TOL. XLVIII.J PHILOSOPHICAL TRANSACTIONS. 35g 



the Rev. F. Pezenas, professor of hydrography to the French king at Marseilles, 

 in which he informed him, that M. Bouguer had read, before the Royal Aca- 

 demy of Sciences at Paris, in the year 1 748, a memoir, in which he describes 

 an heliometcr; which is an instrument consisting of 2 objective glasses, for 

 measuring the diameters of the planets. He said also, that this memoir was ac- 

 tuall\ in the hands of M. de Fouchy, perpetual secretary of the Academy, or at 

 the Ro) al Printing-house ; and that it was registered in the minutes of the Aca- 

 demy for the year 1748. 



Immediately after reading this letter, Mr. S. recollected to have heard a paper 

 on the same subject, from the late Servington Savery, of Exeter, Esq. read be- 

 fore the Royal Society, about the year 1743. He therefore had recourse to the 

 minute book, of the Society for that year, where he found the following minute, 

 which he copied in the presence of Lord Charles Cavendish, then vice-president. 



" A paper communicated from Mr. Savery at Exon, containing a new method 

 for measuring the difference between the apogeal and perigeal diameters of the 

 sun was shown ; and thanks being ordered. Dr. Bradley was desir^ to oblige the 

 Society with an account of its contents. — Oct. 27, 1743." 



On application to Dr. Bradley, he sent the original paper to Mr. Short; on 

 the back of which was a memorandum in the hand-writing of the late president, 

 Martin Folkes, Esq. as a further proof of its authenticity, which runs in these 

 words, 



" Delivered to me by Mr. Graham, sealed up by the author, and then broke 

 open in his presence 5 26th Oct. 1743. M. Folkes." 



Mr. Savery's original paper was as follows: 



A new Way of Measuring the Difference between the Apparent Diametei- of the 

 Sun at the Times of the Earth's Perihelion and Aphelion, or when the Sun is 

 nearer to or farther from the Earth, tvith a Micrometer placed in a Telescope 

 Invented for that Purpose; though the Charge or Magnifying Power of the 

 Telescope is so great, that the whole Suns Diameter does not appear in it at 

 one Fiew. By Servington Savery, of Exeter, Esq. Read Oct. 27, 1743. 

 p. 167. 



Though this may appear impossible, yet Mr. S. has contrived some dioptric 

 telescopes, and a reflecting one; either of which, by representing the object 

 double, will, if well made, answer the design. 



Fig. 1, pi. 10, represents the whole body of the sun, as it ap{)ears double, 

 and magnified in the telescope. Let an be the diameter of the one, and rx of 

 the other image of the sun in perig£BO; so shall nr be the distance between the 

 two images at that time; which measured with the micrometer is equal to, sup- 

 pose, 10 seconds. Let bm be the diameter of the one solar image, and sw of 



