IMPROVED BAROMETERS. 21 



IV. 



A Method of increajing the Senfibility of the Barometer, ad libi- 

 tum. By the Rev. James Wilson, A. M. Communicated 

 by the Author. 



JL HOUGH there have been many contrivances for making Contrivances to 

 barometers with indefinite fcales, they all feem to labour under l" 1 *^ *£* ba- 

 fome difficulty either in their construction or ufe. rometer 



The evaporation of water in that of Des Cartes, the fluggifh J>y ! vater 5 h Y. 



r \ » i i • i i >• i i horizontal or m- 



motion of the mercury m the horizontal and diagonal ones, the c llned columns, 



friction in Dr. Hooke's, and the unfteadinefs of floating bodies by mechanifm 



in Rownings, have rendered thefe ingenious contrivances have not ' ve( j 



fcarce preferable to the common vertical one. ufeful. 



None of thefe defects, I believe, will attach to that which I Description of a 



propofe. It is as follows: (See Fig. 1. Plate II.) A B, a tube AmetYinTrod* 



differing in no refpect from that of the common barometer, floats in the 



but that it is wider and longer, fo that a cylindrical rod, q r. ™ ercur y of a 



& J 7 fyphon barome- 



may float freely on the mercury, and that the lower end is ter. 



connected to a fmall bent tube BCD, inftead of being im- 



merfed in a ciftern. To the lower end of the floating rod is 



fattened a hair, or fine iron wire, (what I ufed was a piece of 



the Indian weed ufed by anglers) which is brought through 



the mercury, and out at D, fo that the rod may be either 



drawn down, or fuffered to afcend at pleafure. 



Upon the tube C D is a mark N, to which by moving the 

 rod we can at any time bring the furface of the mercury. For 

 by drawing it down we force up mercury into each tube, and 

 on the other hand, by fuffering it to afcend, we fufFer the 

 mercury to defcend in each. 



The furface of the mercury then being brought to this mark, and is drawn 



and the rod being fattened, by tying the hair to the pin E, it '^cVr^muL 



on inspecting it afterwards, we find that the furface of the internal rife 



mercury has moved from N, we are fure there has been fome " u 5 es the ,ower 



. ,. . A , . . 1 r , ~ . . ... , _ „ furface to ftand 



variation in the weight ot the atmolphere, viz. if it has fallen, a t a fixed mark. 



we know that this weight has increafed, and vice verfa. 



The variations either at M or N are in a given ratio to the The variations 



variations in the length M N of the fuftained column. For at the j n " er or 

 it f ,. ,, . _ outer furrace arc 



the rile or fall at M, is to the fimultaneous fall or rife at N, in proportioned to 



the inverfe ratio of the bafes, or horizontal fections of the thofe of the 



whole column, 

 mercury 



