'664 JAMAICA. 



tionably expanded, and occupy a larger fpace. This piece of philo o- 

 phy I remember to have feen proved by lame Negroes, wlio had ob- 

 lerved, that when they went with a calk of rum in a Vvaggon, to mar- 

 ket, in the heat of the day, and agitated it as much as polfible, by 

 driving the carriage over the roughed parts of the road, the calk, upon 

 deHvery to the wharfinger, would appear quite full up to^the bung- 

 hole, though they had ftolen fome bottles-full out of it by the way. 



The cfted: likewife upon mercury deferves to be noted; becaufe it 

 materially concerns barometric obfervations j for mercury w^ill expand 

 with heat; and fince, by this expanfion, it muft extend the column 

 upwards in the barometric tube, it is therefore probable, th^t an ex- 

 traordinary rife of it in very hot dry weather is fometime^ o^^;i"g. tQ 

 this caufe, and not to any encreafed prellure or gravity of the atrrjo- 

 f])here. 



Hence it appears, that fome uncertainty mufl inevitably attend the 

 motions of this inllrument, in all hot countries ; for which reafon, upon 

 every unufual rife of the mercury, the flate of the air, at that, time, in 

 refpcft to heat and drynefs, ought carefully to be noted [r]. 



A regard muft alfo be had to the choice of the inftrument; for the 

 tubes of the ordinary, cheap barometers contain fo fmall a quantity of 

 mercury in them, that they are good for nothing : in thefe fmall tubes 

 the attra6llon of cohefion makes the mercury flick to the fides of the 

 glafs, fo as not to rife and fall regularly, according to the variations 

 in the atmofphere, as may be {ttw on comparing thetn with the baro- 

 meters of larger and better ftru£lure. 



Thermometers likewife are not free from irregularities, occafioned 

 by the expanfion of their liquor, which, confequently, may often indir 

 cate a greater degree of heat than is aftually prefent in the air. 



The pendulums of clocks and watches, we find, are lengthened from 

 the fame caufe in this climate, fo as fometimes to require railing very 

 high ; and hence, till they are redified, they mufl continually lofe their 

 time. They are alfo liable to alter their vibrations from the figure of 



[<•] An int;enious ger.tleraan obferved here, that ahhough there was but little or no variation 

 in the life and fall of the mercury, from the Itatenf the weather, there was a tonliuerable one in 

 rlie height of the mercury, in the day and night ; for it rofc every night, and lunk next day, fonie- 

 fimes one ifivifion, and at other times only a poition of cue. A change which it is difficult to ac- 

 Kount for, iinlcfs by fuppoling, that the atnior])here being dcnfer at night occafioned a greater 

 preUiae, than in its more heated and rareficri flatc d.'iing the dny time, even al!cv.ing for tl.c ut- 

 rncitl expanfion by the hear. 



the 



