the Rarefaction and Condensation of Air. 371 



represents the condensing pump, consisting of a cylinder of 

 gun-metal, and of a piston fitted with a plug of oiled leather, 

 which works easily, yet lightly, through a stroke of 8 inches. 

 The cylinder is 10^ inches long. If inch in interior diame- 

 ter, and ^ of an inch in thickness of metal. The pipe A, for 

 the admission of air, is fitted to the lower part of the cylinder; 

 at the bottom of this pipe there is a conical valve, constructed 

 of horn, opening downwards. A copper receiver, R, which is 

 12 inches long, 4^ inches in exterior diameter, \ of an inch 

 thick, and has a capacity of 136g cubic inches, may be screwed 

 upon the pump at pleasure. This receiver is furnished with 

 a conical valve of horn opening downwards, and, at the bot- 

 tom, with a piece of brass, B, along the centre of which there 

 is a bore of ^th of an inch diameter. There is a stop-cock at 

 S which I shall describe more particularly in the sequel. 



Anticipating that the changes of temperature of the large 

 quantity of water which was necessary in order to surround 

 the receiver and pump would be very minute, I was at great 

 pains in providing a thermometer of extreme sensibility and 

 very great accuracy. A tube of narrow bore having been se- 

 lected, a column of mercury, 1 inch long, was introduced, and 

 gradually advanced in such a manner that the end of the co- 

 lumn in one position coincided with the beginning of the co- 

 lumn in the next. In each position ihe length of the column 

 was ascertained to the ^^^^^th part of an inch, by means of an 

 instrument invented for the purpose by Mr. Dancer*. After- 

 wards the tube was covered with a film of bees'-wax, and each 

 of the previously measured spaces was divided into twenty 

 equal parts by means of a steel point carried by the dividing 

 instrument; it vfras then etched by exposure to the vapour of 

 fluoric acid. The scale thus formed was entirely arbitrary, 

 and as it only extended between 30° and 90°, it was necessary 

 to compare the thermometer with another, constructed in the 

 same manner, but furnished with a scale including the boiling 

 as well as the freezing point. When this was done, it was 

 found that ten divisions of the sensible thermometer (occupy- 

 ing about I an inch) were nearly equal to the degree of Fahr- 

 enheit; therefore, since by practice I can easily estimate with 

 the naked eye g'fjth of each of these divisions, 1 could with this 

 instrument determine temperatures to the ^^o^^ P^*^"^ ^^ ^ 

 degree. The scale being arbitrary, the indications of the 



* Of the firm of Abraham and Dancer, Cross Street, Manchester. I 

 have great pleasure in acknowledging here the skill displayed by this gen- 

 tleman in the construction of the different parts of my apparatus; to it I 

 must, in a great measure, attribute whatever success has attended the ex- 

 periments detailed in tliis paper. 



2 C2 



