HUTCHISON'S OBJECTIONS, WITH REPLIES. 463 



5th. If I recollect right, you mention in one of your re- 

 ports, that the heat communicated to the incumbent at- 

 mosphere by some very limited combustion underneath, 

 gave rise to an upward aerial vortex which occasioned a 

 local thunder storm, accompanied with heavy rain. I am 

 inclined to think that if such a limited cause of increased 

 aerial temperature produced the thunder storm in the case 

 referred to, there would be an almost constant upward 

 aerial vortex, accompanied with thunder and rain, over 

 every large city. London, including its suburbs, extends 

 about twelve miles in length, by eight in breadth, and con- 

 tains a population of about 1,800,000. In calm weather, 

 particularly during winter, the increase of temperature 

 communicated to the atmosphere by the combustion of fuel 

 and animal respiration over the central portions of such a 

 large city, cannot be less than eight or ten degrees beyond 

 that of the atmosphere in the surrounding country. But 

 instead of an excess of rain falling over London, the annual 

 amount of rain there is only 22.2 inches, which is less than 

 in almost any other part of England, where observations 

 have been made. The air in the central parts of Glasgow 

 during calm weather, in winter, is usually six or eight de- 

 grees warmer than in the surrounding country. Now, if 

 this excess of atmospheric temperature occasioned an up- 

 ward aerial vortex so as to produce clouds and rain, as 

 should be the case if your theory were correct, the amount 

 of rain that falls at Glasgow should be unusually great. 

 But instead of this being the case, the annual amount of 

 rain collected in the rain gauge kept at the JVTFarlane Ob- 

 servatory, College Garden, when averaged for upwards of 

 thirty years, was only about 22 inches ; whereas the annual 

 amount of rain collected in every one of the rain gauges 

 kept within a limited number of miles of the city, was con- 

 siderably greater. The circumstance of the amount of rain 

 collected in the rain gauge kept at the M'Farlane Observa- 



