60 Time-Pieca. -Engravings on Glafs, 



by the loaded arm CA refling upon I. A pin N proceeds from the pendulum rod to its 

 fituation between the levers. The centre of motion of the pendulum is in the continuation 

 of the axis of the pallets. Suppofe the vibration to be begun, the pin N in its progrefs 

 will lift the pallet D towards L, vi-hile part of the weight B will be fupported by the tooth 

 of the wheel which will follow, and at laft efcape, and fuffer the oppofite tooth F to fall on 

 the pallet E. But in its return or defcent the pendulum will be afted on by the whole 

 weight B, and confequently its vibration v/ill be kept up. V/hen the pin N raifes the 

 pallet E, a fimilar effedl will take place on the fide of the perpendicular towards M, and 

 thus the procefs will go on as long as the wheel GH retains any force. 



The ancient efcapement with the crown wheel, verge, and pallets, has continued in ufe for 

 three centuries, becaufe of the advantage of an almoii direft pulh againft a retiring furface, 

 which enables it to go without oil. In the efcapement laft defcribed, the impulfe being more 

 direifl than on thofe common pallets, it goes more freely without oil ; and inllead of the rc- 

 adion at the ends of the vibrations, the pendulum is perfeftly difengaged during the whole 

 time of repofe. 



From this general view of the principal efcapements, together with that of the compen- 

 fations for temperature before explained*, the philofophical reader may form fome eftimate 

 of the cautions requifite to form the valuable inftrument by which a meafure of time is af- 

 forded. I hope it fcarcely need be added, that 1 have endeavoured to do juftice in the 

 hiftorical narration, and ihall be happy to re£tify any miftakes, if better information (hould 

 dete£l them. 



11. 



Copy of a Letter from Profejfor WlLSON,of GlafgoWf on the Art of multiplying Copies of engraved 



Plates and Stamps in relief f. 



' SiKj Glafgow College, 1^'^ March, 1798. 



1 DOUBT not but you will have the goodnefs readl«y to excufe the liberty which I take 

 upon the prefent occafion of addreffing you by a printed letter. I have been induced 

 to do fo, the more conveniently to communicate to you fome account of what poffibly 

 may be thought entitled to the appellation of a new art, and which, for particular 

 rcafons, it has been thought advifeable to make more or lefs generally known, even at 

 this very early ftage of it. 



Some years ago, upon the difcovery of the fmgular property of the fluor acid in cor- 

 roding glafs, when it was fo common to hand about bits of glafs-plate frofted over by this 

 chemical agent, it happened to ftrike me that we were indulging too long in a barren ad- 

 miration of mere novelty, and overlooking a matter of real importance to which it evi- 

 dently pointed. The general effeft of what then fo much amufed us, when the plates. 



• Philof. Journal, I. p. 5*. 



f.J firft faw this letter at Sir Jofcph Banks's ; bjt having fome doubt as to the pro-n icty of requefting it of 

 hitnfor publication, 1 fent to Mr. Taflie, who favourcl me with another copy for that cxprefs purpofe. N. 



were 



