GEOLOGICAL CLIMATE IN HIGH LATITUDES. 363 



for lack of reason to the contrary, we must believe that it was equally 

 uniform in geological times. It acted as glass does in a green-house ; 

 it retained the heat radiated from the earth's surface, and consequently 

 caused a rise in temperature. This increased in a higher ratio the ca- 

 pacity of the air for water, and that in its turn aided still further in 

 retaining the heat, and of course made the climate warmer. In this, I 

 think, lies the secret of the warm climate in high latitudes in those 

 early times, the otherwise cold polar regions being protected by this 

 double " blanket." The effect of such a covering is well set forth by 

 Professor Tyndall in " Heat considered as a Mode of Motion," pp. 405, 

 406. I quote only one sentence : " The removal, for a single summer 

 night, of the aqueous vapor which covers England would be attended 

 by the destruction of every plant which a freezing temperature could 

 kill." 



In contrast with this, I add one illustration of the temperature 

 possible were the earth covered with a " warm blanket " equal in heat- 

 retaining power to glass. I quote from Professor Langley's summary 

 of work on Mount Whitney to ascertain the amount of heat the sun 

 sends to the earth : " On the summit of Mount Whitney the tempera- 

 ture in a blackened copper vessel, covered by two sheets of common 

 window-glass, rose above the boiling point. With such a vessel water 

 could be boiled in the snow-fields of Mount Whitney by the direct solar 

 rays." 



Besides carbonic acid and water, there probably were in the early 

 atmosphere other gases and vapors. Ammonia would produce thirteen 

 times the effect of CO a at the same density, and marsh-gas four and 

 one half times, and so of others ("Heat as a Mode of Motion," p. 362). 

 Whatever there was of these, their influence tended to increase the 

 " warm blanket." The amazingly slow change of temperature in the 

 early periods finds a reasonable explanation in the effect of those gases 

 and vapors in the atmosphere. 



Professor Tyndall has shown that, commencing with a vacuum, and 

 adding a small number of very small increments, the absorption is sen- 

 sibly proportional to the increments, but, as the quantity increases, the 

 deviation from proportionality augments (idem, p. 356) ; at length a 

 condition is reached in which further increments produce very little 

 effect. The converse must also be true. Commencing with a great 

 amount of the gas, or vapor, a very great number of decrements will 

 be needed to produce any sensible effect ; then a smaller number, and 

 so on, until toward the end, and then the decrement needed will be 

 very small, and the effect comparatively large. 



The following diagram, made from a table on page 35 of the same 

 work, shows this more clearly. The curved line indicates tempera- 

 ture for equal increments of the gas. 



The diagram is for sulphuric ether and olefiant gas. All other 

 (compound) gases and vapors observe the same law, but differ in the 



