CHARACTEES OF THE LEAVES OF THE DATE PAI.M. 13 



THICKNESS OF THE PINN-^ BLADE. 



The pinnae in different varieties vary considerably in texture as 

 well as in actual thickness of the blade. Some are decidedly harsh 

 to the touchy while those of the other extreme have a smooth^ almost 

 silky feel. Relative differences in thickness of the blades would be 

 detected by the careful observer by comparison^ but by the use of a 

 machinist's micrometer this thickness can be made a matter of 

 record. These instruments are graduated to read to 0.001 of an. 

 inch or to 0.01 of a millimeter and should have the improved locking 

 device and safety ratchet for regulating the pressure.* (Fig. 6.) 

 For uniformity the measurement should be made at about the broad- 

 est part of the pinna blade and near the middle of one of the folds. 

 A number of the more familiar varieties have pinnae that range from 

 0.012 of an inch (0.3048 mm.) to 0.020 of an inch (0.5080 mm.) in 

 thickness. Others are distinguished by the greater thickness of the 

 blade, as Thoory, with pinnae from 0.023 of an inch (0.5842 mm.) to 

 0.026 of an inch (0.6604 mm.) or more in thickness. 



USE OF THE FIELD PROTRACTOR. 



For the measurement of the angles formed by the pinnae with the 

 rachisj or rib^ a protractor with rather long arms is essential. As 

 one of the necessary capacity made with the fine graduations called 

 for in engineering is both cumbrous ,and expensive, a single-jointed, 



rule 



degr^ 



found to be a very convenient instrument, giving the angles with 

 sufficient precision and being instantly available for measurements 

 in feet and inches. (See fig» 7.) 



FORMS FOR THE OBSERVER'S USE- 



& 



ruled 



as 



1 These instruments are furnished by tool makers in this country for the use of mechanics cnfja^ed in fine 

 work, graduated in fractions of an inch or decimal equivalents. They may also be obtained by special order 

 graduated in hundredths of a millimeter. 



2 The hinge circle is graduated five-eighths of the way around into spaces of 5 degrees each and figured 

 15°, 30**, 45", etc., from the zero point at the inner angle when the rule is closed. Any anjle a multiple of 5 

 degrees can accordingly be read with accuracy on this hinge circle, but for the degrees between these marks 

 the aid of the vernier on the movable arm is needed. As will be seen when this rule is closed, a 45-degree 

 space on the arm is graduated into 10 parts, so that one of them is equivalent to 4J degrees, or one-half degree 

 less than the 5-degree spaces on the circle. Every second space or the equivalent 1 degree of diHerence is 

 numbered 1 to 5 in order, the 5-line degree coinciding with the 4o-degree line on the circle. Now, if the rule 

 beopened a very little, till the first vernier line marking 4^ degrees coincides with the line of the first 5degrees 

 on the circle, the arm has moved the difference between these, of one-half degree. Open till the second pair 

 of marks colacides, and twice 4^, or 9, degrees of space have been moved to coincide with twice 5, or 10, degrees 

 on the other side, a movement gaining 1 degree. So we may oi>en to 2", 3", 4*, etc. In the same way 

 starting on any even 5-degree mark, as 45**, we may open one- half degree, 1 degree, 2 degrees more. Hence, 

 to read any angle that has been taken, read fust on the hinge circle to the last full 5-degree mark inside of 

 the angle, then add to this the degrees or half degrees to the coinciding line oa the vernier. In figure 7 the 

 rule is opened to an angle of 12 J', 10 degrees being read on the hinge circle and 2 J degrees on the vernier. 



