Temperature on the Viscosity of Air. 209 



the form of a helix, as these facilitate the maintenance of con- 

 stant and uniform temperatures of the bath, and render the 

 use of longer and larger tubes possible. The coils were suc- 

 cessfully made by winding the tubing, as it was drawn from 

 the tube of the glass-worker, upon a wooden drum. The 

 second series was made with a pair of such coils, and 

 consisted of only eight measurements. The results in the 

 measurements of K were so wholly discordant that they were 

 at once rejected. 



Third Series. — A second pair of smaller coils was mounted 

 in a manner similar to that just described, and with the same 

 general results, showing the coils to be utterly useless ; pro- 

 bably, as was to be anticipated, because of changing curvature 

 of the coils owing to expansion or change of position. Several 

 straight capillaries were also studied, varying in size and in 

 the form of tube employed for heating the gas. These expe- 

 riments covered the whole 126 measurements of this series, 

 and resulted in the selection of the form and size given in the 

 general description of the apparatus at page 200. No systematic 

 study was made, however, of the effects of the proportions of 

 the tube on the law of the transpiration, though the apparatus 

 seems well adapted for that purpose. 



Fourth Series. October, November, and December 1878. — 

 The capillaries used in this series were those described in the 

 general account of the apparatus at page 200, viz. straight tubes 

 of about 30 centim. in length and 0*0110 centim. in diameter, 

 and were cut from tube No. I. of the apparatus of 1876. The 

 mercury columns were read to 0*1 millim. by the reading- 

 telescope from steel millimetre-scales placed behind the gauges. 

 Corrections were applied to these scales, but the precision of 

 reading was much less than in the fifth series ; the tempera- 

 ture-measurements were also somewhat less precise ; and the 

 whole series should have much less weight, owing to the 

 conditions under which it was taken, than the fifth series on 

 air, or the second series on carbonic acid. The results ob- 

 tained are given in the original paper. 



The results from No. 49 onwards are of much greater pre- 

 cision than those preceding, chiefly from the use of greater 

 pressures, which increased the precision of measurement. 

 Measurements 49 to 99, when combined by the method of 

 least squares, give for constants in the empirical equation 



^=^ 2 =1+A* + B* 2 

 Vo Vi 



the values, between the limits 2=0° and £=100°, 

 A=0-002821, and B= -0-00000149. 

 These values, however, I have regarded as entitled to so much 



