great a proportion of vitriolic acid be left in the mixture, it may do more mischief than the iron can do good. With this caution it were perhaps much to be wished, (b). that experiments should be made, and attended to, for a long courfe of years; for without fuch continued and repeated trials, it will be impoffible to determine in what manner the application may be made with most advantage. I have only therefore to add, that what has been faid may perhaps receive fome further confirmation, from confidering that the chief constituent parts of all cements for stone are always lime and iron; and that any experiments made in confequence of these obfervations will be likely to fucceed more rapidly in warm climates than in colder ones. Whatever there is of inaccuracy in this short paper will I am perfuaded meet with indulgence both from you and from the Society, whilst whatever truth is brought to light thereby will be received and adopted. I therefore venture to deliver it into your hands without hesitation. I am, &c. (b) The ages of modern buildings are eafily known, and there are none but what have iron rails or bars about them expofed to the weather. If the rain drops from them on Portland or any gritty ftone, it becomes harder. The length of time fuch ftones have been fubjected to this procefs may easily be known, and the effects produced in them may be verified by repeated comparifons of different fragments. (i) See CROUSTEDST's Mineralogy, p. 45. VOL. LXIX. H I have I have added a few rough fketches to explain what has been faid in this letter. FIG I. Represents the mafs of concreted fand and rope from the coaft of Lothian. AB fhews where there is one impreffion of a part of the iron ring. CD fhews were there is also another impreffion of the ring; and EF fhews where there also remain fome impreffions of bits of untwisted rope, so that this whole mass feems to have been formed within the circumference of the ring. FIG. II. Represents a transverse section of the petrefaction within the wooden pipe, from the coal mine in Somersetshire; and G fhews the place of the point of the nail, and the regular curvilinear projection of the petrefaction around it, regularly throughout the whole progress. FIG. III. Is a sketch of the gun, fhewing the parts to which the incruftation adhered. V. Account of Dr. Knight's Method of making artificial Loadtones. By Mr. Benjamin Wilson, F. R. S. TO JOSEPH BANKS, ESQ. P. R. S. SIR, Read Dec. 17, 1778. TH HE method of making artificial loadftones, as it was difcovered and practiced by the late Dr. GOWIN KNIGHT, being unknown to the public; and I myself having been frequently prefent when the doctor was employed in the most material steps of that curious procefs, I thought a communication thereof would be agreeable to you and the philosophic world. The method was this: having provided himself with a large quantity of clean filings of iron, he put them into a large tub that was more than one-third filled with clean water: he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off fuch fmaller H 2 |