36 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



that the sine scale is set with reference to the A scale, whereas the 

 tangent scale refers to the D scale. 



110. In case there is doubt as to the accuracy of the result of any 

 of the slide-rule computations, it may be readily checked by dividing 

 the assumed height of the balloon by the natural tangent of the eleva- 

 tion angle. A table of natural tangents may be found in any book of 

 logarithms. 



111. Plotting board. — In order to obtain the wind direction and 

 velocity from a pilot-balloon observation, some kind of a plotting 

 board is usually employed. The standard Weather Bureau plotting 

 board consists of a wooden base, 42 inches square. Over the central 

 area of the base a circular sheet of cross-section paper is pasted, rubber 

 cement being used for the latter to prevent the stretching of the cross- 

 section paper. At the center of the area, and set into the board, 

 is a brass bearing and pin. From the center of the pin two distance 

 scales (one on either side of a heavy index line) are printed to the 

 edge of the paper. One of these is numbered on the basis of 1 cm. =200 

 meters and the other on the basis of 1 cm. =400 meters. The former 

 extends to 10,000 meters and the latter to 20,000 meters. Over the 

 circular area of the paper and fastened to the brass pin as a center is 

 placed a disk of frosted celluloid with the circumference graduated 

 in half degrees. The celluloid disk or protractor is frosted on the 

 upper surface in order that the points representing the horizontal 

 projection of the balloon's path may be plotted thereon with a soft 

 pencil. In case the surface becomes too smooth with use to readily 

 take the pencil marks, it may be refrosted or roughened satisfactorily 

 by rubbing with very fine sandpaper. 



112. Constructing the horizontal projection oj balloon'' s path. — To 

 plot the computed distances of the balloon for the various minutes 

 of the ascent, or to construct the horizontal projection of the flight, 

 the plotting board should be arranged so that the index line and dis- 

 tance scales are directly in front of the operator. The celluloid 

 surface is then cleared of all previous records by erasing the pencil 

 marks with a piece of soft eraser or art gum. Hard erasers containing 

 any abrasive substance should not be used for this purpose. The 

 celluloid protractor is then revolved about the center until the ob- 

 served azimuth angle for the minute to be plotted is located on the 

 edge of the disk and placed over the index line. Then, taking the 

 computed horizontal distance of the balloon for the same minute as 

 the second factor, a point is plotted on the celluloid protractor directly 

 over the index line and opposite the corresponding value on the dis- 

 tance scale. The point is set off, or made more prominent, by en- 

 circling it. The point should then be numbered to correspond to 

 the number of the minute involved. Only very soft and well-sharp- 

 ened pencils should be used on the celluloid protractor as it is difficult 

 to mark a point accurately on the protractor with a dull point. 



113. The distance scale selected for use in plotting should, of course, 

 depend upon the maximum horizontal distance attained by the 

 balloon. Usually if this is less than 10,000 meters the 10,000 meter 

 distance scale should be used, and if greater than 10,000 meters the 

 20,000-meter distance scale should be used. Both distance scales 

 should be used when the horizontal distances are relatively small 

 during the first few minutes but the maximum distance attained is 

 greater than 10,000 meters. In such cases greater accuracy will 



