Nc iS, 1880] 



NATURE 



61 



froi 'eft to right. As the cylinder turns once in two 

 mil es, there is between each prick and its fellow simi- 

 lar positioned on the next spiral a difference in time of 

 tw( linutes. As stated above, we note by the absence 

 of mo pricks the occurrence of the minutes. By reference 

 to the figures placed (for explanation) along the top and 

 side of the section of the cylinder, we see that the time of 

 the highest prick on the left-hand spiral was gh. 12m. 46s. 

 Following the course of the spiral down towards the 



pricker which is tracing it, we first pass the blank line 

 indicating the occurrence of the next minute, and then 

 come to three observation-pricks at about the 8th and 9th 

 seconds beyond it. Thesp in practice would be measured 

 off and their values determined to the j^th or ,011'^ of a 

 second. There are other observation-pricks at the 22nd 

 second, the 24th, and 25th, and there is a group of others 

 ' . about the 40th. 



When all the ob^el•vations have been reckoned up the 

 paper is put away, and it is not the least advantage of 



the "chronographic" method that in any case of doubt 

 the original observation can be itself referred to years 

 afterwards. 



As the paper is moving whilst being punctured, the 

 prickers have to be mounted on springs to enable them 

 to yield a little. In Fig. 3 is a side view of the pricker, 

 G G being the spring, and c C a portion of the cylinder. 



There is no difficulty in reading off the observations 

 after a little practice : but in order to facilitate the eye 



in following the sequence of the punctures — before the 

 paper is used a continuous spiral line is ruled upon it which 

 shall exactly correspond with their course. This is done 

 in the following way (see Fig. i) :• -At T are two clutch- 

 wheels, which connect the screw and cyhnder with the 

 clock-work L. By moving a lever near them the clock- 

 work is thrown out of gear, and simultaneously the winch 

 on the left is thrown into gear with the screw. On the 

 carriage K (see Fig. 2) is a little roller E, and by moving 

 a handle this is sprung up against the paper on the 

 cylinder. The winch before referred to is now turned, the 

 cylinder rapidly revolves, and the carriage quickly tra- 

 verses the screw, the spiral line meanwhile being traced 

 by the pressure of E upon the paper of the cylinder. To 

 prevent damage to the prickers during the operation, the 

 act of disengaging the clock-work breaks their electrical 

 communication, and they can neither of them be raised 

 until the clutch-work is restored. The cylinder moves 

 very swiftly whilst the line is being traced, and were it 

 brought to a standstill suddenly great dainage would be 

 done. To prevent this it is arranged that when the 

 carriage k is approaching either extremity of the screw 

 it shall work a brake arrangement b b, which brings the 

 machinery quietly to repose. The act of putting the 

 clutch-wheels T into position again also releases the brake. 

 The clutch-wheels T are mounted on a spring, so that 

 should their teeth not correspond when they are put into 

 gear, one will give and wait for the other to overtake it. 



It is desirable that the clock L should drive C C with 

 great uniformity ; and as the time of a conical pendulum 

 is affected in a very great degree by any variation in the 

 force of the clock-train, a special governing arrangement, 

 the invention of the Astronomer- Royal, is employed. 

 U (see Fig. 4) is a trough filled with glycerine and water. 

 Power reaches the pendulum by means of its connection 

 with the vertical spindle seen at the centre of the trough, 

 which rises from the clock-work. In this connection 

 there is a joint, and a dipper d forms part of it. Too- 

 much power drives out the pendulum, and it would then 

 go faster, were it not that the dipper, entering the glycerine 

 and water more deeply, checks its motion. On the other 

 hand, whenever the power falls off the pendulum performs 

 a smaller circle, thereby lifting the dipper a little more 

 away from the liquid, and diminishing the resistance in 

 exactly the same proportion as the force. The pointer x 

 is a very delicate index of the angle the pendulum is 

 swinging. The compensatory action of this governor is 

 very considerable ; doubling the power produces no 

 perceptible difference in the time. The quickness with 

 which it works is surprising ; an infinitesimal change in 

 the power is immediately indicated on the scale a, 

 showing how well the apparatus is doing its work. 



To prevent damage to the governor and the more 

 delicate clock-wheels by any sudden check to the cylinder, 

 a ratch-wheel arrangeinent has been introduced, which, 

 when the cylinder is suddenly stopped, enables the 

 pendulum to run on until it comes to rest gradually, 

 by want of power. 



Fig. I gives a very faint idea of the dimensions of the 

 apparatus. The cylinder cc is 12 inches in diameter, and 

 30 inches long. Its weight is about 70 lb. The space 

 between each seconds prick is yg inch, and the distance 

 between the successive turns of the spiral of pricks Wjj inch. 

 There is room on the paper for 200 spirals, and as 

 each is more than one yard long, we can get more than 

 200 yards (65 hours) of continuous observation without 

 disturbing the instrument. There is always a spare cylinder 

 covered with paper kept ready to replace the first. 



The iron base-plate on which the instrument is mounted 

 weighs over 3 cwt., the rails on which the carriage K runs 

 are cast in one piece with it, and, along with all other 

 bearing surfaces are planed. The pendulum, which weighs 

 some iS lb., and is compensated, goes round once in 

 two seconds. Its suspension-piece weighs 2 cwt. As 



