TBANSACTIONS OF SECTION A. 277 



7. On an Instrument for Determining the Sensible Warmth of Air. 

 By Professor G. Forbes, F.B.S. 



8. On Synchronism, of Mean Temperature and Rainfall in the Climate of 

 London. By H. Courtenay Fox, M.B.C.S. 



My object is by tbe examination of a long series of facts to ascertain whether there 

 be any law which regulates the occurrence at the same time of extremes of tempera- 

 ture and rainfall, so far as we can ascertain it in the English climate. 



The facts which I have used are the rainfall and mean temperature as for the 

 Royal Observatory in each month and season for 66-67 years. The mean 

 temperature from 1813 to 1840 is that computed by James Glaisher, Esq., F.R.S. 

 {vide Philosophical Transactions, 1850, part 7) ; and from 1841 to the present time 

 it is from direct observation. The rainfall from 1813 to 1840 is derived from 

 sundry observations about London collated by George Dines, Esq., F.M.S., and 

 from 1841 to the present time it also is from direct observation at the Greenwich 

 Observatory. 



I have constructed tables/or each month, in which the sixty-seven (or sixty-six) 

 years are arranged in the order of the mean temperature of that month, beginning 

 with the coldest and ending with the warmest, and also arranged in like manner 

 in the order of their amount of rain. The sixty-seven years are then divided, as 

 nearly as can be, into five equal sections, of which the middle section is termed 

 average years ; the division on each side of the average I term cold and warm, 

 dry and rainy, respectively ; while the extreme sections I qualify by the word very, 

 calling them very cold, very warm, very dry, and very rainy, respectively. We 

 have thus a pretty fair division of the series of years in both these characters. 

 What I have done for each month has been also done on exactly similar principles 

 for each season and for the whole year. 



1. In the winter months, cold tends to be synchronous with dryness, warmth with 

 large rainfall. — In January so strong is this tendency that the synchronism of cold 

 with dry is without marked exception (that is, there was no instance of a very dry 

 month being also a very warm one). 



2. In the summer months, cold tends to be accompanied by much rain, warmth by 

 dryness. — The synchronism of warm with dry in July, and that of cold with wet in 

 August, are both without marked exception. 



3. To put this in popular language, rain brings warmth in winter and cold in 

 summer — that is (if rain be cause, which is by no means proven), it mitigates the 

 special character of each extreme season, winter and summer. 



4. But the peculiar laws of summer and winter are found to extend a little 

 over the adjoining months in the following manner. In November there are the 

 synchronisms, cold with dry, warm with wet ; and both October and March have a 

 slight tendency to the combination of cold with dryness, although there is in these 

 months indefinite relation between excess of rainfall and temperature. So that 

 there are six months, from October to March, of which four possess strongly the 

 winter character of cold with dry, warm with wet, and two have it to the extent 

 of slight cold with dry. On the other hand, the summer synchronism of warmth 

 with dryness obtains in April and to a small extent in May. The connection 

 between large rainfall and temperature in these months is ambiguous, but upon the 

 whole the balance is in favour of the union of cold with wet. Consequently we 

 have five months, from April to August, the last three of which possess the 

 summer character, warm with dry and cold with wet, whilst the first two exhibit 

 the same tendency in a much slighter, though still perceptible, degree. The only 

 definite tendency in September is to the synchronism of dry with warm, which so 



