XXI. Tables of temperatures of the sea at different depths beneath the surface, reduced and collated from the various observations made between the years 1749 and 1868, discussed. With map and sections
This communication, the result of an inquiry having originally reference to the bearing of the subject on certain geological questions, was commenced more than twenty years ago, but abandoned for a time, partly owing to the pressure of other engagements, and partly waiting more accurate information of the range of life at depths. The great impulse given to these questions by the more recent expeditions of the 'Lightning’ and 'Porcupine,’ culminating in that of the ‘Challenger,’ has not only again directed attention to the subject of deep-sea temperatures, but has led to such improved methods of observation, that it may now seem late to bring forward the less accurate experiments of former observers. It might therefore seem almost a work of supererogation, now that the subject in connexion with these later voyages has been so ably and zealously taken up by my friend Dr. Carpenter, to introduce these more variable older elements into the discussion. Still the older observations, though restricted to comparatively limited depths, have a wide range; and in the case of the Arctic voyages they were obtained under conditions of so much difficulty and danger, that it may be long before similar experiments are repeated; while many of the original opinions evidently deserve great consideration. It was, moreover, always my intention to complete the task I had begun when time and opportunity offered; and as Dr. Carpenter’s work commences with the observations made by him on board the ‘Lightning’ in 1868, it may not be out of place to have a record of all that was done in temperature-soundings up to that time, even as supplementary to the more exact work of later voyages. I may also notice that, notwithstanding the superiority of the more recent observations and the inaccuracy of many of the older ones, there are a certain number of the latter which were made with great care, and which may vie with recent experiments in exactness; while with respect to the others, the errors are such as may in most cases be computed and allowed for; or merely taking the old observations as they are, the comparative temperatures recorded at corresponding depths with the same or similar instruments have their own special value. The older observations are also so scattered through various narratives of voyages and in scientific periodicals, that no one can, without much difficulty of search, form an idea of their number and interest, or of the progress which the subject had made at the hands of the eminent men who had from time to time engaged in the inquiry on the Continent. I purpose, therefore, to show the state of the question at the time of the ‘Lightning’ expedition. For all that has been done since, I would refer to the exhaustive papers of Dr. Carpenter).