86 Prof. J. W. Draper on the Fixed Lines in the 



days, and the direction of the current was reversed. It will 

 be seen that the value of this quantity increases slightly with 

 the temperature, as we should expect from the slight difference 

 in size of the two tubes used. The values of as will be seen to 

 agree quite closely, with the exception of experiments 2 and 21. 

 A comparison of these results with those of Meyer, Max- 

 well, Puluj, or von Obermayer will show the superior accuracy 

 of this method. Such a comparison can be most easily made 

 by means of a graphical construction. Let j]=;ct x be the 

 general form of the equation ; then 



log 7] = log c + x log r, 



which is of the form of the equation to a straight line referred 

 to rectangular axes, and making an angle whose tangent is oc 

 with the axis of X, the value of log c being the intercept on 

 the axis of Y. Therefore, if we plot the various values of 

 log 7} as ordinates, and of logT(T = — 273° C.) as abscissae, 

 we shall obtain points lying along a straight line, from whose 

 tangent with X the value of x may be determined. An in- 

 spection of the lines thus obtained from the data of various 

 experimenters furnishes the most ready means of comparing 

 the accuracy of their results. By such an examination it will 

 be seen that, while Meyer obtained values of x from x = 2'3 to 

 ^ = 0-21, and Puluj from # = 0*65 to ^ = 0*47, the above Table 

 shows variations from ,# = 0*799 to ^ = 0*738 only in these 

 preliminary experiments. 



As a result, then, of these experiments, it would appear that 

 the viscosity of air increases proportionally to the 0'77 power, 

 nearly, of the absolute temperature between 0° and 100° C. 

 But more determinations at temperatures between these limits 

 are necessary to prove the law of this variation. 



XIII. On the Fixed Lines in the Ultra-red Lnvisible Region 

 of the Spectrum. By John William Draper. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



I DESIRE to call the attention of those experimenters who 

 are at present occupied in investigating the less-refran- 

 gible end of the spectrum, to a paper illustrated by an en- 

 graving in the Philosophical Magazine for May 1843. From 

 this it will be seen that in the preceding year I had made 

 photographs, not only of the Fraunhofer lines, but also of 

 many others at both ends of the spectrum, and in exploring 

 the less-refrangible region had found three great lines far 



