On the Bnrometef. 141 



should be first accurately placed, the barometer then smartly 

 rapped, in order to reduce the mercury to a level, and the 

 index again accurately adjusted. 



The principle of a baronicter is so perfectly uniform in 

 its effect, that although, for want of precision in their con- 

 struction, any two of them may not, under sni)ilar circum- 

 stances, respecting the slaie of the atmosphere, and the ele- 

 vaiion in whicli ihev are placed, stand precisely at the same 

 height ; yet all will agree in this — they will rise or sink at 

 the same time. 



The ordinary upright barometer is the only one in use, 

 among scientific persons, of which there are two kinds, viz. 

 the cistern or bason barometer, and the syphon baroineter: 

 the former of these requires a Jloaiing gage, with an ad- 

 justing screw, in order to show the precise length or height 

 of the colunm of mercury in the tube ; which is essential 

 in accurate observations, and particularly in the measure- 

 ment of altitudes by the barometer ; but which may be 

 dispensed with in ordinary observations. 



The only objectionable circumstance in the perpendicular 

 barometer is the shortness of its scale; but, since there are 

 no means of extending the scale of a barometer without, in 

 some degree, injuring its principle, this defect is supplied 

 by a noiiiu.i or veriiisr. 



Barometers intended for the purpose of measuring alti- 

 tudes, are fitted up in a peculiar njauner adapted to that 

 intention. 



The attractive power of the snn upon the earth's surface, 

 is to that of the moon, as 2 to 3. 



With respect to the direction of the wind, it is tolerably 

 uniform within the tropics, being more immediately under 

 the influence of the two prmcipal causes of currents of air, 

 viz. tirst the sun, which, in its course heating the places 

 it successively passes over, causes a very considerable rare- 

 faction of the air there, which is constantly followed by a 

 current of air, to restore the equilibrium; — and secondly, 

 I he current of wind, which is produced by the attractive 

 influence of the .»/« and moon ou the atmosphere, as the 

 ♦liflfereni places pass under them. 



Near the poles, the wind is probably more uniform than 

 it is in the middle or temperate climates ; the lower portions 

 of the air being constantly inclined towards the equator, iu 

 con^equcllCC of the increasing heat in that direction ; and 

 the upper portions being renewed, from the more elevated 

 regions of the inter-tiopicul currents. 



Whereas, in the middle or temperate cliinateS, besides 

 •ihtr causes unknown to us, the direction of the wind be- 

 comes 



