Thermmetrlcal and Bayometrical Firji'inivef!. ^ 173- 



. The philofophical world is aware, that hygrometers have been made, on this principle! 

 with cat-gut, hair, whalebone, and other materials. It feems probable, that the firft of thefe 

 fubftances would exert confiderable force as a firft-mover, but it would fcarcely prove durable ; 

 and what is ftill worfe, the variations of moifture in the atmofphere are little fuited to operate 

 upon machinery prefervcd in a cafe in an apartment. 



If the increafe of the fpace moved through by the expanfion of metals upon the principle of 

 Ellicott's pendulum, fliould be adopted inflead of the compound bars in fig. 4, the projeft 

 of fig. 5, may be followed. A bar of fle"el, A K, is fixed befide a bar of brafs B I, the 

 joinings, I K, being inflexible; but thofe at A, B, in the lever A F, being jointed, the 

 difference of e.xpaniion between the two metals will be magnified at F, in the proportion of 

 A F to A B. The lines, L C, G D E, reprefent a fecond combination of the fame 

 kind, in which the point G will have a fimilar and equal motion to that of F, but the bar 

 E L, being prolonged to F, fo as to bear upon the lever E F, the whole of the fecond 

 combination will be puflied forward by the expanfion of the firfl; on account of which, the 

 motion of G will, in fa6l, be double that of F; by the addition of a third combination, the 

 motion will be tripled, &:c. A fuificicnt number of thefe, properly placed in the cavity, be-» 

 tween the two cylinders, fig. 4, would afi-ord the fame confequences ; but it may be doubted, 

 whether any contrivance of this laft kind could afford the fame power in as fmall a fpace as 

 that occupied by the compound bars. 



I (hall now proceed to form an ellimate of the quantities of force communicated by thefe 

 feveral contrivances. 



The apparatus, fig. i, plate VI. or barometrical clock, is driven by a force which may be 

 eflimated in its annual quantity from the fum of the deviations of the barometer taken from a 

 Meteorological Journal, fuch as that in the Philofophical Tranfadtions ; together witii the 

 quantity of mercury fo moved, which may be derived from the, dimenfions of the furfaces i:i 

 the tube and bafon. From thefe variations, fcverally, muft be taken the quantity of what 

 mechanics call loft time, or the ineffcdlual movement between the dircfl and retrograde a£lions 

 on the machinery. The whole power will be meafured by the entire column of variation, 

 fuppofed to defcend through half its height; for this will be the cafe when a perpendicular 

 tube empties itfelf by the dcfcent of any fluid contained in it. I have not taken the trouble 

 to colleft thefe elements ; but it may eafily be imagined, that the fum of all the variations 

 during the year, would amount to no great quantity. I underfland, from the ingenious 

 conftruftor of this apparatus, that the accumulated power was not fufficient to allow the 

 deck to be maintained by a force equal to that which drives a common watch, namely, fix 

 ounces, with the daily fall of twelve inches. 



In the inveftigation of the power of expanfion, in fig. 4, a variety of curious objects of 

 phyfical and mathematical refearch offer themfelves to our confideration. In a former part 

 of this Journal*, it has been (hewn, that the cunziture afTumed by a flraight compound bar, 

 having each of its parts uniformly thick, will be circular ; whence it follows, from the nature 



• I. 61. 

 Vol. IIL—JuLY 1799. A a of 



