68 APPENDIX TO REPORT ON OYSTER CULTURE 



APPENDIX E. 



Report on the Temperature of the Surface of the Sea on the 

 Coasts of Great Britain and Ireland, and on the West Coast 

 of France, by Henry Hennessy, f.r.s., Vice-President of the 

 Royal Irish Academy. 



I have examined the records of the observations on the temperature 

 of the surface of the sea in deep and shallow water surrounding the 

 British Islands, and off the west coast of France, made for the purpose 

 of elucidating the physical conditions of the sea with reference to oyster 

 culture, and I beg to submit to the Royal Commissioners the following 

 report, which embodies the results I have obtained. These observa- 

 tions were made partly during the last three weeks of October, 1868, 

 and partly during the last three weeks of May, and the months of June 

 and July, 1869. Those taken on the British and Irish coasts were made 

 by the coast-guards, and those on the French coast by officers and men 

 attached to the department of fisheries, observers from whose character 

 and habits accuracy might be fairly expected. 



In order to draw correct conclusions as to the comparative conditions 

 of temperature in different localities, we must be assured that the obser- 

 vations have been made at the same hours of local time, with similar 

 precautions, and with instruments accurately corresponding in their con- 

 struction. I have reason to believe that the two latter conditions were 

 attended to, though I have no means of forming an independent judg- 

 ment on the question, as I was not consulted when the observations were 

 set on foot. 



The hour of observation seems to have been uniformly about noon. 

 Why this hour should have been selected I cannot understand, unless 

 because it is the hour of maximum sunshine though not of maximum 

 temperature for either air or water.* For determining the mean tem- 

 perature of the sea observations made between 9 and 10, a.m., and be- 

 tween 9 and 10, p.m., would be more efficient, and maximum and mini- 

 mum thermometers would have bee a the most suitable for ascertaining 

 the highest and lowest temperatures. This is of less consequence, how- 

 ever, regarding the sea than the air, as the diurnal range of water tem- 

 perature is small, on account of its peculiar thermal properties*, whereby 

 it slowly acquires and parts with heat. From the same causes the time 

 of greatest water temperature occurs later both in the day and the year 

 than the time of maximum air temperature. 



As the hour of the day at which' most of the observations were made 

 during the last three weeks of October, 1868, is not distinctly mentioned, 

 and as in some cases it seems to have greatly varied, I have been able to 

 make use of the records of only a few stations in connexion with the 

 general subject of the distribution of heat at the surface of the sea. The 

 observations made during the summer months being directly connected 

 with the important question of the conditions of temperature under which 



* The weight of the results of the British and Irish observations is less than that of the 

 French on account of the suspension of the former on Sundays, a circumstance which has 

 possibly arisen from the coincidence of the hour of observation with that of public worship 

 in England, and this furnishes an additional objection to the selection of noon whenever 

 only one observation during the day can be made. 



