Manchester Memoirs, Vol. lix. (19 15), No. 12. -$j 



Sheets 66 and 67. 



Relate to the cause of the long-continued and irregular 

 sound of thunder. About the year 1808 or 1809 (see 

 " Meteorological Essays," 2nd Edition, Appendix, p. 202), 

 it occurred to Dalton that if he could assume an electric 

 discharge to be made instantly from one cloud to another, 

 the distance apart of the clouds being say 12-14 miles, 

 then the sound will be first heard from the nearest point, 

 then from equidistant points and then from the furthest 

 points. He gave this explanation at the Royal Institution 

 lectures in 18 10. No. 66 is a small (20" X 17") diagram 

 that has been carefully made and painted. A cottage 

 with a lightning conductor, a tree and a man are depicted. 

 The suggestion has been made that this was drawn and 

 coloured for Dalton by some artist friend. No. 67 is a 

 much larger diagram (43" X 27"), and is a rough copy of 

 66. The explanation of the diagram is given on p. 203 

 of the "Essays." The clouds between which the lightning 

 passes are supposed to be 14 miles apart. On the last 

 page (p. 244) Dalton says that he has been informed that 

 the above explanation was previously given by Boscovich. 

 The name should have been Beccaria. 13 



Sheets 68 and 69. 



The first of these (i6"x2o") shows the gradual 

 diminution of the density of the atmosphere as the height 

 increases by the aid of a distribution of black dots. 

 These are close together at the bottom of the diagram 

 and the number in a given space is made to decrease 

 gradually. In No. 69 (i9"x 2f) the decrease of density 

 is shown by horizontal lines, these are close together 

 at the bottom of the diagram and are drawn further 



1:1 Lettere dell* elettricismo artificiale e naturale. Turin, 1753. Sec 

 " History of Kiectricity," by Joseph Priestley, 4'h Edition, 1775. 



