MEASURING SCREW. 24-7 



bafes liad no afllon, were produced. For the analjiis I 



ufed lime water and nitrous gas, prepared with great care. 



Near Santa Fee, at an elevation of 2890 yards, are found Large foffil 



an immenfe number of foffil bones of elephants, both of the derab'le\ei<'ht '" 



African fpecies and of the carnivorous kind, difcovered ip and nearly from 



North America. I have fince received others from a part of^"^^'';'^"^'^ °^ 



•^ America to the 



the Andes, about the latitude of 2"^ from Quito, and from othero 

 Chili : fo that thefe animals muft have exifted from Patagonia 

 to the Ohio. 



VIII. 



jMcfkod of mecifuring any Aliquot Part of an Inch hy a Screiv, 

 itfhich gives no fuch Part in its Turn; and Obfsrvation on an 

 Error of Ediuurds in placing the Eye Stop of refied:ing Tele-r 

 fcopes. In a Letter from Mr. J. C . Ho r. n e lower . 



To Mr, NICHOLSON. 

 Dear Sir, 



.OEING on a vifit about fifteen years ago in the confines of Gonilruaion of 

 the principality, (a region fcieace never yet explored) I was in {auarc. 

 want of a reticulated fquare to be placed in the focus of the 

 eye glafs of an optical inftrument, and by the difappointuient 

 ufually attending jobs of this kind when done at a diftance, I 

 refolved on accomplifliing it myfelf, though I forefaw it would 

 be a difficult undertaking, efpecially as I am but an indifferent 

 •workman even with the befl tools, and with fuch as lay before 

 me, I could not anticipate much pleafure in my tafk. 



The firfl: thing which occurred to me was, the conflru6lion A fcrew was 

 of the old fafliiqned micrometer, and I found a tap which pro-"?^^^^y ^P' 

 mifed well for the purpofe; but unfortunately it had no deter- 

 minate number of threads; but neverthelefs I was refolved to 

 proceed, and by this tap I cut a pair of dies, and by them I 

 cut a fcrew on a piece of large brafs wire, which being more 

 homogenous than fleel in general, had a better chance of the 

 accuracy my inflruraent required, however I fet the nut in a 

 jimbal, and on trying the fcrew, I found it had 26,6 threads in 

 an inch, and as I wanted tenths of an inch, I muft neceffarily 

 provide n>eans to correct the excefs or deieO: in the fcrew, 

 which I did thus: 



