OP ARTS AND SCIENCES. 25 



Critique of the Method. 



Since the variation to be studied in the present investigation is one of 

 quite small amount, and is determined as the result of a somewhat com- 

 plex measurement, any systematic error of even quite small magnitude 

 might mask the quantity sought. It is essential, therefore, to show how 

 far such errors of sensible magnitude have been avoided or eliminated. 

 In the following pages I have given a somewhat detailed discussion of 

 the possible sources and magnitudes of error in my measurements, and 

 of the accuracy attained in the various component measurements. 



The advantages of the method arise from its simplicity and direct- 

 ness. The only measurements of precision required are of the tem- 

 perature of the second capillary, and of the lengths of several mercury 

 columns. No measurements of volume are involved. The two capil- 

 laries through which identically the same masses of gas are successively 

 transpired are as nearly as possible alike, and under the same conditions 

 except as to temperature. The gas is transpired through each under 

 nearly the same pressure difference, and at a pressure not far from 

 one atmosphere. The flow of the gas is perfectly uniform, except for 

 slight accidental fluctuations, throughout the whole of a measurement or 

 set of measurements, tending thus to the elimination of a class of errors 

 possible in such methods as that of oscillating plates, or of transpira- 

 tion when the driving pressure falls from a given initial to a less final 

 amount. The disturbing effect of eddies, or other special action, at the 

 entrance to or exit from the tube, would be in part eliminated in this 

 method, since the measurements give the ratios of the resistance in 

 two tubes under constant and nearly identical conditions. No com- 

 plete discussion of these effects has been yet given, and their experi- 

 mental elimination certainly seems more easy than their mathematical 

 treatment. It seems quite possible that they are still sensible sources 

 of deviation from the assumed law of transpiration, but as the magni- 

 tude of the disturbance is small, and cannot be widely different in the 

 two tubes used in this apparatus, and since the determination of the 

 constants of the tubes dependent on their diameters, lengths, «&;c. are 

 experimentally made by the same process and under conditions identical 

 with the subsequent work, the I'esulting error must be small. It should 

 be noted, also, that the debated question as to the slip of the gas over 

 the inner surface of the glass tube lias an influence in my results only 

 to the extent by which this slip is alfectod by the temperature. 



The freedom of the method from constant error is indicated by the 

 close accordance of the results on dry air as obtained by the first, fourth, 



