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As you have turned my attention to this fubjed, I now fend 



you fuch remarks as have occurred to me ; fome of which may 



poiTibly be ufeful to thofe who are employed in barometrical 



obfervations. 



I am, dear Sir, 



Your's, &c. 



H. HAMILTON. 



Dublin., February 6th., 1792. 



To the Rev. BoSlor J. A. Hamilton. 



POSTSCRIPT. 



I FIND the mercury in my portable barometer (now a con- 

 fiderable time in ufe) varies as freely, when the cover is fcrewed 

 clofe to the top of the box, as it could do in any open barometer ; 

 for I never could perceive the leaft alteration in the height 

 of the mercury upon opening the hole in the box after it had 

 been a long time clofed, fo that the air muft have free accefs 

 to the box though it is clofe enough to retain the mercury 

 perfectly. The fame thing may happen in other clofe barometers, 

 and when it does happen it is an advantage, as it faves the 

 trouble of turning the cover and bringing together the holes 

 in it and the box, whenever we would know the height of the 

 mercury. I therefore thought this circumftance worth mentioning. 

 This kind of barometer will ferve juft as well at fea as at land, 

 and will fupply what has been much wanted ; as none of the 

 contrivances for a marine barometer have been found to anfwer 

 the purpofe fufBcicntly. 



