358 THE niYSICAL CONDITION OF THE OCEAN. 



lias been shown to be lollowed by a dift'erence of a foot in the mean 

 level of the se;i, and in ])arts of the world wlicie the mean height of the 

 barometer varies mncli with t?lie seasons and the tidal range is small, 

 this effect is very marked. 



Of any seenlar changti in the level of tlie sea little is known. This 

 can oidy be measnred by com[)arison with the land, and it is a qnestion 

 ■which is the more unstable, the land or the water — i)robably the land, 

 as it has been shown that the mass of the Uxnd is so trilling, compared 

 with that of the ocean, thjit it wonld take a great deal to alter tlie gen- 

 eral mean level of tlie latter. 



All the points connected with the sea that I have had the honor of 

 bringing before you form part of the daily observation of the marine 

 surveyor when he has the chance, but I can not refrain from also men- 

 tioning other duties, which are indeed in the present state of our knowl- 

 edge and of the practical requirements of navigation, the principal 

 points to which he has to pay attention, as it may explain why our 

 knowledge on so many interesting details still remains very imperfect. 



Working as we do in the interests of the vast marine of (h-eat 

 Britain, the paramount necessity of good navigational charts requires 

 that the production of such charts should be our })rincipal aim. It is 

 difticult for a landsman and difficult even for a sailor who has never 

 done such work to realize the time that is necessary to make a really 

 complete marine survey. The most important part, the ascertainment 

 of the depth, is done, so to speak, in the dark — that is to say, it is by 

 touch and not by sight that we have to find the different elevations 

 and depressions of the bottom of the sea. In making a map of the 

 land an isolated rock or hill stands up like a beacon above the surround- 

 ing laud, and is at once localized and marked, but a similar object under 

 the sea can only be found by patient and long-continued sounding, and 

 may very easily be missed. 



When it is considered that marine surveying has only been seriously 

 undertaken for about one hundred years, with a very limited number of 

 vessels, we shall, T think, understand how in the vast area of the waters, 

 taking only those bordering the shores, many unsuspected dangers 

 are yearly discovered. Very, very few coasts have been minutely sur- 

 veyed, and, setting aside for a moment the great changes that take 

 place off shores where sandbanks prevail, I should be sorry to say 

 that even on our own coasts charts are perfect. Yearly around Great 

 Britain previously unknown rocks come to light, and if this is the case 

 at home, what are we to think of the condition of charts of less-known 

 localities ! Our main efforts therefore are directed to the improvement of 

 charts for safe navigation, and the time that can be spared to the eluci- 

 dation of purely scientific i)roblems is limited. jSTeverthcless, the daily 

 work of the surveyor is so intimately connected with these scientific 

 problems that year by year, slowly but surely, we add to the accumula- 

 tiou of our knowledge of the sea. 



