o70 LECTURE XLVl. 



produce considerable errors in the inferences which might be deduced from 

 these measurements. For example, a degree measured at the Cape of Good 

 Hope, in latitude 33" south, was found to be longer than a degree in France, 

 in latitude 46^ north, and the measurements in Austria, in Nortli America, 

 and in Eugland, have all exhibited signs of similar irregularities. There ap- 

 pears also to be some difference in the length of degrees under the same la- 

 titude, and in different longitudes. We maj-, however, imagine a regular 

 elliptic spheroid to coincide very neariy with any small portion of the earth's 

 surface, although its form must be somevvhat different for different parts : 

 thus, for tiie greater part of Europe, that is, for England, France, Italy, 

 and Austria, if the measurements have been correct, this osculating spheroid 

 must have an ellipticity of txs-- 



The earth is astronomically divided into zones, and into climates. The 

 torrid zone is limited by the tropics, at the distance of 23° 28' on each side 

 of the equator, containing all such places as have the sun sometimes vertical, 

 or immediately over them; the frigid zones are within the polar circles, at 

 the same distance from the poles, including all places which remain annually 

 within the limit of light and darkness, for a whole diurnal rotation of the 

 earth, or longer; the temperate zones, between these, have an uninterrupted 

 alternation of day and night, but are never subjected to the sun's vertical 

 rays. At the equator, therefore, the sun is vertical at the equinoxes, his 

 least meridian altitude is at the solstices, when it is 66° 32', that is, more 

 than with us at midsummer, and this happens once on the north and once 

 on the south side of the hemisphere. Between the equator and the tropics, 

 he is vertical twice in the year, when his declination is equal to the latitude 

 qf the place, and his least meridian altitudes, which are unequal between 

 themselves, are at the solstices. At the tropics, the meridian sun is vertical 

 once only in the year, and at the opposite solstice, or the time of midwinter, 

 his meridian altitude is 43° 4', as with us in April, and the beginning of Sep- 

 tember. At the polar circles, the sun describes on midsummer day a com- 

 plete circle, touching the north or south point of the horizon ; and in mid- 

 winter he shows only half his disc above it for a few minutes in the opposite 

 point; that is, neglecting the elevation produced by refraction, which, in 

 these climates especially, is by no means inconsiderable. At either pole, 

 the corresponding pole of the heaven being vertical, the sun must annually 



