256 



2ndly. With the invariable pendulum, the disc subtended a some- 

 what less angle than the tail-piece, so that the inferred number of vi- 

 brations in 24 hours was diminished about -rVths of a second. But 

 experiments with the invariable pendulum being intended to be in the 

 strictest sense of the word comparative, this constant difference will 

 no way affect the ultimate result. But as the most direct way to 

 remove any doubts which may be entertained on the subject, the 

 author has computed from the whole of Captain Sabine's observa- 

 tions, the successive differences in the vibrations at the various sta- 

 tions visited by him, by the two methods, viz. that of employing the 

 disappearances and re-appearances, and the disappearances alone. 

 The results only in one instance differ so much as a tenth part of a 

 vibration. They are indifferently in excess and defect, and the mean 

 of their discrepancies is exactly nothing. From this he concludes, 

 that if the observations be made as nearly as possible under similar 

 circumstances, the method of observing by disappearances alone is 

 productive of no perceptible error in practice, in experiments with 

 the invariable pendulum, while in those with the convertible pendu- 

 lum the equal apparent size of the disc and tail-piece precludes the 

 possibility of any, either in practice or theory, from this cause. 



The standard of Sir George Shuckburgh having been found iden- 

 tical with that of Bird, in the custody of the clerk of the House of 

 Commons, adopted as the imperial standard unit of extension, the 

 length of the pendulum already determined is fixed with the same 

 degree of precision in parts of the Imperial standard yard. 



A repetition of Sir George Shuckburgh's experiments on the weight 

 of given volumes of distilled water, and a re-measurement of the cube, 

 sphere, and cylinder used by him, were found to give no material 

 variation from his results; and these being stated in terms of the 

 mean of several standard weights kept at the House of Commons, 

 the troy pound nearest the mean has been adopted and declared by 

 the legislature to be the original unit of weight under the denomina- 

 tion of the Imperial standard troy pound. 



The relation between this pound and the cubic inch of distilled 

 water at 62 Fahrenheit, barometer 30 inches, has been ascertained 

 by the Commissioners of Weights and Measures, who find that the 

 latter contains 252,458 grains, each grain being the 5760 part of the 

 standard troy pound. 



The avoirdupois pound is fixed for assigning its proportion to the 

 standard troy pound, so as to contain exactly 7000 such grains. 



The Imperial standard gallon is defined by stating its contents un- 

 der the same circumstances of temperature and pressure at 10 pounds 

 avoirdupois ; and the bushel, by its containing 80 such pounds. 



The author having, in compliance with a request of the Lords 

 Commissioners of His Majesty's Treasury, undertaken to superin- 

 tend the construction of, and to adjust the principal standards to be 

 deposited at the Exchequer, Guildhall, Dublin, and Edinburgh, Mr. 

 Dollond was directed to prepare those of linear measure, and Mr. 

 Bate those of weight and measure, the proper quality of metal for 



