[f n n 



Mr. CavaLlo fliow's in the faftid pSpeft that an angle of 'one* 

 minutre includes a- choi'd of fix ftet'at 20626 feet < diftance ; } 

 whence it follows from the nature of fimil&r' triangles, that at' 

 the diftance of 52800 feet or tea miles, the- chord of the fame 

 ahglewiffbe 15^ feet ne'arly, and '7- feet 8 inches, the length 

 of the divifions of ' the fignai line, will be feen under an angle 

 of half a minute nearly, at ten miles diftance, which may be 

 taken with great certainty, firtce the inftrument- is • capable of 

 nieiafirring a fourth part of this 'angle ; another caufe which muft 

 contribute' materially to the exaiStitude of the obfervation is,' 

 that both the telcfcope and micrometer are to remain perma- 

 nently fixed, as well in their diredlion as 'in the relative pofition^ 

 of all their pirts, and if the fcale of the micrometer be gradua- 

 ted by' the fignals of the next ftation, nothing but fome difturb- 

 ance of the machinery can occafion error, fince the* fame im- - 

 moveable mark on the fcale is always referred to the fame im- 

 moveable obje<fl. 



It is evident, from the preceding mfcthod, that an'y Ibrt of 

 ilgnals capable of communicating all the numbers under ten" 

 thoufand will enable the correfpondents to denote words and"' 

 fentences thereby, and it feems poflible to iccomplifh this by an' 

 inftrument fomething like what we conceive the telegraph to be, 

 is reprefented in the drawing, fig. 2. It is an upright pole with 

 two immoveable tranfverfe arms, CD and E F, at the extre- 

 mities- 



