54 PHILOSOPHICAL TRANSACTIONS. [aNNO l66d, 



be distinctly seen, and serve for measuring small distances to minutes and 

 seconds ; let each of them, thus furnished, observe the visible way of the moon 

 among the fixed stars, by taking her exact distance from any fixed star that lies 

 in or very near her way, with the exact time of such appearing, and also the 

 apparent diameter of her disk : continuing these observations every time for two 

 or three hours, so that two exact observations of her apparent place among 

 the fixed stars being made, at two places distant in latitude, and nearly under 

 the same meridian, hence her true and exact distance may be collected, not 

 only for that time, but at all other times, by any single observer viewing her 

 with a telescope, and measuring exactly her apparent diameter. 



It is also desirable, when there happens any considerable eclipse of the sun, 

 that they would observe exactly the measure of the greatest obscuration, com- 

 pared with the apparent diameter of his disk. For by this means, after the 

 distance of the moon has been exactly found, the distance of the sun will easily 

 be deduced. 



The fittest time for making observations on the moon will be when she is 

 about a quarter or somewhat less illuminated, which is about four or five days 

 before or after her change ; because then her light is not so bright but that 

 with a good telescope she may be observed to pass close by, and sometimes over 

 several fixed stars. Or else, at any other time when the moon passes near or 

 over some of the larger fixed stars, which may be easily calculated and foreseen. 

 Or, best of all, when there is any total eclipse of the moon ; for then the smallest 

 telescopic stars may be seen close to her body. 



j4n Observation on Saturn. iV"" 9> i^. 152. 



This observation was made by Mr. William Ball, accompanied by his brother. 

 Dr. Ball, October 13, l665, at six o'clock, at Mainhead near Exeter in Devon- 

 shire, with a very good telescope near 38 feet long, and a double eye-glass, as 

 the observer himself takes notice, adding, that he never saw that planet more 

 distinct. This observation has induced the supposition that Saturn is surround- 

 ed, not by one circular body or ring only, but by two. And the further observa- 

 tion of this appearance is earnestly recommended to the author (Huygens) of the 

 System of Saturn. 



Of Barometers, and some Observations made with them. By Dr, Beal. 



N" 9, p' 153. 



The barometer is an instrument for measuring the weight of the atmo- 

 sphere, in order to determine the changes of the weather. It is founded on 

 the Torricellian experiment, so called from its inventor Torricelli ; and is only 



