504 



SCIENCE. 



[N. S. Vol. II. No. 42. 



This training in numerical work should 

 be a prominent feature of the whole course 

 in astronomy, and without more than a be- 

 ginning in such work the student may pass 

 to a consideration of the different kinds of 

 time in the order, sidereal, apparent solar, 

 mean solar time, aud should learn the use of 

 the ephemeris in so far as it deals with the 

 concepts he has had occasion to employ. 

 He will learn that the various quantities 

 contained in the ephemeris are all variable 

 with the time; that their values which 

 he is to use must be interpolated from it 

 for the instant at which the observation in 

 question was made, and that this instant 

 must be expressed in Washington or Green- 

 wich time. This seems an exceedingly ele- 

 mentary matter, but it is the writer's ex- 

 perience that students are frequentlj^ per- 

 plexed bj^ it and that a little care is required 

 for its elucidation. 



The order in which the student shall take 

 up his practical problems is not a matter of 

 primary importance, but it has been found 

 convenient in practice to assign first the 

 determination of time from a single altitude, 

 or series of altitudes, of the sun, measured 

 with the sextant, showing the student how to 

 use the instrument and explaining its chief 

 sources of error without going into their 

 mathematical theory. The reduction of 

 these observations bi'ings the student to a 

 consideration of the fact that the altitude 

 which he has measured cannot immediately 

 be employed with the latitude and declina- 

 tion of the sun for the solution of the as- 

 tronotnical triangle, but must be first trans- 

 formed from an apparent into a true alti- 

 tude by correcting for the effect of refrac- 

 tion and parallax. 



The theory of the parallax may be briefly 

 given, neglecting the earth's compression, 

 but it will usually be better to give arbitra- 

 ril}' the refraction formula than to attempt 

 its derivation. The student will usually 

 have difficulty in determining which limb 



of the sun he observed, and his perplexity 

 may be used to emphasize the advantage of 

 observing both the vipper and lower limbs. 

 So also he will usuallj^ require some stimulus 

 to secure the bestowal of sufficient attention 

 upon the determination of the index correc- 

 tion. 



The next step in his progress may be a 

 rapid revision of the theory of the theodo- 

 lite or engineer's transit which he encoun- 

 tered, but usually did not master, in con- 

 nection with his course in surveying. This 

 work should include the measurement of 

 angles by repetition, the effect of a reversal 

 of the instrument in eliminating its errors, 

 the method of employing its plate and stri- 

 ding levels, and the mode of eliminating the 

 effect of graduation errors. If an instru- 

 ment with micrometer microscopes is avail- 

 able, instruction in its use may be given at 

 this point aud the instrument first employed 

 for measuring the zenith distance of a ter- 

 restrial mark. It is advantageous to throw 

 the alidade level somewhat out of adjust- 

 ment, in order to impress upon the student 

 that the reversal of the instrument elimin- 

 ates all defects of this kind from the 

 measured zenith distance. 



Although not strictly germane to a course 

 in astronomy, the subject of trigonometric 

 levelling with the effect of refraction and 

 the curvature of the earth's surface may be 

 introduced here with advantage. 



In his measurements of zenith distance 

 the student should be taught to bring the 

 level bubble somewhere near the middle of 

 its sca,le, but not be allowed to spend much 

 time in getting a nice adjustment of it, read- 

 ing the level and subsequently applying its 

 indications as a correction to the circle 

 readings. This requires a knowledge of 

 the value of a level division, and the student 

 should be required to determine this value 

 by whatever method his instructor deems 

 most convenient. 



Passing on to astronomical uses of the 



