26 Reynolds and Moorby, Equivalent of Heat. 



made, and it was arranged that, after the trials, experi- 

 ments should be made to determine this. For, although 

 the limits of this error were within 0"03 per cent., they 

 were much the largest in the research. 



For the standard of length a series of Whitworth 

 gauges and a brass scale, by Elliott, were used ; and from 

 these a bar was prepared by Mr. Foster having parallel 

 plane ends 30 inches apart, to be used for reference in all 

 measurements, and preserved. 



The standard of temperature being the interval 

 between the temperature of ice melting under the pressure 

 of the atmosphere and that of water boiling at the sea 

 level in latitude 45^, under a pressure of 760 mm. of ice- 

 cold mercury, reference to the barometer was necessary. 



To verify thermometers at the higher temperatures 

 with facility at any time, irrespective of the pressure of 

 the atmosphere, and to secure ready verification of the 

 absolute distance between the upper and lower surfaces of 

 the mercury, the author designed a special barometer, in 

 which the light over each of the surfaces of the mercury 

 is cut off by the separate adjustment of two cylindrical 

 brass curtains, the upper curtain screwing down over a 

 slotted cylindrical prolongation of the lower curtain, which 

 encases the tube and the lower vessel, screwing down on 

 the latter. So that when the truly turned lips of the 

 curtains are adjusted to read the barometer, say at 30'^ 

 the lower curtain can be screwed off the barometer and 

 placed vertically over the '^o" bar standing on end on a 

 surface plate, so that the lip of the curtain rests on the 

 plate, when, as the relative positions of the curtains have 

 not been altered, the light over the top of the bar should 

 be just as it was seen over the mercury. The lower vessel 

 is a cylindrical cast-iron bottle, with parallel plate-glass 

 windows, and with the tube passing out through a stuffing- 

 box, there being an aperture furnished with a nozzle for 



