268 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



mass (for instance, a heavy waggon rolling down a hill), or large velocity (for instance, 

 a bullet speeding through the air). A force continuously applied to a moving mass 

 produces a continuous increase in its rate of movement ; this is termed acceleration, 

 and force may be defined as the rate of change of momentum ; it can be measured, 

 therefore, by observing the amount of momentum it generates in a measured time, 

 and dividing by that time. If a gramme is taken as the unit of mass, a centimetre 

 as the unit of length, and a second as the unit of time, the unit of force 



_ momentum _ gramme-centimetre per second 



Time. Time in seconds. 



= gramme-centimetre per second, per second = 1 dyne. 



The unit which corresponds to the dyne in the measurement of work is called an 

 erg, that is, the work done in lifting a gramme weight through the height of one 

 centimetre ; the weight of a gramme is 981 dynes, and the work done in lifting it 

 one centimetre is 981 ergs. 



The kinetic energy of a body moving with velocity v is x mass 

 or for one gramme Jv 2 ; hence if all the work that liquid can do 

 is spent in giving kinetic energy to it, the velocity with which it will 

 flow out is given by putting the kinetic energy = work done. In 

 other terms : 



= h ; hence v = lh or h = !L 



A liquid, however, has not necessarily a free surface, but may be 

 completely enclosed, as is the water in a system of hydraulic pressure 

 mains, or the blood in the circulatory system. The pressure in such 

 a system at any point may be measured by inserting at that point 

 a vertical tube at right angles to the blood-vessel ; the blood would 

 rise in it to a point, and would form a free surface a certain distance 

 up this tube ; the head h in the above calculation must be reckoned 

 from this free surface downwards. If, instead of using a tube of fine 

 bore for this purpose, we employ a wider tube, say of ten times 

 greater area, the height or head to which the fluid rises will be the 

 same as in the narrow tube, though naturally the actual weight of 

 fluid supported will be ten times greater ; but the weight per unit of 

 area is the same in both cases. When, therefore, we measure the 

 pressure of fluid in terms of the height of a column of fluid, such as 

 mercury, which it will balance, we really mean that the force of the 

 blood is equal to the weight of the mercury it supports per unit of 

 area, and this will naturally be proportional to the height of the 

 column. 



Let us next consider the simple case of a fluid flowing from a 

 reservoir, E (fig. 236), along a tube, which we will imagine is open at 

 the other end. 



In the course of the tube we will suppose three upright glass 

 tubes (A, B, and D) are inserted at equal distances. Between B and 

 D there is a bladder, which may be divided into a number of channels 

 by packing it with tow to represent the capillaries, and between B and 



