78 



greater excentricity of the orbit formerly than it is at present. 

 But even the excentricity of the orbit is a doubtful question ; 

 the only proof brought forward that the orbit is excentric is the 

 variable diameter of the sun. The atmospheric lens not only 

 refracts the rays when not in the direction of the radius, i. e. in 

 the zenith, but also augments the size of bodies, such as the sun, 

 the maximum size being in the horizon and the minimum in the 

 zenith. According to a series of observations made by the 

 author for nine years within the tropics, the sun's apparent 

 diameter is throughout the year the same when measured in the 

 zenith; therefore its observed variable diameter is an optical 

 deception, and not from any excentricity of the orbit. That this 

 is the case must be well known to those who have taken observa- 

 tions in the southern hemisphere with delicate instruments, and 

 especially when the diameter of the sun formed an important 

 amount in the measurement*. 



To return from this digression, we find that the variable nature 

 of climates from pole to pole arises principally from the obliquity 

 of the rays and the height from the sea, and not from internal heat; 



* Santa Ana, N.L. 5° 10' 0". The sun's diameter, according to direct ob- 

 servations taken in the meridian from the south tropic to the north. 



South tropic <r61° 22' 37" altitude. 31' 30" diameter. 



Equator 84 50 „ 30 45 „ 



Zenith 90 „ 30 36 „ 



North tropic 71 35 20 „ 31 12 „ 



The following measurements exhibit the apparent variation of the sun's diameter 

 in the plane of the orbit during its daily apparent path, from the rising to the 

 setting ; the latitude of observations and the sun's declination being equal. 

 Diameter. Diameter. 



The sun rising < 0° 32' 55" Noon S. in zenith <90° 30 r 36" 

 „ 15 32 35 „ 75 31 2 



30 32 18 „ 60 31 30 



„ 45 31 55 „ 45 31 50 



„ 60 31 32 „ 30 32 15 



„ 75 31 6 „ 15 32 32 



The sun setting ... 32 51 

 If it is maintained that the orbit is an ellipse, because the sun appears larger 

 at a lower altitude in December than in June at a higher elevation ; on equally 

 good data we may state that its diurnal path is an ellipse, and that our 

 approach to the sun morning and evening is compensated by the obliquity of 

 the rays and greater velocity, thus forgetting the consequence of such de- 

 ductions at those places under the sun in the opposite hemisphere. When 

 Kepler established the excentricity of the earth's orbit little was known of re- 

 fraction. 



