Ox THE MECHAXICAL EQUIVALENT OF HEAT 3f>3 



stant, it is exactly eliminated from the equation. The depression for 

 this diameter is about -08 mm., but depends upon the height of the 

 meniscus. The height of the meniscus was generally about 1-3 mm.; 

 but according as it was a rising or falling meniscus, it varied from 

 1-4 to 1-2 mm. These are the practical values of the variation, and 

 would have been greater if the barometer had not been attached to the 

 wall a little loosely, so as to have a slight motion when handled. Also 

 in use the instrument was slightly tapped before reading. The varia- 

 tion of the height of the meniscus from 1-2 to 1-4 mm. would affect 

 the reading only to the extent of -01 to -02 mm. 



The only case where any correction for capillarity is needed is in 

 finding the temperatures of the steam at the 100 point, and will then 

 affect that temperature only to the extent of about 0-005. 



The scale of the instrument was very nearly standard at C., and 

 was on brass. 



At the centre of the brass tube which surrounded the barometer, a 

 thermometer was fixed, the bulb being surrounded by brass, and there- 

 fore indicating the temperature of the brass tube. 



In order that it should also indicate the temperature of the barome- 

 ter, the whole tube and thermometer were wrapped in cloth until a 

 thickness of about 5 or 6 cm. was laid over the tube, a portion being 

 displaced to read the thermometers. This wrapping of the barometer 

 was very important, and only poor results were obtained before its 

 use; and this is seen from the fact that 1 on the thermometer indi- 

 cates a correction of -12 mm. on the barometer, and hence makes a 

 difference of 0-04 on the air thermometer. 



As this is one of the most important sources of error, I have now 

 devised means of almost entirely eliminating it, and making continual 

 reading of the barometer unnecessary. This I intend doing by an 

 artificial atmosphere, consisting of a large vessel of air in ice, and 

 attached to the open tube of the manometer of the air thermometer. 



The Thermometers 



The standard thermometers used in my experiments are given in 

 the following table on the next page. 



The calibration of the first four thermometers has been described. 

 The calibration of the Kew standard was almost perfect, and no cor- 

 rection was thought necessary. The scale divided on the tube was to 

 half-degrees Fahrenheit; but as the 32 and 212 points were not cor- 

 rect, it was in practice used as a thermometer with arbitrary divisions. 



