INTRODUCTION TO PNEUMATICS. Ixix 



piston formed the bottom. This common pump is, therefore, called the 

 sucking and lifting pump, as it is constructed on both these principles. 



The forcing 1 pump consists of a forcing power added to the sucking 

 part of the pump. This additional power is exactly on the principle of 

 the syringe : by raising the piston, the water is drawn up into the pump ; 

 and by making it descend, it is forced out. The large pipe, A B (Jig. 3), 

 represents the sucking part of the pump, which differs from the lifting 

 pump only in its piston, P, being unfurnished with a valve, in consequence 

 of which the water cannot rise above it. When, therefore, the piston 

 descends, it shuts the valve, y, and forces the water (which has no other 

 vent) into the pipe, D : this is likewise furnished with a valve, V, which, 

 opening outwards, admits the water, but prevents its return. The water 

 is thus first raised in the pump, and then forced into the pipe, by the 

 alternate ascending and descending motion of the piston, after a few 

 strokes of the handle to fill the pipe, from whence the water issuesat the 

 spout. 



SECTION II. On Wind and Sound. 



WE are now to give some account of the nature of Wind and Sound. 

 Wind is the motion of a stream or current of air, which may be produced 

 by a variety of causes ; but the most common one is a partial change of 

 temperature in the atmosphere : for when any one part is more heated 

 than the rest, that part is rarefied ; and thus, becoming lighter than the 

 air around, it rises, and the surrounding air presses in towards that part, in 

 order to restore the equilibrum : this spot, therefore, receives wind from 

 every quarter. Those who live to the north of it experience a north wind ; 

 those to the south, a south wind; and those who live on the spot where these 

 winds meet and interfere have turbulent and boisterous weather whirl- 

 winds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs 

 most frequently in the torrid zone, where the heat is greatest: the air 

 being more rarefied there than in any other part of the globe, is lighter, 

 and consequently ascends; whilst the air about the polar .regions is 

 continually flowing from the poles, to restore the equilibrium. This 

 motion of the air, did no obstacles interfere, would produce a regular 

 and constant north wind to the inhabitants of the northern hemi- 

 sphere, and a south wind to those of the southern hemisphere ; but 

 these winds do not meet without previously changing their direction. 

 The atmosphere accompanies the earth in its diurnal motion : it travels, 

 therefore, with greater or less velocity as it is nearer the equator, 

 or more distant from it. When therefore the air flows from the north 

 or south to restore the atmospherical equilibrium at the equator, this 

 air, not having acquired the velocity of the equatorial regions, cannot 

 keep pace with the earth, which, travelling faster, passes through it ; and 

 as the earth moves from west to east, its motion through the air produces 

 a regular east wind at the equator. The winds from the north and south 

 combine with this easterly wind, and form what are called the trade- 

 winds. The composition of the two winds north and east produces a 

 constant north-east wind ; and that of the two winds south and east pro- 

 duces a regular south-east wind. These winds extend to about thirty 



