286 Royal Society : — 



neutral ; and therefore if the green be made a hhie green at the same 

 time that the red is a yellow red, they become quite as distinct to 

 the colour-bUnd as to the normal-eyed. 



The colouring of geological maps is very perplexing to the colour- 

 blind, and it is recommended that engraved marks, to distinguish 

 the diiferent strata, should alvrays be added to the colours. 



In conclusion, the author gives hints which he considers useful for 

 the examination of colour-blind persons, and states the importance of 

 collecting further evidence on the subject, of an accurate and definite 

 nature. 



"On the Thermal Effects of Fluids in Motion." By J. P. Joule, 

 Esq., F.R.S., and Professor W. Thomson, F.R.S. 



On the Temperature of Solids exposed to Currents of Air. 

 In examining the thermal effects experienced by air rushing through 

 narrow passages, we have found, in various parts of the stream, very 

 decided indications of a lowering of temperature (see Phil. Trans. 

 June 1853), but never nearly so great as theoretical considerations 

 at first led us to expect, in air forced by its own pressure into so 

 rapid motion as it was in our experiments. The theoretical investi- 

 gation is simply as follows : — Let P and V denote the pressure and 

 the volume of a pound of the air moving very slowly up a wide pipe 

 towards the narrow passage. Let p and v denote the pressure and 

 the volume per pound in any part of the narrow passage, where 

 the velocity is q. Let also e— E denote the difference of intrinsic 

 energies of the air per pound in the two situations. Then the equa- 

 tion of mechanical effect is 



^=(PV-j,.) + (E-e). 



since the first member is the mechanical value of the motion, per 

 pound of air ; the first bracketed term of the second member is the 

 excess of work done in pushing it forward, above ^he work spent by 

 it in pushing forward the fluid immediately in advance of it in the 

 narrow passage ; and the second bracketed term is the amount of 

 intrinsic energy given up by the fluid in passing from one situation 

 to the other. 



Now, to the degree of accuracy to which air follows Boyle's and 

 Gay-Lussac's laws, we have 



if t and T denote the temperatures of the air in the two positions 

 reckoned from the absolute zero of the air-thermometer. Also, to 

 about the same degree of accuracy, our experiments on the tempera- 

 ture of air escaping from a state of high pressure through a porous 

 plug, establish Mayer's hypothesis as the thenno-dynamic law of 

 expansion ; and to this degree of accuracy we may assume the in- 

 trinsic energy of a mass of air to be independent of its density when 

 its temperature remains unaltered. Lastly, Carnot's principle, as 

 modified in the dynamical theory, shows that a fluid which fulfils 



