916 



PHILOSOPHICAL THANSACTIONS. 



[anno 1771. 



adjusted in the meridian, to a mark 700 feet distant. Mr. W. mentions these 

 particulars, because the observations show that even so small an instrument is 

 capable of tolerable exactness : and it is for that reason he set down the result of 

 all the transits he had taken for a year past; though much fewer would have suf- 

 ficed for showing the rate of the clock. 



In the middle of February, when the first change was, the frost was intense; 

 and the pendulum did not, for some days, throw out so far by about 7' as it 

 generally did; which was about 1° 37' on one side, and 1° 40' on the other. At 

 the change in August, Mr. W. observed no difference. It appears by these 

 trials as if the clock gained in warm and lost in cooler weather: but this is not 

 clear. It began to gain before the weather grew warm. Whether this be 

 owing to damp, or any other causes, longer experience and abler observers may 

 discover. From the observations made by Mr. W., it appears that the rate of 

 the clock was as follows : 



Mr. W. here adds a few other observations made since he settled in Chisle- 

 hurst, the lat. of which is 51° 24' 33" north, and the longitude is 18'.5 in time, 

 east of the observatory at Greenwich. These additions consist of observations of 

 the occultations of stars by the moon, and of the eclipses of Jupiter's satellites, 

 not necessary to be here repeated, 



LIL Of a Pure Native Crystallized Natron, or Fossil jilkaline Salt, found in 

 the Country of Tripoli in Barbary. By Donald Monro, M.D., F.R.S., (sfc. 

 p. 567. 

 It is well known that the nitre, or nitron, of the ancients, which they used 



for making of glass, * and in their baths,-f- and for other purposes, was not the 



• See an account of the making of glass with nitre and sand in C. Piinii Secundi Hist. Natural. 

 toin. in. lib. xxxvi. cap. 26, and an account of it« medicinal virtues, ibid. lib. xxxi. cap. 10. And 



