228 DISCUSSION OF THE DESICCATION QUESTION. 



divided into two groups of twenty-five years each, the mean of the first 

 period was found to be 48°.61 ; that of the second, 49°.18, a difference of 

 0°.57 (Fahrenheit). Taking the observations made from 1770 to 1860, a 

 period of ninety years, and dividing this into three equal iiortions, the mean 

 temperature of the first thirty years was found to be 47°.73 ; that of the 

 second period of the same length, 48°.47 ; that of the third, 49°.03 — an in- 

 crease of 0°.74 as between the first and second periods, and of 0°.54 between 

 the second and third.* 



In estimating the value of these results, notice must be taken of the fact 

 that the locality where the observations were made is one in close proximity 

 to a city of five millions of inhabitants, and in the midst of a region most 

 densely populated. And it must be remembered that the change in respect 

 to number and density of popiilation which has taken place since the begin- 

 ning of the period embraced within the investigations of Mr. Glaisher has 

 been enormous.! It is a well-known fact that cities are considerably warmer 

 than the more thiidy inhabited country otherwise under similar climatic con- 

 ditions. Statistics prove this to be true ; and there could be no doubt that 

 such would be the effect of an immense aggregation of population within 

 a limited space, even if there were no statistics bearing on this question. 

 Many millions of tons of coal are burned in and about London during every 

 year ; and the whole mass of brick of which the city is built is heated during 

 the entire winter, and more or less in the summer, many degrees above the 

 natural temperature. There can be no question that conditions such as are 

 here indicated vitiate all observations made in or near large cities, with a 

 view to tlie determination of any possible secular variation of the tempera- 

 ture ; and that, in consequence, Mr. Glaisher's results are not to be accepted 

 as throwing any light on the problem before us. 



To sum up what has here been brought forward in regard to the determi- 

 nation of the question of a possible decrease of temperature by means of the 

 records of instrumental observations, it has to be admitted that, at the pres- 

 ent time, there is little or no liope of reaching in that way any result which 

 can be depended on, or accepted even as probable. The length of time 



* This abstract of Mr. Glaisher's results is taken from Cosmos, 2"= Serie, Tume V. p. 676, the oiiginal author- 

 ity not being at hand at the moment of the present writing. 



t The population of London has increased, since the beginning of the present century, from a little less than 

 one million to nearly five millions (included within the Metropolitan and City Police districts). During the year 

 1880-81 no fewer than 24,945 new houses were built, ami seventy miles of streets added. The increase in the 

 consumption of coal has been in a much greater ratio than that of the population. 



