to find a right angle at some other spot. 18. When proficient in finding the required angle, the learner will again be taken to the point X, figure 1, and, using the second prism, he will note how the target 0 is reflected to his right toward O ' . He will then be directed to find the point A by retiring in line with the picket X and the aiming point Z>, and noting that, as he approaches A, the image of O approaches I). 19. Then, using the tape provided, he will measure the base AX. If the tape used is 20 yards (60 feet) long, each such length, using the first and second prisms (multiplier 50), is equivalent to 1,000 yards of range. 20. When the learner is proficient in this preliminary practice, he may be directed to take short ranges, which are known, and the cause of any error pointed out. 21. In using the second prism the ob- server should be especially cautioned to fix his attention on the far-off aiming point, and not on any intermediate point, which serves to guide the eye in a straight line to the aiming point. When the observer comes to take long ranges, he will note that the re- flection of the target approaches the aiming point more slowly than it did when taking short ranges; and as, after repeatedly observing the object on one side of the aiming point, the eye is apt to be satisfied with what is in reality only an approximate coincidence, he should retire, or advance, until the target appears on the other side of the aiming point, and then proceed to take off what is necessary. For extremely long ranges it might be safer to take a mean between two estimations — one slightly under and the other slightly over. Aiming point. D O’ 12 IV. RANGE FINDING. 22. If, in range finding, a good natural aiming point can not be found, a marker or picket may be placed to give an aiming point. The distance that this mark should be placed from the observer de- pends on the distance of the target. Thus, for a range of 1,000 yards, an aiming point may be 100 yards away ; but for 2,000 or 3,000 yards the mark should certainly not be nearer than 200 yards. It is a maxim that the farther the aiming point the more accurate is the measurment of a range, all other circumstances being equal. Should an observer have an assistant, he may send him 100 yards to the left to serve as an aiming point, while he himself goes the same distance to the right, or vice versa; thus very little time is lost in finding an aiming point for a long range. Problem 1 . — To Find the Range or an Object with the Aid of Pickets or Men Placed as Markers. (Fig. 8.) 23. The observer near X looks in the first prism (90°) and sees the image of O nearly on some distant point or marker, D ; he moves, as before explained, until the reflection of O falls true on D ; he. plants a picket at this point, X, and retires on the line DX prolonged toward A; when he thinks he has gone far enough, i. e., nearly one- fiftieth of the estimated range, he stops to observe with the second prism, say at O, where he sees in the second prism O on his right of the aiming point D. This shows he has not gone far enough, so he moves on, say to F; he now sees O reflected to his left of D. He soon finds A where 0 is reflected true along the line XD. The base AX is now found, and it is one-fiftieth of the range XO. Problem 2 . — To Measure Ranges by Using Second Prism Only. 24. Use the point from which the range is to be measured as an aiming point. Hold the range finder by method (a) or (6) (para- graph 9 above), and find, by using second prism, the point A, figure 2, such that direction of O seen through prism coincides 0 with direction of D seen di- rectly under or over the sec- ond prism. Mark point A by a stake. The line DA should be approximately at right an- gles to line DO. 25. Proceed along AD prolonged beyond Z>, holding the range finder by method (b) or (a) (paragraph 9 above) until point B is reached, from which the reflection O seen in the second prism coin- cides with D or A seen under or over the prism. The range DO is 25 times the base BA. 13 Problem 3 . — To Calculate a Long Base by Measuring a Short Base one-fourth its Length. 26. If the base XA is very long, or the ground prevents accurate use of the tape, the base should be measured with the third prism, thus: The points X and A (fig. 8) being marked by pickets, the observer attaches the measuring tape to the picket A, and moves backward on the line OA , running out the line until he reaches a point B, where he sees in the third prism the picket X reflected true on 0 along the line BAO. This short base BA is one-quarter of the base AX. Every yard of the short base, therefore, represents 200 yards of range. 27. The advantage of using the third prism to measure the long base AX is that it compensates for any error that may have been made in determining the position of the point A , for if this point be erroneously determined so that AX is longer than one-fiftieth of the range, then the angle BAX , figure 8, of the subsidiary triangle ABX will be enlarged, while the other, found by the third prism, remains constant ; consequently, the base AB will be found less than one-fourth of the erroneous base AX and more nearly the true range will be determined. Conversely, if the position of the point A be erroneously determined so that AX is less than one-fiftieth' of the range, the angle BAX will be lessened and the base AB will be found more than one-fourth of AX and more nearly the true range will be determined. Conversely, if the original base AX be made less than one-fiftieth of the range, the angle BAX will be lessened and the base AB will be found more than one-quarter of AX, or nearly the true range. Problem Jf. — To Find the Range of the Object without the Aid of Pickets. 28. The observer at A, figure 3, marks his position, and, using the second prism, reflects the target O on some distant point, D. Then, selecting some intermediate object in line to assist him in marching straight, he moves toward D, halting occasionally to observe with the first prism, and makes sure he has not overshot the point where, as at X, the target O is aligned on D. When he has found X he measures the distance from X to A and thus obtains the range. D 14 Problem o. — To Detekmine the Distance between any Two Points without occupying them. 29. The observer being at X. figure 4. to ascertain the distance between A and B , he sets up a picket at X. and from it, as a fixed point, with an aiming point, F, at right angles to BX , lays off and 3 E Fig. 4. marks the extremity of the base XB as though he were taking the range XB. (Problem 1.) Again, from X, with an aiming point X, he lays off and marks the base XC required to find the range XA. The distance between the two extremities C and I) will be one-fiftieth of the distance between A and B. War Department, # Office of the Chief of Ordnance, Washington, September 2, 1911. Revised, March 1, 1906. Revised February 1, 1909. Revised September 2, 1911. Form No. 1908. Ed. June 13-17 — 1,000. 9847 /188-Encs. 1 and 2. O 14 Problem 5 . — To Determine the Distance between any Two Points without occupying them. 29. The observer being at X, figure 4, to ascertain the distance between A and B , he sets up a picket at X, and from it, as a fixed point, with an aiming point, F , at right angles to BX , lays off and B marks the extremity of the base XD as though he were taking the range XB. (Problem 1.) Again, from X, with an aiming point E , he lays off and marks the base XG required to find the range XA. The distance between the two extremities G and D will be one-fiftieth of the distance between A and B. War Department, Office of the Chief of Ordnance, Washington , September 2, 1911. Revised, March 1, 1906. Revised February 1, 1909. Revised September 2, 1911. Form No. 1908. Ed. June 13-17—1,000. 9847/188— Encs. 1 and 2. O 104720. (Face page 14.) No. 1 104720 — 17. (Face page 14.) No. 2 Fig 8.