APPENDIX A. THE PITOT-DARCY GAGE. 



SCOPE OF APPENDIX. 



Many measurements of velocity were made 

 with the Pitot-Darcy gage, but as only a few 

 have been finally utilized in the preparation 

 of the report, a discussion of the instrument 

 seemed not appropriate to the main text. 

 Certain phases of our experience, however, 

 are thought worthy of record because they 

 have practical bearing on the utility and the 

 use of such gages, and these are the subject of 

 the appendix. 



FORM OF INSTRUMENT. 



Darcy developed the Pitot tube by adding 

 a second tube, differently related to the cur- 

 rent, and by connecting the two above with a 

 chamber from which the air was partly ex- 

 hausted. The water columns in the two tubes 

 were thus lifted from the vicinity of the water 

 surface to a convenient position, where their 

 difference in height could readily be measured. 

 In the gages constructed for our use the 

 aperture of one tube was directed upstream 

 and that of the other downstream. The 

 tubes were borings in a single piece of brass, 

 which was shaped on the outside in smooth 

 contours, designed to interfere the least possible 

 with the movement of the water. The form 

 first given was afterward modified, and figure 

 85 shows the third and last design, with which 

 most of the work was done. The openings 

 had a diameter of 0.1 inch. At the opposite 

 or upper end of the brass piece were stopcocks, 

 and above these connection was made with 

 rubber tubes, which led to the complementary 

 part of the apparatus, where the difference in 

 height of the two water columns was observed. 

 It is convenient to call tiie member exposed to 

 the current the receiver, and the complemen- 

 tary member the comparator. 



In the comparator were two glass tubes, 

 straight and parallel, with internal diameters 

 of about 0.8 inch. At the top they were 

 connected by an arch, and at the summit of the 

 arch was a branch tube, with a pet cock, used 

 in regulating the amounts of air and water. 



At the bottom they communicated with the 

 rubber tubes through a brass piece, in which 

 were two stopcocks, connected by gearing so 

 as to open and close together. These parts 

 were mounted on a board which also carried a 

 scale of inches and decimals. A sliding index 

 was arranged so that it could be set by the 

 meniscus of a water column and its position 

 then read on the scale; and there was a fixed 

 mirror behind the tubes to aid in avoiding 

 error from parallax. The board was sup- 

 ported in an inclined position, the slope given 

 to the tubes and scale being that of 2 hori- 

 zontal to 1 vertical. This had the effect of 



0.1 



o.a root 



FIGURE 85. Longitudinal section of lower end of receiver of Pitot-Darcy 

 gage No. 3, with transverse sections at three points. 



making the movements of the columns 2.69 

 times as great for the same change of pressure 

 as they would be if the tubes were vertical. 



In preparing for observation, the internal air 

 pressure was so adjusted that the columns 

 stood near the middle of the scale. The 

 receiver was then held, by a suitable frame, in 

 the selected part of the current, and the stop- 

 cocks were opened. In the glass tube con- 

 nected with the receiver opening facing up- 

 stream the column rose; in the other tube it 

 fell. When the full effect of the current had 

 been realized, the stopcocks at the bottom of 



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