My opinion on the construction I need not say ought not to be quoted: but you are quite welcome to found any general statement on it; or perhaps it may guide you in further enquiries. To make it clear, I must speak rather generally upon the subject. There are three ways in which a breakwater may be constructed. 1. By building a strong wall with perpendicular face from the bottom of the sea. 2. By making a bank with nothing but slopes towards the sea. 3. By making a sloping bank to a certain height and then building a perpendicular wall upon it.--Now if the 1st of these constructions could be arranged, I have no doubt that it would be the best of all, because a sea does not _break_ against a perpendicular face, but recoils in an unbroken swell, merely making a slow quiet push at the wall, and not making a violent impact. But practically it is nearly impossible. The 2nd construction makes the sea to break tremendously, but if the sloping surface be made of square stone put together with reasonable care there is not the smallest tendency to unseat these stones. This is the principle of construction of Plymouth Breakwater. In the 3rd construction, the slope makes the sea to break tremendously, and then it strikes the perpendicular face with the force of a battering ram: and therefore in my opinion this is the worst construction of all. A few face-stones may easily be dislodged, and then the sea entering with this enormous force will speedily destroy the whole. This is the form of the Cherbourg Digue. From this you will gather that I have a full belief that Plymouth Breakwater will last very long, and that the Digue of Cherbourg, at least its upper wall, will not last long. The great bank will last a good while, gradually suffering degradation, but still protecting the Road pretty well. I was assured by the officers residing on the Digue that the sea which on breaking is thrown vertically upwards and then falls down upon the pavement does sometimes push the stones about which are lying there and which weigh three or four tons. I saw some preparations for the foundations of the fort at the eastern extremity of the Digue. One artificial stone of concrete measured 12'9" × 6'7" × 5'7", and was estimated to weigh 25000 kilogrammes. CHAPTER VI. AT GREENWICH OBSERVATORY--1846 TO 1856. 1846