388 FERGUSON'S LLC1UKES. 



LECT. be placed true, whenever the sun shines, without the 

 ' ^ help of any other instrument. 



Let d (see engraving, p. 376) be the sun's place in 

 the stereographic projection, x dy z the parallel of the 

 sun's declination, Z d a vertical circle through the sun's 

 center, P d the hour-circle ; and it is evident, that the 

 diameter N S of this projection being placed duly north 

 and south, these three circles will pass through the 

 point d. And therefore, to give the dial its due position, 

 we have only to turn its gnomon toward the sun, on a 

 horizontal plane, until the hour on the common gnomonic 

 projection coincides with that marked by the hour-circle 

 P d } which passes through the intersection of the shadow 

 Zd with the circle of the sun's present declination. 



The Babylonian and Italian dials reckon the hours, 

 not from the meridian, as with us, but from the sun's 

 rising and setting. Thus, (plate 8) in Italy, an hour 

 before suu-sel is reckoned the 23d hour ; two hours 

 before sun-set, the 22d hour ; and so of the rest. And 

 the shadow that marks them on the hour-lines, is that 

 of the point of a stile. This occasions a perpetual varia- 

 tion between their dials and clocks, which they must 

 correct from time to time, before it arises to any sensi- 

 ble quantity, by setting their clocks so much faster or 

 slower. And in Italy, they begin their day, and regu- 

 late their clocks, not from sun-set, but from about mid- 

 twilight, when the Ave Maria is said ; which corrects 

 the difference that vrould otherwise be between the clock 

 and the dial. 



The improvements which have been made in all sorts 

 of instruments and machines for measuring time, have 

 rendered such dials of little account. Yet, as the theory 

 of them is ingenious, and they are really, in some res- 

 pects, the best contrived of any for vulgar use, a general 

 idea of their description may not be unacceptable. 



