Celestial Photography. 135 



of the second is to allow the first to remain undisturbed when 

 its position gives an accurate rate, and the cylinder has "become 

 empty. 



The great impediment to the production of uniform move- 

 ment by such an apparatus is, of course, that as the amount of 

 superincumbent pressure determines the rapidity of the escape 

 of the water, and the consequent descent of the weight, this 

 force is continually decreasing with the diminishing quantity 

 of water in the cylinder. In Dr. Draper's clepsydra, this 

 defect is remedied in the following manner. The plug of the 

 principal stop-cock is not perforated by a round hole as usual, 

 but by a slit, which reduces or augments the flow of water 

 through it in a more uniform ratio. To the handle of the stop- 

 cock is attached a strong rod in a vertical direction, reaching 

 up the outside of the cylinder, and pressed by a spring against 

 an excentric attached to the side of the cylinder, so that the 

 turning round of this excentric regulates the movement of the 

 stop-cock with great precision and delicacy, and the rating 

 requires only a few moments. The position of the rod is 

 shown by a divided arc. " Although it may be objected," Dr. 

 Draper says, • ' that this contrivance seems to be very trouble- 

 some to use, yet that is not the case in practice ; even if it 

 were, it so far surpasses any prime mover that I have seen, 

 where the utmost accuracy is needed, that it would be well 

 worth employing." 



It seems very probable that an arrangement similar in 

 principle to either the sand-clock or the clepsydra might be 

 employed to supply a cheap and convenient driving power for 

 equatorially-mounted telescopes. Those who have been habi- 

 tually subject to the annoyance of the rapid motion of an 

 object through the field of a deep eye-piece, will readily appre- 

 ciate the comfort and pleasure of using a driving apparatus 

 which, by causing the telescope to follow the apparent motion 

 of the heavens, keeps the object immovable in the centre of 

 the field; and contrivances to effect this object, which is 

 especially desirable for micrometric purposes, have been in use 

 for a considerable time. There is more difficulty in accom- 

 plishing this point than might be apparent to a novice, from 

 the circumstance that no reciprocating motion- — such as that of 

 a pendulum or balance-wheel — is admissible, because it causes 

 the whole wheel-work to move by jerks, which would not only 

 be perceptible but very annoying under the magnifying power 

 of the eye-piece. Flies, therefore, such as are used in the 

 striking train of a clock, or the mechanism of a musical box, 

 have been adopted to act upon the resistance of the air ; and 

 these have been made capable of regulation by varying the 

 position of the vanes at pleasure, or causing them to dip more 



