Oct. 



1 2. 



1876I 



NA rURE 



529 



PRINCIPLES OF 



TIME-MEASURING APPA- 

 RATUS^ 

 I. 

 "\^E cannot measure time in that sense in which we 

 *' measure other things. Time has been very 

 happily defined as the great independent variable of all 

 change ; and it is by watching matter in motion, which 

 is the simplest form of change with which we are ac- 

 quainted, that we estimate its progress. Thus, the mo- 

 tion of the earth around its axis furnishes us with that 

 well-defined interval, the day ; and the motion of pendu- 

 lums (which swing against the earth's attraction) and of 

 watch balances (which swing against the attraction of the 

 particles of matter of which their springs are composed) 

 furnish us with its subdivisions. I mention this at 

 starting, because during our discussion, I want you per- 

 petually to bear in mind that pendulums and watch 

 balances are not mere appendages or terminations to the 

 mechanism of time-measuring apparatus, but are them- 

 selves the true time-measurers ; and in general, the 

 question of accurately constructing such apparatus re- 

 solves itself into the problem of obtaining an uniform 

 impulse — ^just such an impulse, neither more nor less, 

 which shall exactly restore to the pendulum or watch 

 balance that amount of motion, of which it has, during 

 its preceding swing, been deprived, by the friction of its 

 connections, and the resistance of the atmosphere. 



Our natural time-measures, the sidereal and solar 

 days, are determined respectively by the passage of a 

 star or the sun across the plane of the meridian. The 

 solar day is three minutes fifty-six seconds longer than 

 the sidereal day, the reason of which will be obvious from 

 the accompanying diagram (see Fig. i). During the 



i'r- -:• - 



ti/' :(f 



Fig. I. 



time of rotation, the earth, E, has advanced a little distance 

 upon its annual journey round the sun, s. Therefore, any 

 place upon its surface will have to proceed just a little 

 further (through the angular space Sr Eg S) in order to 

 get the sun opposite to it, than it would have had to 

 have done, had the earth been stationary. The sidereal 

 day is practically the time of one exact rotation of the 

 earth upon its axis ; the distance of the stars being so 

 indefinitely great, that their rays throughout the width of 

 the earth's orbit may be considered to continue parallel. 



The measure employed in our ordinary every-day reckon- 

 ing of time is mean solar time, which we derive in this way. 

 Through sundry astronomical causes, the time of the earth's 

 rotation with respect to the sun is not exactly uniform, 

 solar days differing at certain periods of the year by as 

 much as half an hour. In order to avoid the practical 

 inconvenience which it would occasion by having days, 

 hours, and minutes of different lengths, at different seasons, 

 we add the time of all the days of the year together, and 

 dividing by their number (which is fractional) we obtain 

 the average length or mean of the days, and we refer to 

 this and its sub-divisions as days, hours, minutes, of mean 

 time. 



Hour-glasses, candles, and water-glasses, were the 

 instruments used by the ancients to indicate the passage 

 of time. It was not till a comparatively recent date that 

 apparatus consisting of a moving body, impelled through 

 the medium of a combination of wheels (which also served 

 to register the body's progress) was introduced for the 



' Lecture by Mr. H. Dent Gardner, a the Loan Collection, South 

 Kensington. 



purpose. We have a very good illustration of such early 

 mechanism in the clock from Dover Castle (see Fig. 2). 



A rope supporting at its extremity a weight, w, is 

 wrapped around a cylinder or barrel, B, and by its means 



Fig. 



drives the wheel GG. This wheel is engaged with a 

 pinion, P, and through it impels the escape-wheel S. The 

 teeth of the escape-wheel operate upon two tongues or 

 pallets. Pi P2, set at an angle to each other upon the stem 

 carrying the moving body or time-measurer, M M. The 



Fig. 3. 



action of the wheel upon the pallets is exceedingly simple ; 

 the tooth A (see Fig. 3^) is now pushing the pallet P^ to the 

 right. It will presently have pushed it out of the way 



^ For comparison with Fig. ?, imagine the wheel to be moving in the 

 reverse direction, and the letters Pj F2 interchanged. 



