270 Traxsactions of the American Lxstitute. 



made by the same means used by surveyors for detecting terrestrial 

 heights and distances, by measuring the paralhix — i. e., the ditFerence 

 of direction in which the same object appears from different points 

 of view. Mars and Venus, as being the nearest to the earth, offer 

 the greatest advantages for determining parallax, and many observa- 

 tions have therefore been made of these planets at two well deter- 

 mined points, distant from each other on the earth's surface. The 

 base line between them being well known and the difference in direc- 

 tion in which the planet appears to the observers being carefully 

 noted, it has been possible to compute the length of the other sides 

 of the triangle, which gives us the distance of the planet, and thence 

 can be deducted the distance of the sun, as we know the ratios of 

 their mean distances to that of the earth. The actual mean distance 

 of the sun, corresponding to the mos^ probable value of the paral- 

 lax is 92,380,000 miles ; and since his apparent diameter at this dis- 

 tance is 32' 3" (somewhat more than half a degree) we readily find 

 the corresponding true diameter to be of the collossal size represented 

 by 861,232 statute miles. This is nearly 109 times that of the eartli. 

 So that the earth, if seen from tlie sun, would appear but little more 

 than 1-12,000 part as large as the sun appears to us. The velocity 

 of light in stellar space, corresponding to this value of the sun's par- 

 allax, is at the rate of 185,600 miles in a second of mean time. 



To form some idea of the stupendous distances, magnitudes and 

 velocities with which we thus become acquainted, let us compare 

 them with some of those with which we are familiar. If we repre- 

 sent the eartli by an ordinary rifle bullet, the sun would be rep- 

 resented on the same scale by a globe of four and a half feet in 

 diameter at a distance of thirty rods ; but if we denote the earth by 

 an orange of average size, the sun would appear as a sphere twenty- 

 five feet in diameter, half a mile off. We have seen that light, even 

 with its inconceivable velocity, requires eight minutes, three seconds 

 to traverse this enormous distance. An electrical signal, travelling 

 with the sjieed usual upon our telegraph wires, would put a girdle 

 round the earth in about one and three-fourth seconds, yet it would 

 require one hour fifty minutes to reach the sun. A locomotive 

 engine, moving forty miles an hour, would consume 263^ years in 

 traversing the distance ; and, could we supi)o.sc an iron bar to extend 

 from the earth to the sun, a blow or pull given at either end, no mat- 

 ter how violently, could not reach the other extremity for more than 

 31:5 days, although vibrations are transmitted through iron at the 



