194 Mr. S. P. Langley's Experimental Determination of 

 large number of observations carefully repeated on different 



occasions:- 





Maximum pressure borne 

 without flaring before reach- 

 ing bottom. 



Mininum pressure required 

 for sensitiveness. 



Meth. spirit 

 and water, 1 : 1 



Between 21 inches and 30 

 inches. 



Above 21 inches. 



Meth. spirit 

 and water, 1 : 3 



About 12 inches. 



From 7 to 8J inches. 



Meth. spirit 

 and water, 1 : 7 



From 8 to 9 inches. 



About 2i inches. 



Cold water 

 12°-5 Cent. 



4^ inches. 



2J inches. 



Hot water, 

 61° Cent. 



About -J inch. 



About -J inch. 



Hot water, 

 84° Cent. 



About J inch. 



About J inch. 



It will be seen that the effect of varying the viscosity is 

 very distinct. In the extreme cases a jet which will not bear 

 a pressure of more than \ inch without flaring when the 

 liquid is water nearly at the boiling-point, requires from 20 

 to 30 inches to make it flare when we pass to the mixture of 

 alcohol and water in equal proportions. It is probable that 

 with the hot water the results are somewhat disturbed by the 

 impossibility of getting the liquid perfectly quiet owing to the 

 formation of convection-currents; but the differences are so 

 great that the general conclusion cannot be disturbed by this 

 circumstance. The comparisons between the cold water and 

 the alcoholic mixtures, which were all used at the tempera- 

 ture of the room, are entirely independent of this source of 



error. 



XXV. Experimental Determination of Wave-lengths in the 

 Invisible Prismatic Spectrum. By S. P. Langley*. 



[Plates VIII. & IX.] 



Note. — The following investigation was made at the expense of the 

 Bache Fund, and is published here by the permission of its Trustees. It is 

 the subject of a still unpublished memoir presented in April 1883 to the 

 National Academy of Sciences, in whose Transactions the unabridged 

 commimication will be found. 



IN September 1881, while engaged upon Mount Whitney 

 in measuring with a linear bolometer the heat in the 

 invisible spectrum of a flint prism, I came upon a hitherto 



* Communicated by the Author. 



