53 



serration, which, not only on account of the rarely recorded occurrence 

 of such ice in our island, but from the manner in which all the accom- 

 panying circumstances combine to throw a clear light upon the real 

 causes of the phenomenon, induces me to communicate to the Academy 

 the facts which I observed, and the conclusions to which I have been 

 led. It is important to distinguish two well-defined periods of cold 

 weather which occurred in the month of January, 1867 : the earliest 

 was continued from the first to the fifth ; the second occupied the in- 

 terval from the tenth to the nineteenth. The ice formed on ponds dur- 

 ing the longer period having been more permanent, this period was 

 popularly considered as that of the greatest frost. Thermometrical 

 results show that the lowest temperature was attained during the first 

 period. 



Observations taken within the city of Dublin, or at a station close 

 to the sea, would not furnish results from which we could draw any just 

 conclusions as to the lowest temperature to which the Dodder was 

 exposed. Observations taken at a station situated about the same dis- 

 tance from the sea, and at nearly the same height as the middle portion 

 of the course of the river, would give the nearest approximation to the 

 information required. In the " Transactions of the Association for Pro- 

 moting Social Science," for 1860, p. 662, 1 have shown that the winter 

 temperature of a large town must sensibly increase in going from the 

 outskirts towards the centre. This conclusion was first established by 

 observations made in London ; and it seems to be fully confirmed with 

 regard to Dublin by a comparison of observations, recorded at Trinity 

 College, and the Phoenix Park, and especially from those made by Mr. 

 Yates, in Grafton-street, with similar results obtained in the suburbs.** 

 The observations on the low temperature of January by Mr. Arthur Pirn, 

 at Monkstown, which he has kindly communicated to me, fully esta- 

 blish the correctness of the remark as to the influence of the sea on 

 winter temperature ; and the Monkstown results seem also to show 

 that the minima temperatures decrease very rapidly in going from the 

 sea inland. | About seven miles and a quarter measured along the course 

 of the river above the point where I observed the ground ice, the 

 level of the water in the Dodder is 474 above the sea ; and the station 

 which from its position most nearly realizes our requirements is the 

 Ordnance Meteorological Observatory at the west end of the Phoenix 

 Park. This is 1 59 feet above the sea level — a height which corresponds 

 to a part of the Dodder near Bushy Park, about one mile and three 

 quarters above the place where the ground ice was observed. It also 

 seems that the observations on minima temperatures at the Phoenix 

 Park were made under circumstances approaching more closely to the 

 actual conditions of a thermometer over an open stream, than those 

 made at other stations in Dublin and its environs. 



* I find that the same general law has been long since distinctly recognised by Dr. 

 Lamont at Munich. See his essay " Ueber die Temperatur- Vernal tnisse in Bayern," 

 " Annalen der k. Sternwarte bei Miinchen," vol. iii. 



f See " On the Distribution of Heat over Islands," " Atlantis," vol. i., p. 396. 



