244 LECTURE XXVII. 



employed for measuring the velocity of the wind, with the assistance of 

 a watch.* 



The principal methods of applying the force of fluids to useful purposes 

 are to employ their weight, their impulse, or their pressure. The weight of 

 water may be applied, by collecting it in a reservoir which alternately 

 ascends and descends, by causing it to act within a pipe on a moveable 

 piston, or by conducting it into the buckets of a revolving wheel; its 

 impulse may be directed either perpendicularly or obliquely against a 

 moveable surface ; and its pressure may be obtained, without any imme- 

 diate impulse, by causing a stream to flow horizontally out of a moveable 

 pipe which revolves round an axis. The force of the air can only be applied 

 by means of its impulse, and this may be employed either perpendicularly 

 or obliquely. 



When water is collected in a single reservoir, which serves to work a 

 pump or to raise a weight, the mode of its operation may be determined 

 from mechanical considerations only ; and it is obvious that if we are de- 

 sirous of preserving the whole force of the water, we must employ a second 

 reservoir to be filled during the descent of the first, which may either 

 descend in its turn, or empty itself into the first when it has ascended 

 again to its original situation. The action of a column of water inclosed in 

 a pipe, is of a nature nearly similar to that of such a reservoir, excepting 

 that the apparatus is more liable to friction ; the arrangement of its parts is 

 nearly similar, although in an inverted position, to that which is more com- 

 monly employed for raising water by means of pumps. But both these 

 methods of employing the weight of water are in great measure confined to 

 those cases in which it is to be procured in a small quantity, and may be 

 allowed to descend through a considerable height, and when the circum- 

 stances do not allow us to employ machines which require a greater space. 



We have seen that in order to determine the effect of any force employed 

 in machinery, we must consider not only its magnitude, but also the velo- 

 city with which it can be brought into action, and we must estimate the 

 ultimate value of the power, by the joint ratio, or the product, of the force 

 and the velocity. Thus, if we had a corn mill, for example, in which we 

 wished the millstone to revolve with a certain velocity and to overcome a 

 given resistance, and supposing that this effect could be obtained by means 

 of a certain train of wheels from a given source of motion ; if the velocity 

 of the motion at its source be reduced to one half, we must double the 

 diameter of one of the wheels by which the force is communicated, in order 

 to give the millstone the desired velocity, and thus we must introduce a 

 mechanical disadvantage, which can only be compensated by a double in- 

 tensity in the force at its origin. 



If we apply this estimation of effect to the motion of an overshot wheel, 



* Huygens, Mach. Approuvees, i. 71. Sir C. Wren's "Weather-wiser, Birch's 

 Hist, of the Roy. Soc. i. 341. Hookes in his Philos. Experiments, &c. edited by 

 Derham, p. 41. Whewell's, Trans, of the Camb. Ph. Soc. vol. vi. Osier's, Report 

 of the Br. Ass. vol. vii. Sections, p. 33, and Description of a Self-registering Ane- 

 mometer, &c. 4to, Birmingham, 1839. 



