It is not possible to give any formula for the composition of mortar which will apply generally; so much depends upon the quality of the limestone, the mode of its treatment in burning, the use of clean sharp sand and pure water. When these ingredients are judiciously selected, duly apportioned, and well beaten together, they will immediately form a paste of some tenacity, which will ultimately take bond and give a consistency to the work. From all the experiments that have been made, it seems to be essential to the composition of the best water-cements, that the limestone should contain about one-seventh of alumine or clayey matter. But as this description of limestone admits of a less proportion of sand in the mortar compounded of it, than that which is more purely calcareous, it is not so much in request for the common operations of building, as being less economical. It may, however, be stated, as a pretty general maxim, that where comparatively pure calcareous matter is met with, at least three measures of clean sharp sand, free of earthy particles, mixed with one of burned lime, in the state of powder, and a due portion of pure water, well beaten together, will form good mortar for common use. [Sidenote: Mortar of the Edystone.] From the similarity of the situations and the buildings upon the Edystone and Bell Rocks, and from Mr Smeaton’s celebrity as an engineer, his Narrative of the former work became a text-book to the writer in the erection of the latter. In considering the importance of this subject, with a view to the erection of the Bell Rock Light-house, the judicious remarks, and numerous experiments, of Dr Higgins on water cement, were carefully examined, and they will always be consulted with interest by the professional reader. The composition of the Edystone mortar consisted of equal portions, by measure, of Aberthaw lime and Pozzolano earth, both in the state of powder, mixed up with sea-water. When the writer first visited Plymouth in the year 1801, the Edystone Light-house had been erected about 42 years; and he was informed by Mr Tolsher, agent for the establishment, that the original pointing of the joints of that building had never required repair. [Sidenote: Bell Rock Mortar.]