*hu* by flip quantify d, which added to the time at the beginning* of thff 

 interval, gives the apparent time at Greenwich corresponding to the 

 Driven true distanrfe of the moon from the sun or star. 



Having- thus found the time at Greenwich, compute the time at the 

 place of observation from the corrected altitude of the sun or star, the 

 sun's or star's north polar distance (furnished by Tables) and the lati- 

 tude. 



The difference between this latter time and the time at Greenwich, is 

 the longitude. 



The other methods of finding- the longitude are, by an occupation of 

 a fixed star by the moon ; by a Solar eclipse j and by the passage of the 

 *noon over the meridian. 



LOOKING Glass, method of judging of. (" Coddington. ) 

 To mid the thickness of a looking glass, bring a pin or other slender 

 object into contact with the fore surface of the glass, and observe its 

 image, as shown by reflection ; then the thickness of the glass will be 



q 



equal to -ths of the apparent distance between the objects and its image. 



In a looking glass it is not only necessary that each plane should b 

 perfect, but they must be also parallel to each other. If the images of a 

 candle seen very obliquely, and under different degrees of obliquity, and 

 from all parts of the glass, do not always keep pretty nearly at equal dis- 

 tances from one another, it is a proof that the sides of the glass are neither 

 plane nor parallel. 



Another method of trying the goodness of a glass is as- follows : Stick 

 a pin or slender wire in the bar of a window sash, so that the pin may 

 be nearly horizontal, and in the plane of the window. Then hold the 

 looking-glass, and turn it about so as to see the image of the pin very ob- 

 liquely and from all parts of the glass. In this case two images will be 

 visible ; and if these images keep always straight, parallel, and at regu- 

 lar distances one from another, the glass may be considered as being well 

 figured. These phenomena will be more conspicuous if two pins be 

 stuck parallel to one another, and at a small distance asunder. 



With respect to the polish of a glass ; we may observe, caeteris pari- 

 bus, that the darker the colour of the glass of the speculum is, the better 

 generally is the polish. 



For the theory of plane mirrorswe Reflection. 



LUNAR inequalities.' See Moon, 



ITS 



