Your favor of the 3d is duly recieved, & I freely offer you what occurs to me respecting the inventions therein mentioned. that of the fireproof cieling is entirely new, as far as I know, and will in many situations, I think be useful. I take for granted that the triangles H. K. F. in the drawing (which I return you) are either of the solid of the joist A. or are strips of hard wood firmly planted on it, as a shoulder for the arch. I will state to you three kinds of fireproof cielings which I have known. the first is by planting slips of lath on the inside of every joist at bottom1 laying short planks closely on these and filling the interval between them & the floor with brick & mortar. this is usual to the Northward under the name of countersealing, and is used in my own house, and has once saved my house by the floor resisting the taking fire from a coat of sawdust spread over it, and burning on it a whole night. the 2d was that of David Hartley the British minister. he laid, on his sleepers and joists, floors of sheet iron, and a floor of planks close on them, nailing them thro’ to the joists by holes punched thro’ the iron. he built a room in the fields near London, & in the presence of the royal family and a great concourse of spectators made a large fire on the lower floor of the room, which burnt out without communicating the fire to the naked planks below or cieling above. I had this relation from himself. he had a patent for it, and I believe it has been practised in London. the 3d method I saw in use in Holland. it was nothing but an arch of common brick turned between the joists. your method has the advantage of covering the underside of the joists, and saving the expence of lathing to plaister on.

With the construction of filtering machines I am but little acquainted. I know that the best drinking water commonly used in Paris is filtered through large cisterns of sand, being recieved at the bottom of the sand. I made, about 2 years ago, 4 cisterns of brick of 8 feet cube, each2 & sunk in the earth; and always intended, in order to have potable water, to sink a well 2. or 3.f. from one of them, of 2.f. diametter and of the depth of the cistern. to fill the well to a certain height with sand, and have a pipe of communication at the bottom, expecting that the water rising thro’ the sand in the well3 would be sufficiently purified. I have kept my cisterns empty thus long to harden, & am only now preparing to let water into them. your plan unites the Parisian with mine by the descending as well as ascending filtration. as to the refrigerating power of the machine you propose, you would do well to ascertain by actual experiment whether any substance which is not itself cooler than the water will refrigerate it. it is believed that charcoal is a better purifier than sand, but I do not know that it has been found a better cooler. my ideas however on this question are not the result of experiment. it would be easy to take a box of sand, & another of charcoal, of the same temperature, to pour water of the same temperature into both, and try with the thermometer whether either & which would become cooler than the other. I hazard these loose thoughts at your request, and pray you to recieve them with indulgence, and with my best wishes for the success of your inventions.

Th: Jefferson