Barometric and Thermometrtc Firji-movers. an 



to increale, and the confequence will be, that a portion of the mercury being forced from the 

 vefTel C D into A B, will render this laft heavier, and caufe it to defcend ; while C P at 

 the fame time rifes. And on the other hand, when, by a diminifhed preffure of the external 

 air, the mercury fubfides in A B, the veflel C D will preponderate, and A B will rife. 

 Now the frame E F, which is interpofed between the barometer and the pullies I K, is jointed 

 at the corners and alfo at the places where it is attached to the chain and the barometer ; and 

 the inner edges of the upright pieces E, F are formed into teeth like thofe of a faw, the flopes 

 of which lie in oppofite diredions, as is (hewn in the figure. The wheel G, which is placed 

 between thefe bars, is alfo toothed in the fame manner ; and its diameter is fuch, that when the 

 teeth on one fide, as for example E, are engaged, thofe on the other fide, F, may be free ; but 

 it is too large to admit of both fides being difengaged at once. The wheel G is prevented 

 by a clicic from moving in the diretSion oppofite to that which may be produced by theadtion 

 of the bars E and F. Hence the play of the machine is evident. When the preflure of the 

 atmofphere diminifties and the barometer rifes from its ciftern, the fide E of the frame will 

 move the wheel G through a greater or lefs fpace, according to the variation ; and when, on 

 the contrary, it falls, the teeth E will be drawn out of their bearing, and thofe of F will be 

 thrown into the wheel, and ftill produce a motion of the fame kind ; the joints of the frame 

 E F allowing it to change its figure enough for this purpofe. It is hardly necefi'ary to remarii:, 

 that this wheel G being connefted with the clock, ferves to wind it up, and that the cloclc is 

 conftrudled to go for a much greater number of days than the barometer has ever been known 

 to remain ftationary. 



The ingenious mechanic will readily form a notion of many other methods, of applyino- the 

 variations of the barometer to fimilar objeds. The wheel-barometer of Robert Hook, as 

 well as another contrivance, in which the barometer and its ciftern are placed at the different' 

 extremities of an inclined lever, may likewife be ufed for this purpofe. 



Several artifts have exerted their induftry, in attempts to apply the variations produced by 

 change of temperature in bodies as a firfl mover. If a thermometer be fufpended by its centre 

 of gravity is fuch a manner, that the tube may lie nearly horizontal ; the daily variations in the 

 bulk of the mercury will caufe a preponderance on the one fide or the other, accordingly as tfe" 

 temperature is higher or lower, than it was at the original fixing of the centre of fufpenfioW; 

 The thermometer may contain mercury or any other fluid, or it may confift of air confined \yf 

 mercury, as in the manometer. In this contrivance, the great and frequent ranges of variation 

 affords much promife of utility. The limits of convenient or practicable power from change 

 of equilibrium in a fluid thermometer, will hereafter be examined. A much greater force 

 feems to offer itfelf, in the power by which the expanfion is produced ; but the difficulty of 

 forming a pifton or other apparatus for confining fluids, will probably conftitute an infurmount- 

 able impediment to this method. 



The folid thermometer does not prefent the fame difficulty. Fig. 2, reprefents a feries of ex- 

 panfion-bars, each conufting of a plate of brafs, foldered to another of fteel, and poffefling the 

 property of bending by change of temperature, according to the laws already explained in this 



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