PORTABLE BAROMLTr.U. ^ 



describe, was constructed several years since by Dr. Ilugli Usi- 

 jnilton, and is by him described at large in the littli volume of 

 the Transactions of the Irish Academy, I saw the instrument 

 in his hands, nearly sixteen years ago, and v.as much pleased 

 Avith its performance. I do not know, however, that any more 

 were then made. I have lately constructed the barometer, —•'^"npl'fi«t'l^y 



■ , ... T 1 11 • 1 • 1 • -11 I tilt; author, it 



whose description I shall now give, which is still more siniplc j^ jjg.],^^ |i,.,„^ 



th^n Dr, Hamilton's, and much cheaper; and which, in many ^"'^"'tasy m tht: 



trials I have made of it, appears to unite solidity, lightness, 



and ease of observation to as great a degree as can be wishccl. 



The barometer tube is about 33* inches in length ; its bove T^csn-iption. 



, ^ . , . ,. , , , ,. The barometer 



is one-tenth ot an inch in diameter, and the external diameter tube is e.lued 



is three-tenths of an inch. This sized bore is fully sufficient to '"^"^ box <".v- 



■^ linde-, the bore 



allow the free motion of the mercury. The cistern is of box, of whicb ex- 

 turned truly cylindrical, and is an inch in its internal diamc- ^f'^'^^ ^^^^^ °f 



, .,.,,*, ... .tlietubeaslOO 



tcr, and an inch in depth, A short stein projeets iroin its top, to 9 in area. 



(the instrument beins in the position for making an observa- ^^"^ ^^'^'^ 

 \ . . reaches to the 



J:\on,) for the purpose of giving a firmer hold to the tube, middle of the 



.This stem is ])erforated with an hole sufficiently large to admit ^^^rii^ of the 



, ■ , • 1 , • • 1 1 " 1 rrM , L-yliiidcr, the 



the tube, which IS glued to it in the usual mode. 1 he tube other end of 



projects into the cistern exactly to half its depth. The bottom ^''"''"'^^'^ cylinder 



• 1 • -1 ,1 1- . r, 1 • 1 IS closed with a 



ol the cistern is closed by a strong lid oi box, which screws on screw-cap, anci 



,thc cistern, and pressing against a leather glued to the inside ^'^^'^''<?''- When 

 r I ,-i 11 11 r 1 • ■ . *''^ tube is ill- 



ot the lid renders the whole perfectly impervious to the mer- !ed, luercurv 



cury in every position. The tube being filled and boiled in I'* ^^^ pom-ed 

 , 1 1 • , 1 1 • 1 . '"*o thec-vlin- 



^the common way, and the instrument held inverted in a per- der, so as" to 



,pendicular position, mercury is poured into the cistern till it is o^^"Py tlnee- 

 fillcd within two-tenths of an inch of the top. The lid is then length, and 

 firmly screwed on, and secured from being opened by idle curi- tlieretore c.o- 

 . . , „ . , , . , , ^'^-'I'S the end of 



osity by a small screw passing through its side. The essential the tube in all 



part of the instrument is now finished. The end of the tube position.s. 

 ... , , 1 , ■ . When this ba^ 



m the cistern can never be uncovered by the mercury in any rouieter is ,';et 



possible positif)n, and of course no air can ever enter into it • "l""'?ht the at- 



. ' niosphere acts 



and as the areas ot the cistern and tube arc as the squares of thro' the pores 



the diameters, the diameter of the bore of the tube being ,1 , of the wood, 

 its external diameter ,3 , and the diameter of the cistern 1 ,0 , 

 the area of the cistern is 100 — 9 = 91 ; and there being two- 

 tenths of an inch left empty in the cistern, the mercury must fall 

 ] S2-tenths, or 1 S inches and 2-tenths, before the cistern is quite 

 full : a space adequate to the measure of greater heights tiiun 

 B 2 any 



