48 PROCEEDINGS OF THE AMERICAN ACADEMY 



Lest, however, an error might occur in the last reduction fi'om a 

 difference between the coefficient of expansion of the bore of a capillary 

 tube and of its lineal expansion, I have carefidly measured both, and 

 find that the coefficient for the bore is 0.0000075, while for the linear 

 expansion I find 0.0i)00080 per degree centigrade, a difference too 

 slight to affect the results in my use of it ; I have thought it best to 

 use the value 0.0000075 as it entered in the fourth power, while the 

 other entered only in the first power. The tubes used have also been 

 calibrated to insure the selection of those of uniform bore, and their 

 dimensions have been accurately measured by mercury and a micro- 

 meter screw. The dimensions of the two tubes used in ihe experi- 

 ments to be described, were, for tube No. I., }. = 1272.3 mm., R = 

 0.1098 mm. ; for tube No. II., ;i= 1274.1 mm., ie = 0.1115 mm. 



To make an experiment with this apparatus, it is merely necessary 

 to start the jet of wafei" and allow the exhaustion to proceed until the 

 mercury columns in J^ and J^ have come completely to rest. Read- 

 ings are then taken of the heiglits of these columns by means of a 

 cathetometer from a steel scale placed beside the gauges. Tlie reading 

 of the barometer corrected for instrumental error gives the pressure 

 at A. All these are reduced to the freezing point, and A' and i^are 

 corrected for capillaiity by the tables of Delcros. <rhe temperature of 

 the baths is also taken by thermometers in various positions in the 

 troughs. This must be kept constant throughout the experiment, and 

 I have, therefore, principally used the temperatures of melting ice and 

 boiling water. In the experiments of which the following table gives 

 the results, advantage ha^s been taken of the four methods of checking 

 the results of one experiment by another, by reversing tiie direction of 

 flow of the air through the tubes and heating alternately? in each case, 

 first one and then the other trough. In the table, column first gives the 

 number of the experiment; column second, the direction of flow of the 

 air, which entered at the tube whose number is first given and passed out 

 from the other ; columns three, four and five give the pressures at 

 A, B and D respective'ly ; columns six and seven show the temperatures 

 in centigrade degrees of the baths around tubes I. and II. respectively ; 



column eight shows the values of the ratio j^-^ at different tempera- 



tures ; column nine, the values of — , i.e. of ri at the higher to ?/ at the 



lower temperature ; column ten shows the values of the exponent x in 

 tlie equation )^= cr^. This is the quantity which it was the object of 

 the experiments to obtain. 



