REACTIONS OF CERTAIN MOIST FOREST MAMMALS. 1 87 



was tedious and consumed much time, that it was decided to 

 install some device, whereby the flow at any time could be readily 

 obtained. 



The subject of air in motion presents one of the most confusing 

 branches of the study of the flow of fluids. The internal eddies, 

 cross currents, and general intricacy of motion of the particles 

 among each other, occurring in a pipe transmitting air, are 

 almost entirely defiant of mathematical expression. It is neces- 

 sary to adopt as a basis of mathematical investigation the simple 

 assumption that the particles move side by side in such a way 

 that those which at any instant form a lamina or thin sheet, 

 perpendicular to the axis of the pipe or orifice, remain together 

 as a lamina during the further stages of flow. Experiment is 

 then relied upon to make good the discrepancies between the 

 indications of the formula resulting from theory and the actual 

 results of practice. 



The laws governing the flow of all fluids are based on the 

 assumption that the density remains constant throughout the 

 flow, as has been stated above. In considering the flow of a gas 

 such as air, however, the laws referred to do not strictly hold. 

 The velocity in an air duct of uniform size varies due to a loss or 

 decrease in pressure which causes an increase in volume and a 

 consequent increase in the velocity. The flow of air, due to a 

 large difference in pressure is most accurately stated by the ther- 

 modynamic formula for air discharge under conditions of adia- 

 batic flow. The usual method in stating and measuring small 

 pressures is by noting the height of a column of water which the 

 pressure will maintain in equilibrium or balance in a "U ' tube, 

 or manometer. 



From this we can analyze the different pressures acting when 

 air flovvs through a pipe. The flow of air through a pipe or duct 

 is under the influence of three distinct pressures, namely, the 

 velocity, static, and dynamic or total pressures. The velocity 

 head or pressure is defined as that pressure which is required to 

 accelerate the mass from a state of rest to the final velocity 

 attained. The static head or pressure, also termed the frictional 

 or resistance pressure or maintained resistance, is that pressure 

 required to overcome the resistance offered to the flow. This, 



