VOL. IV.] PHILOSOPHICAL TRANSACTIONS. . 377 



slated houses in or about London. For the second, the most experienced 

 helliers or slaters affirm that some sorts have continued on houses several liun- 

 dred years, and are yet as firm as when first put on. And as to the third, the 

 computation of charges may give some idea of the comparative expenses, being 

 little more than tiling, and much less than leading. To know the goodness ol' 

 any slate, take and strike it agamst any hard body ; then if the sound be distinct 

 and clear, that sort of stone is firm and good. Or, if in hewing it does not 

 break before the edge of the sects, the slater's hewing instrument, you may not 

 doubt of the firmness of the slate. But if, after it has been exactly weighed, it 

 be put for 2, 4, or 8 hours under water in a vessel, and afterwards taken up and 

 wiped very clean with cloths, if then it weigh more than before, it is of that 

 kind which imbibes water, and therefore not so fit to endure any considerable 

 time without rotting the laths and timber. These stones may be pretty well 

 guessed at, whether they be of a close or loose texture by their colour : For the 

 deep blackish blue is apt to take in water ; but the lighter blue is always the 

 firmest and closest. To which may be added the touch; for a good stone feels 

 somewhat hard and rough ; whereas an open stone feels v'ery smooth, and as it 

 were oily. Another way is to place the stone longways perpendicular in the 

 midst of a vessel of water, no matter how shallow the water be, so it exceed 

 half a foot in depth, and so let it remain a day, or half a day, or less. If it be 

 a good firm stone, it will not draw water above half an inch above the level of 

 the water, and that perhaps but at the edges only, the parts of which might be 

 somewhat loosened in the hewing. But a bad stone will draw water up to the 

 very top, be the stone as long as it will, all over. 



It is worthy of observation, that if a side wall happen to take wet by the beat- 

 ing of the weather, or the like, when nothing else will cure it, our kersing with 

 slate will quickly remedy it. 



Some Observations concerning the odd Turn of some Shell Snails, and 



the darting of Spiders. By an ingenious Cantabrigiaii. In a 



Letter to Mr. J. Ray. N" 50, p. 1011. 



I have found two sorts of shell-snails, easily to be distinguished from each 

 other, and from all besides; because the turn of the wreaths is from the right 

 hand to the left, contrary to what may be seen in common snails. They are 

 very small, and might therefore well escape so long the more curious naturalists; 

 neither of them much exceeding, at least in thickness, a large oaten corn. 



The first I thus describe: the open of the shell is pretty round, the second 

 turn or wreath is very large, and the rest of the wreaths, about the number of 

 six, are still lessened to a point. This turhen or conical figure is near a quarter 

 of ah inch, the colour of the shell is duskish, yet when the animal is shrunk 



VOL. I. 3 B 



