APPLIED M 



IWIXD-POWBB. 



moleeulM; even if it do not consist of actual solid 

 particle*, we may, at all events, suppose it to consist of 

 a number of separate muses, as small and light as we 

 please ; and though rare and attenuated as compared 

 with solid bodies, yet certainly as much as they, subject 

 to the general laws which govern all matter. Now, if 

 we double the speed with which any one of these masses 

 imivi'*. we douMe the force with which it strikes on any 

 body placed in its way ; if we triple its speed, we triple 

 the force of its blow ; and so on, the force being always 

 exactly proportional to the velocity, when auy one mass 

 or particle is taken into account But when we extend 

 our conception to the motion of an unlimited number of 

 such particles, which constitute a continuous fluid, we 

 observe that, by doubling the speed of the fluid's motion, 

 we double the number of mosses striking any body 

 opposed to it in a given time ; by tripling the speed we 

 triple the number striking ; and so on, the number of 

 particles striking, as well as the force of each, being 

 proportional to the velocity of movement. 



Upon the whole, then, the pressure on any surface, 

 arising from a continuous series of blows from masses of 

 air, must be as the force of each, multiplied by the 

 number in a given time ; and as, on doubling the velocity, 

 the number is doubled, the force or pressure is 2 X 2, 

 or 4 times, on tripling the velocity the pressure is 3 X 3, 

 or 9 times, and so on, the pressures being always, as we 

 have stated above, proportional to the velocities mul- 

 tiplied by themselves, or the squares of the velocities. 

 It is convenient to estimate the pressures of wind cur- 

 rents on some unit of surface, such as one square foot, 

 while the velocities are generally stated as being at a 

 certain number of miles per hour. The following table 

 gives the pressures per square foot of surface produced 

 by winds of different degrees of strength, as distinguished 

 by their respective names, with the corresponding ve- 

 locities in miles per hour. 



Tattle of the Velocities of Winds in miles per hour, and 

 UuAr pressure in Ua. on a surface of one square foot. 



of the pale on 1 square foot being nearly 2 Ibs. , we have 

 600 sq. ft. X 2 Ibs. 1200 Ibs., the total pressure on the 



The following is a simple rule for determining very 

 nearly the pressure per square foot, when the velocity in 

 miles pr nour is known : Multiply the velocity by 

 itself, halve the product, and point otf two figures to the 

 right 



Example 1. Required the pressure of a wind blowing 

 at 40 miles per hour. 40 X 40 = 1600 ; half is 800 ; 

 pointing off 2 figures, we have 8 Ibs. , which nearly agrees 

 with 7 '873, the pressure in the table. 



in 1,1 r. 2. Required the pressure of a tempest at 

 70 miles per hour. 



70 

 70 



2)4900 



24 50 Ibs. 



The pressure of a wind on any surface, such as a 

 square foot, being known, it is easy to estimate that on 

 any other surface ; for if, instead of 1 square foot, 2 

 square feet were opposed to the wind, we should find the 

 pressure doubled in amount ; on 3 square feet it would 

 be tripled ; and so on in regular proportion. Knowing, 

 then, the velocity of the wind, we compute its pressure 

 on any known surface by multiplying the pressure on I 

 square foot by the surface in square feet. Thus, if we 

 wished to ascertain the pressure of a gale at 20 miles per 

 ! boar on a ship's sail 20 feet wide by 30 feet high, since 

 20 X 30 - 000 square feet of surface, and the pressure 



When the wind is made use of as a motive force, the 

 surface on which it blows must move with it, for if it 

 simply rested opposed to the air, it would sustain pros- 

 sure, but develop no motive force. The sail of a ship at 

 anchor is certainly pressed on by the wind, at. 

 force the vessel from its moorings ; but until it does 

 force it, no motive power is developed, for no motion in 

 produced. But when a ship in in motion, the sail is, to 

 a certain extent, going with the wind, ami therefore re- 

 ceives a pressure due only to the excess of the vdoriiy of 

 the wind over that of the ship. Suppose the wind were 

 blowing at the rate of ten miles per hour, and the ship 

 sailing before it at the rate of seven miles per hour, the 

 actual velocity with which the wind strikes the sail is 

 only three miles per hour, the difference between its rate 

 and that of the ship. The pressure on the sail is, there- 

 fore, only that due to three miles per hour. If we sup- 

 pose, now, that the velocity of the wind increased t-> 

 twenty miles per hour, the speed of the vessel propelled 

 would also be increased probably to fourteen milos per 

 hour, and the pressure on the sail would be that due to 

 six miles per hour, the excess of twenty over fourteen. 



Theoretically, as the resistance to the vessel's motion 

 is that of the fluid in which it moves, the pressure due to 

 increased velocity should follow the some law as that of 

 the wind, and the example given above would be a pro- 

 bable case. In practice, however, the form of the vessel, 

 the disturbance of the surface of the water, the bagging 

 of the sails under increased pressure, and other circum- 

 stances, atiect the regularity of the laws of water-ri 

 ance and wind-pressure so much, that we are not in a 

 condition to compute, accurately, the ratio of the speed 

 of the vessel to the velocity of the wind. 



\\Vre the sail of a vessel fixed in a middle position, 

 and the vessel itself capable of moving with equal ease 

 in any direction, as it would be if it were made quite 

 circular, instead of being long and narrow, the vessel 

 would be propelled exactly in the direction in which the 

 wind might blow. But as this result would by no means 

 suit the views of men desiring to sail in some particular 

 direction, it is necessary to contrive the form of the 



Fig. 101. 



vessel, and the position of tho 

 sails, so that the pressure of the 

 wind in one direction may be con- 

 verted into a pressure in another 

 direction. The vessel is, there- 

 fore, made long and narrow, so 

 that it may be subject to much 

 less resistance from the water, 

 when moving in the direction of 

 its length, than if it wore to 

 move sideways ; the sails are 

 made capable of being tmm-.l 

 about to different angles with tlio 

 length of the vessel, so as to re- 

 ceive the wind obliquely ; and, 

 to direct the vessel and overcome 

 any tendency of the wind to blow 

 the front or back part of the 

 vessel round out of its proper 

 course, a rudder or he!m is 

 mounted at the stern, to balance, 

 by its resistance in the water on 

 one side or the other, the force 

 tending to moke the vessel devi- 

 ate. Vilien the wind strikes on a 

 surface presented to it obliquely, 

 its pressure is diminished. Sup- 

 pose a certain surface five feet wide (Fig. 101) exposed 

 directly to the wind, 'or at right angles to the direction 

 of the wind, the pressure on it may be considered to bo 

 made up of five pressures each on one foot of the width. 

 But if the same surface be turned obliquely to the wind, 

 so that while the surface itself still remains five feet 

 wide, the quantity of air intercepted is to be measim-d 

 by throe feet in width, tho length of tho line drawn :>t 



