294 



ON STJAM-ENCINES. 



Obfervations on (2.) A difk of £ of a fecorid in diameter, whether fpurloug 

 thcplanetJuno. orrea ,^ m Qrder t0 be feen as a round> we ll denned body^ 



y ' requires a difiindr, magnifying power of 5 or 6 hundred, and 

 mutt be fufficiently bright to bear that power. 



(3.) A real difk of half a fecond in diameter will become 

 fo much larger by the application of a magnifying power of 

 5 or 6 hundred, that it will be eafily diftinguiflied from an 

 equal fpurious one, the latter not being affected by power in 

 the fame proportion as, the former. 



(4.) The different effects of the infkle and outride rays 

 of a mirror, with regard to the appearance of a difk, are a 

 criterion that will (how whether it is real or fpurious, pro- 

 vided its diameter is more than £ of a fecond. 



(5.) When difks, either fpurious or real, are lefs than | of 

 a fecond in diameter, they cannot be diftinguiflied from each 

 other; becaufe the magnifying power will not be fufficient to 

 make them appear round and well defined. 



(6.) The fame kinds of experiments are applicable to tele- 

 fcopes of different forts and fizes, but will give a different 

 refult; for the quantity which has been {rated at | of a fecond 

 of a degree. This will be more when the inflrument is lefs 

 perfect, and lefs when it is more fo. It will alfo differ even 

 with the fame infirument, according to the clearnefs of the 

 air, the condition, and adjuftment of the mirrors, and the 

 practical habits of the obferver. 



^ 



Mr. Woolf *s 

 improvements 

 in fleam-en- 

 gines. 



XIX. 



Account of fame new Improvements on Steam-Engines. 

 Arthur Woolp. 



By Mt. 



I 



N our eighth volume, p. 262, we gave a fhort account of 

 a former improvement made by Mr. Woolf on the fteam- 

 engine, founded on a difcovery that fleam, of any higher 

 temperature than that of boiling water, if allowed to pafs 

 into another veffel kept at (he fame temperature as the fleam 

 itfelf, will expand to as many times its volume, and Jftili be 

 equal to the preffure of the common atmofphere, as the num- 

 ber of pounds which fuch fleam* before being allowed to ex- 

 pand, could maintain on each fquare inch of a fafety-valve 



expofed 



