VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 327 



occasionally introduced are mentioned in the observations and experiments on the 

 Georgium Sidus, as they follow, in the order of time in which they were made, 

 viz. on the light, the diameter, and the magnitude of that planet. 



And first, on Oct. 22, 1781, the planet was perfectly defined with a power 

 of 117; had a fine, bright, steady light; of the colour of Jupiter, or approach- 

 ing to the light of the moon. Next, on a great many days, in the course of 

 that and the following year, and with several variations in the apparatus and mode 

 of observing, Dr. H. deduced the several measures of the planet's apparent 

 diameter, as in the annexed table. His method of deducing the ap- 5 « g 

 parent diameter from the observation, was commonly thus: dividing 5.03 

 the breadth of the image, in inches, by its distance from the eye, / l8 

 gives the tangent of the magnified angle; then the angle answering 3. 63 

 to this tangent being divided by the number denoting the power / J, 

 of the telescope, gives the apparent diameter of the planet sought. 3. 77 

 For instance, if the breadth of the magnified image measure 2.4 inches, ^' f ' 



at the distance of 431 inches from theeye: then — ^ = .0055684, the *• ° 6 



J 431 



tangent of 19' 8"; which divided by 227, the power of the telescope, gives 

 5*.o6 for the planet's apparent diameter. 



Dr. H. then concludes: I intend to pursue these experiments still further, 

 especially in the time of the planet's opposition, and am therefore unwilling as 

 yet to draw a final conclusion from the several measures. In a subject of such 

 delicacy we cannot have too many facts to regulate our judgment. Thus much 

 however we may in general surmise, that the diameter of the Georgium Sidus 

 cannot well be much less, nor perhaps much larger, than about 4 seconds. 

 From this, if we would anticipate more exact calculations hereafter to be made, 

 we may gather that the real diameter of that planet must be between 4 and 5 

 times that of the earth: for by the calculations of M. de la Lande, contained in 

 a letter he has favoured me with, the distance of the Georgium Sidus is stated at 

 18. 913, that of the earth being 1. And if we take the latter to be seen, at the 

 sun, under an angle of 17", it would subtend no more than ."898, when removed 

 to the orbit of the Georgium Sidus. Hence we obtain — — = 4.454 ; which num- 



° .ay a 



ber expresses how often the real diameter of the Georgium Sidus exceeds 

 that of the earth. 



///. Conclusion of the Experiments and Observations concerning the Attractive 

 Powers of the Mineral Acids. By Richard Kirwan, Esq., F. R. S. p. 15. 



Having found, as exactly as I was able, (says Mr. K.), the quantity of each 

 of the mineral acids taken up at the point of saturation by alkalis and earths, 

 and also that taken up by phlogiston, when these acids are by it converted into 



