5S8 lECTURE XLV. 



a whole circle, depending- on this change, would also vary; the other 'that the 

 sun's motion in the ecliptic is actually variable, consequently the diurnal 

 chano-e of right ascension is liable to adouble inequahty. Hence it happens 

 that the solar time agrees at four instants in the year only with the mean 

 time, derived from supposing the whole 365 days to be divided into as many equal 

 parts; the difference is called the equation of time, and amounts sometimes to as 

 much as 16 minutes. The term equation is commonly applied in astronomy 

 to any small quantity, which is to be added to, or subtracted from, another, 

 quantity; thus it is usual, in calculating the place of a planet, to find from the 

 tables of its motion, the mean place, in which it would be found if its orbit 

 were circular, and thence to derive the true place, by means of various cor- 

 rections called equations. In France the solar time is considered as the 

 true time, and is used for all civil purposes, so that the clocks are sometimes 

 embarrassed with a complicated apparatus, calculated forimitating the inequali- 

 ties of the actual apparent motion of the sun. (Plate XXXV. Fig. 503.) 



The art of dialling consists principally in projecting, on a given surface, 

 such lines as will coincide with the shadow of an index or gnomon parallel 

 to the earth's axis, at intervals corresponding to the different hours of the 

 day: so that nothing more is necessary for the construction of a dial, than 

 to determine the intersections of the surface on which the dial is to becon- 

 structed, with planes passing through the edge of the gnomon, and situated 

 at equal angular distances from each other: thus, supposing the plane of the 

 dial perpendicular to the gnomon, and parallel to the equinoctial, the hour 

 lines of the dial will be at equal distances from each other; but in other cases 

 their distances will be unequal, and must be determined either by calcula- 

 tion or by construction. A point may also be used as a gnomon, as well as 

 aline; but in this case the hour lines must cover a larger portion of the 

 surface, in order that the shadow of the point may always fall among them. 

 (Plate XXXV. Fig. 60i . . 506.) 



The changes of the seasons depend on the return of the sun to the same 

 position Avith respect to the equator, or on the length of the tropical year, so 

 called from the tropics, which are circles supposed to be parallel to the 

 equator, and between which the sun travels continually backwards and 

 forwards, appearing to remain for some time, when he is near them, with 



