^ PORTABLE BAROMETER. 



Obsf-.n-ation by against any fixed body, or if no such occurs, by kneeling oa 

 toe barometer, one knee ; the cistern should be let down so as to touch the 

 ground, the left hand holding the barometer in a vertical po- 

 sition, which a little practice will render very easy. The in- 

 dex must then be moved by the.knob till its under surface, 

 as before stated, is tangent to the mercury, A few light taps 

 should be given to the tube to ascertain that the mercury is 

 fallen .as low as it can. The height being then read off and 

 registered together with that of the attached thermometer, the 

 brass tube is turned back, so as to cover the slits ; the instrui* 

 mcnt gently inverted; and the whole is finished: all this may- 

 be done in two minutes. 

 !Deduction of The most convenient mode for deducing the heights froni 

 the heiglits to the barometrical observations, is certainly by the common lo^ 

 KSHai'biMhclo- §3.rithmic tables; and it is unnecessary here to detail the me- 

 garitlimjc ia» thod, which may be found in numerous books. It is, however^ 

 necessary for this method, to carry the tables of logarithms, 

 or by other which is sometimes inconvenient. The engraved table, formed 

 methods. by Mr, Ramsden, is on a single narrow sheet, and extremely 



portable, besides being very easy in its use; but it may be lost 

 or mislaid when wanted. Several ingenious formulae have 

 been devised, which may either be engraven on the instru- 

 ment itself, or committed to memory. Of the former, Sir G. 

 Sluickburg has given a very concise one in his second paper on 

 the measurement of heights by the barometer, in the sixty-eighth 

 vol. of the Philosophical Transactions ; and Mr. Professor 

 Leslie has invented a very elegant one of the latter sort : but 

 these, though very simple in form, require a considerable nuni^ 



Computations bcr of figures in the operation, and are on that account incon- 



inay be made • ^ t- ^ .^ r r .■ .■> 



on "a journey ^'^'^cnt. I'or the purpose therefore of computing on tne spot, 



very near the and very near to the truth, any observations made on a jour- 



tmth, from a i .1 i • , ,, . „ . . 



table here ^^Y^ '^"^ '"^^ almost without the necessity of writing at all, 



piven, wliioh is I have caused the fo]lo\^ ing short table to be engraven on the 

 the barometer, ^^ale of the barometer. It expresses the value of the difl'crcnce 

 of a tenth of an inch in the height of the mercury, at the tem- 

 perature of freezing, in English feet. 



TABLE. 



