W I X 



some limitations. Mr Smeaton was led from experiment to conclude 

 that overshot wheels do most work, when their circumferences move at 

 the rate of 3 feet in a second, but this determination is also to be under- 

 stood with some latitude. 



3. In an overshot wheel, the machine will be in its greatest perfection ; 

 when the diameter of the wheel is % of the height of the water above 

 the lowest point of the wheel. 



4. The power of the overshot wheel is greater, caeteris paribus, than 

 that of the undershot, nearly in the ratio of 13 to 5. 



WIND. 



Winds may be divided into constant, or those which always blow in the 

 same direction ; periodical, or those which blow half a year in one direc- 

 tion, and half a year in the contrary direction, which last are called mon- 

 soons ; and variable, which are subject to no rules; 



I. Constant or Trade Winds. 



The trade wind at the Equator blows constantly from the east : from 

 the Equator to the northern tropic, or cren as far as the parallel 25 or 

 30, it declines towards the N.E., and ^ c further you recede 



from the Equator: and from the Equ; , -ithern tropic, or to 



the parallel 25 or 30, it has a S.E. direction. The line however that se- 

 parates the opposite trade winds is not precisely the Equator, but the 

 second or third parallel north. To a certain extent also they follow the 

 course of the sun, reaching a little further into the southern | sphere, 

 and contracting their limits in the north, when the sun is on the south 

 side of the Equator ; and making a reverse change when he declines to 

 the north. In a zone of variable breadth iu the middle of this tract, ralms 

 and rains prevail, caused probably by the mingling and ascending of the 

 opposite aerial currents. The phenomenon of the trade winds may be 

 thus explained. The air towards the poles being denser than that at 

 the Equator, will continually rus'i towards the Equator ; but as the ve- 

 locity of the different parts of the earth's surface, from its rotation, in- 

 creases as you approach the Equator; the air which is rushing from the 

 north will loot continue upon the same meridian, but it will be left be- 

 hind ; that is, in respect to the earth's surface, it will have a motion 

 from the east; and these two motions combined produce a N.E. wind on 

 the north side of the Equator. And in like manner there must be a S.E. 

 wind on the south side. The mr which is thus continually moving from 

 the Poles to the Equator, being rarified when it comes there, ascends to 

 the top f Hit? atmo-puc-re, tuid thf:i returns bark to the Poles, 



