SUN-DIALS. 



passing through the style and the sun would always be carried 

 raund the style with an uniform motion by the diurnal motion of 

 the sun, and that at all seasons this plane would at the same hours 

 have the same position. 



It is for this reason that the gnomon of sun-dials is placed at 

 such an inclination with the plate of the dial, that when the dial 

 is properly set the gnomon will be directed to the north pole of 

 the heavens, and being so placed, its shadow will fall upon the 

 same lines of the dial at the same hours, whatever be the season 

 of the year. 



5. It is evident, therefore, that dials must be differently con- 

 structed for places which have different latitudes. We have shown 

 in a former Tract * that the elevation of the celestial pole is equal 

 to the latitude of the place, and consequently the inclination of 

 the gnomon of a sun-dial must be also equal to the latitude of 

 the place where the dial is intended to be set. It follows, there- 

 fore, that a dial constructed for London would not be suitable for 

 York, Newcastle, or Edinburgh. 



The position of the plate of the dial upon which the shadow of 

 the gnomon is projected is quite unimportant. All that is really 

 important is the direction of the gnomon, which must always be 

 that of the celestial pole, whatever be the position of the plate of 

 the dial. Thus the plate of the dial may be either horizontal, 

 vertical, or oblique. Its position will depend upon the place 

 where it is to be erected. If it be in an open space, as in a garden 

 or field, having a clear exposure on all sides, it will be generally 

 most convenient to make it horizontal ; and, hence, in such cases, 

 it is usual to fix it upon the top of a column of three or four feet 

 high, so that it may be easily observed by a person of ordinary 

 height standing near it. Sometimes it is convenient to place it 

 upon the wall of a building, such as a church. A wall with a 

 southern exposure is in that case the most convenient; but to 

 indicate the hours of the early morning in the spring and 

 summer, an eastern exposure would be required, and to in- 

 dicate those of the late evening a western exposure would be 

 necessary. 



Where these vertical dials are erected, it is therefore frequently 

 the practice to establish them at the same time on different walls 

 of the same building. 



Whatever be the position of the plate of the dial, the position 

 of the hour-lines upon it is a matter of mere technical calculation, 

 for which the formula and principles of spherical trigonometry are 

 necessary, but which is not attended with any difficulty. 



* Vol. i. page 102. 



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