Defcr'tption ef a Seath-compafs. ^9 



§. 4. By this examination, if I have not verified I have at leaft preferved Mr. Whitchurft's 

 ftandard ; and, for the prcfent, I {hall confider this meafure of the difference of the length of 

 the two pendulums, vibrating 42 and 84 times in a minute of mean time, as corre£l:. On this 

 prefumption I fhall proceed to the examination of weight. 



§. 5. From the opinion of different fkilful perfons with whom I have conferred, as well as 

 from the refult of my own confiderations, I am inclined to believe, thsre is hardly any body 

 in nature, with which we are familiarly acquainted, that is of fo fimple and homogenous a 

 quality as pure diftilled water, or fo fit for the purpofes of this enquiry ; and I have concluded, 

 that if the weight of any quantity of water whofe bulk had been previoufly meafured by the 

 abovementioned fcale, could be attained under a known prelTure * and temperature of the 

 atmofphere, we fhould be in pofleiEon of a general ftandard of weight. 



§. 6. With this view, I direded Mr. Troughton to make, in addition to the very fenfible 

 hydroftatic balance before mentioned, a folid cube of brafs, whofe fides were 5 inches ; and alfo 

 a cylinder of the fame metal, 4 inches in diameter and 6 high. From St. Thomas's hofpital, 

 by favour of Dr. Fordyce, I procured 3 gallons of diltilled water ; with thefe I made the 

 following obfervations : but before I relate the experiments, I will defcribe the apparatus. 



Mr. Whitehurfl's machine for meafuring the pendulum, has been fufEciently explained in 

 his pamphlet mentioned above ; my divided fcale, which was a new inflrument, was as follows : 

 - §, 7. Defcr'tption of the Beam-compafs, or Divtded-fcale of equal parts. 



ah (Plate V. fig. i,) is a block or beam of mahogany, 6 feet 3 inches long, 6 inches 

 deep, and 5 wide, upon which are laid two brafs rules, t d e zxAfg-, each divided into 6cS 

 inches and tenths. The former of thefe, called the fcale, is for a time kept immoveable 

 by the finger-fcrews c e d^ and Is furnifhed with fine hair-line divifions, intended to be viewei 

 «nly by the microfcopes, hi: the latter, called the beam, has no motion but by means of the 

 fcrew g, and bears ftronger divifions upon it, with which the Aiding pieces or indexes at k 

 and m may readily be compared by the naked eye, and is intended only to fet the microfcopes, 

 or rather the wires in their focus, to the required diflance nearly, viz. to within tItw or ^J* 

 of an inch. The microfcopes are compound, and fimilar to thofe defcribed by the late General 

 Roy in his account of his large theodolite. (See Phil. TranC vol. LXXX.) The one 

 at h contains only crofs-wires fixed in its focus ; the other at / has a micrometer alfo, by means 

 of which its crofs-wires may be moved to the right or left, or over the image of the divlfion* 

 of the fcale, any given fpace not exceeding ^'-^ inch ; and the quantity fo moved may be 

 meafured by the divifions on the •fcrew-head pafEng under the index at 0. The divifions on 

 Aefe rules have been called inches and tenths ; it was not necefTary that they fhould be more 

 than equal parts ; but they were in faft laid down by Mr. Troughton from a fcale of the 

 late excellent artifl Mr. J- Bird, who had divided into Inches feveral fcales of different lengths ; 

 one of which, 4a inches long, belonged to the late General Roy ; a fecond, of 5 feet, was 



* I do not here mean to infer any opinion refpefting the compreiTibility of water, but only to fay, that 

 where water or any thing elfe is weighed in air, the denfity of that medium, as fitewn by the barometer and 

 tbeimometer, rouft be known in order to make allowances for it if necefTary. 



O a purchafed 



