274 



METEOHOLOGY. 



[Sect. IX. 





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1 



of the tube, which plunges into the cistern in well-con- 

 structed marine barometers, is contracted so as to diini- 



shi p's 



nisli 



the amount of oscillation produced by the 



motion. 



The instvaiiitent should be suspended out of the 

 reach of sunshine, but in a good light for reading, as 

 near midships, and in a place as little liable to sudden 

 changes of temperature and gusts of wind as possible. 



The light should have access to the back of the tube, so 

 as to allow of setting the index to have its lower edge 



mercury 



In 



a tangent to the convex surface of the 



O 



+ 



w%ll-c6nstructed harometers the slider has its lower 

 p£irt tubular, embracing the tube, and can be made to 

 descend by the rack-motion of the vernier till it be- 



comes an upper tangent 

 being on its exact level, a reflected light by day, or 

 white paper strongly illuminated from behind at night, 

 will throw the light properly for setting the vernier 

 correctly. The exact height of the cistern above the 



to the mercury : the eye 



■.^ 



hi 



ip 



r^ 



water-line bliould be ascertained and entered on 



tl 





register. 



The attached thermometer ought to indicate a tem- 

 perature the exact mfean of that of the whole barometric 



column. Its bulb, therefore, ought to be (though it 

 seldom is) so situated as UY afford the best chance of its 

 doing so, that is to say, fifteen inches above the cistern, 

 enclosed within the wooden case of the barometer, nearly 

 in contact with its tube, and wdth a stem so long as to be 



read off at the upper leveL To ensure a fair average 

 and steady temperature, it were w ell to enclose the whole 

 instrument, thermometer and all, in an outer case of 

 leather, over a wTapper of flannel, leaving only the scttmg 



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