660 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 7(52. 



NV the difference of the semichords rv and rv, cv and cv = the difference of the 

 semidiaineters of the sun and Venus. 



In the right-angled triangles crv and crv, two sides are given, therefore the 

 other sides rv and rv may be found; the difference of these two sides nv being 

 reduced to time, by the horary motion of Venus in her path, will give the time 

 answering to the parallax of latitude. The parallax of longitude being added to 

 or subtracted from the parallax of latitude, as the case requires, will give the re- 

 tardation or acceleration of the contact of the place of observation, after or before 

 the contact as seen from the centre of the earth. 



In all the above calculations, I have considered the place of Venus, with 

 respect to the centre of the sun, both in right ascension and declination ; and 

 I have neglected the fractions of seconds in the results of the parallaxes of longi- 

 tude and latitude, having always taken the second that was nearest to the frac- 

 tional part. 



I take this opportunity of acquainting the Royal Society, that I have, by 

 means of an achromic object-glass micrometer of 40 feet focus, adapted to a re- 

 flecting telescope of two feet focal length, measured the least and greatest dia- 

 meters of the sun, and I find the apogeal diameter = 31' 28', and the perigeal 

 diameter = 32' 33^'. 



CI. Some Suggestions concerning the Preventing the Mischiefs which happen to 

 Ships and their Masts by Lightning. By !Vm. fVatson, M.D., F.R.S. p. 629. 



These observations were drawn up in consequence of what had happened to 

 the Harriot packet, in her passage to New York. * This vessel was struck with 

 lightning, which split the mainmast, main top mast, and main top gallant mast 

 in pieces, ripped up the partners of the main mast, broke down the bulk head 

 between the steerage and the hold, tore off the locks from the cabin doors, burnt 

 the tarpaulin off the main hatches, made several holes between the coomings of 

 the hatches and the deck, rendered all the compasses useless, broke one of the 

 beams between decks, stove the boat, wounded one of the men very much, and 

 the rest were stunned for some time. Most of the rigging was burnt off the 

 mast head. The whole caused such a smoke in the ship, that taking her to be 

 on fire below, they threw water a considerable time into the cabin; but provi- 

 dentially no other damage was done.' 



A few years since a ship belonging to Capt. John Waddel, was almost beaten 

 to pieces by thunder and lightning, of which a particular account is published in 

 the Phil. Trans.* And very lately the main mast of the Bellona, a 74 gun ship, 

 was split in pieces by the lightning, which happened in Jan. J 762. What hap- 



• VoL xlvi. p. 1 1 1. — Orig. 



\ 'i i 



