NO. 3 EUROPEAN AERONAUTICAL LABORATORIES — ZAHM 7 



formula V= J lE_(po — p), p„ — p being the pressure difiference 



between the room and chamber, V the speed ' of inrush, and p the 

 nearly constant density. The method has since been adopted at 

 Eiffel's laboratory and at the National Physical Laboratory." This 

 for the speed of flow ; the direction may be shown by fine silk threads 

 moored in the current, or by floating- particles, fine streams of smoke, 

 etc. In passing it may be mentioned that the direction of flow in the 

 unchecked current is parallel to the tunnel walls truly to a fraction 

 of one degree. 



The pressure difference in question is found by connecting the 

 interior of the timnel wall by means of external nipple and hose, to 

 one branch of a U tube manometer whose other branch opens into 

 the quiet air of the room ; then observing the difference of level of the 

 licjuid in the two arms. Manometers are made in many forms, accord- 

 ing to the accuracy desired. The English one, known as the " Chat- 

 tock tilting gauge,'' made public in 1903. measures barometric 

 pressure differences truly to one five hundred thousandth of an 

 atmosphere. My gauge, made in 1902 on a dift'erent principle, was 

 graduated to millionths of an atmosphere and for the most accurate 

 measurements of static pressure differences was always read to 

 tenths of a graduation. At Eiffel's Laboratory, and at various 

 other places, a less accurate, but somewhat simpler, manometer 

 gauge is used. It consists merely of an inclined alcohol tube suitably 

 mounted beside a graduated scale. The latter instrument, a long 

 known type of gauge, I would recoinmend for its convenience ; but 

 where great precision is required the English gauge or mine would 

 perhaps serve better. 



^ More strictly speaking, V is the increase of velocity of the air as it flows 

 from the room into the tunnel ; but as the air starts from near rest, the 

 increase of velocity is practically the whole velocity of inflow. A consider- 

 able error may ensue if V be taken as the true speed of inflow for the case of 

 a tunnel of goodly section as compared with that of the room. Thus for the 

 new English tunnel the cross-section is 7 x 7 feet in a room whose section is 

 30 X 40 feet. Hence the average speed of flow through the room is 4 per cent 

 of the speed through the tunnel. Hence something like 4 per cent must be 

 added to the speed computed from the true static pressure difference in 

 question. 



^ At the National Physical Laboratory, the velocity along the axis of the 

 tunnel as computed from the pressure dift'erence inside and outside the tunnel 

 wall is corrected by use of a small calibration constant obtained by placing a 

 Pitot tube in the center of this tunnel before the place where the models are 

 tested. 



