126 Defer tpt'ton of a Cleck moved by the Barometer, 



VI. 



Concerning ihofe Perpetual Motions which are producible in Machines, by the Rife and Fall of 

 the Barometer or the Thermometrical Variations in the Dimenfions of Bodies. — [TV. N.) 



I 



N a former communication, I have given an account of fome of the delufive proje£ls for 

 obtaining a perpetual motion, from an invariable power *. In that paper I remarked that the 

 flow of rivers, the viciflitudcs of tides, the variations of winds, the thermometrical expanfioiis 

 of folids and fluids, the rife and fall of the mercury in the barometer, the hygrometric changes 

 in organized remains, and every other of thofe mutations, which never fail to take place 

 around us, may be applied as firft movers to mills, clocks, and other engines, and keep 

 them going till worn out. Many inftances of this kind of perpetual motion are feen in water- 

 mills, and other common engines, which are neceflarily confined to certain local fituations. — 

 The wind-mill, though lefe confined with refpecl to place, is the fubjedt of a much more va- 

 riable power ; other inftruments, ftill lefs confined with regard to fituation and expofure, have 

 been made, which are capable of continuing their motion without ceafing. Such was the 

 clock, or perpetual motion, in Cox's mufeum, which was (hewn about twenty years ago in 

 London. My former paper was written to (hew the value of the perpetual motion, ftriftly fo 

 called, which has for the mofl: part been purfued by men of little information. In the prefent 

 memoir, I fhall endeavour to afcertain that of this fecond kind of motion, which, becaufe more 

 promifing, and of nearly the fame apparent pradtical value, has been followed at fome expence 

 by men of higher claims. For this purpofe, I (hall firft defcribe a few fchemes, and then in- 

 veftigate the quantity of power they are likely to afford. 



Fig. I, Plate 6, is a fketch of the firft mover in a clock, which formed part of Cox's mu- 

 feum, which was fold by public lottery, about the year 1776, if my recollection be accurate. 

 A B reprefents the furface of the mercury in a barometer, the glafs velTel of which had the 

 form of a bottle or chemical matrafs. The diameter of the upper furface of the mercury was, 

 I think, about twelve inches. C D reprefents the bafon or receptacle, into which the aper- 

 ture of AB was plunged. I fuppofe, of courfe, that the lower furface of mercury, which was 

 expofed to the preffure of the atmofphere, was nearly the fame as the upper A B, as in fadt it 

 appeared to be. From the intervention of the cafe, and other parts of the apparatus, I could 

 only conjefturc the manner in which the effedt was produced ; but this was afiierwards ex- 

 plained to me by Mr. Rehe f, who contrived and made it. The bafon C D is fufpended 

 by two chains K L, which pafs over the pullies or wheels H I, and are attached to the frame 

 E F ; which laft is fixed to the barometer A B. Let us now fuppofe the apparatus to be at 

 liberty, and it will be clearly feen, that if the two malTes attached to the oppofite ends of the 

 chains K L be not precifely equal, the heavieft will defcend, and caufe the lighteft to rife. — 

 The malTes muft, cherefore, be brought nearly to this ftate of equality, by the adjuflment of 

 weight added to one or both of them. In this ftate, fuppofe the prefl'ure of the atmofphere 



• Philof. Journal, I. 375. 



•f This gentleman is it prefent one of the board of infpeftion of naval works at the'Admiralry. 



to 



