144 LECTURE XVII. 



logwood, sulfate of iron, and a little acetate of copper, or verdigris ; and 

 the stiffening is a thin glue. 



The texture of paper is scarcely different from that of felt, except that 

 its fibres are less visible to the naked eye. To make white paper, linen rags 

 are ground with warm water in a mill, into a paste of the consistence of 

 cream : a portion of the paste is taken up in a wire sieve, which is passed 

 obliquely through it, and this, being a little shaken, subsides into a sheet, 

 which is turned out on a piece of flannel ; a number of sheets being thus 

 formed, they are then pressed, first w r ith the interposition of flannel, and 

 afterwards alone, while they are still moist. For thick paper, two or more 

 sheets are laid on each other before the first pressing. To fill up the pores 

 of the paper, and to increase its strength, a size is employed, which is 

 generally made by boiling shreds of parchment or untanned leather. 

 Sometimes the size is added after printing on the paper, but this is only 

 done in works of inferior elegance, and in this country not at all. 



Such are the principal cases of the union of flexible fibres, for the 

 different purposes of strength or of convenience. Their importance is such 

 that they might be esteemed worthy of a more detailed consideration ; but 

 we are not likely to make any material improvements in these departments 

 of mechanical art by the application of theoretical refinements.* 



LECTURE XVII. 



ON TIMEKEEPERS. 



THE measurement of time by clocks and watches is a very important 

 and interesting department of practical mechanics. The subject is inti- 

 mately connected with the consideration of astronomical instruments, but 

 it is not essentially dependent on astronomical principles. 



Time is measured by motion ; but in order that motion may be a true 

 measure of time, it must be equable. Now a motion perfectly free and 

 undisturbed, and consequently uniform, is rendered unattainable by 

 the resistances inseparable from the actual constitution of material sub- 

 stances. It becomes therefore necessary to inquire for some mode of 

 approximating to such a motion. Astronomical determinations of time, 

 which are the most accurate, can only be made under particular circum- 

 stances, and even then they assist us but little in dividing time into small 

 portions. 



The first timekeepers somewhat resembled the hour glasses which are still 

 occasionally employed ; they measured the escape of a certain quantity, not 

 of sand but of water, through a small aperture. In these clepsydrae, "it 

 * For additional authorities, see Lect. XIX. 



