of the under one, as a tenon into a mortice, or a box into the lid ; these pipes ought to be

laid even with quick lime, quenched and dissolved in oil. The least level to carry and command water up hill from the descent is 100 feet, but if it be conveyed by one canal only, it may be forced to mount 240 feet. As touching the pipes by means whereof the water is to rise aloft, they ought to be of lead this is also to be observed, that the water ;

always ascends of itself at the delivery, to the height of the head whence it flowed. If it

12 PRELIMINARY ESSAY. be fetched a long distance, the work must rise and fall often, that the level may be still maintained the pipes ought to be 10 feet long ; the pipes were named from the number ;

of finger's breadths of which the sheet of lead was formed before it was turned into the

shape of a pipe, and they were also to be of different thicknesses. In every turning and winding of a hill, the pipe should be five finger's round, and no more, to repress and break the violence of the water in the current."

From this description of the mode in which the Roman aqueducts were constructed, it is obvious that the principles and precautions, which, as is sometimes supposed modern science has discovered and applied to such structures, were known and used at that early day. The declivity given to the channel was indeed greater than that usual in more modern conduits, but in other respects, few or no improvements or alterations in the manner of building and securing such works, seem to have been made.