gQ ON PENDULUMS. 



riments, or might be kept fo by means fimilar to thofe pointed 

 out by our ingenious author, or his block of ftone itfelf might 

 have been ufed. 

 Ramfden's That great artifl Ramfden, whofe mechanical (kill and 



pyrometer. clearnefs of intellect were fo varioufly difplayed in his works, 



and ftill more in his converfation to thofe who remember and 

 regret that he has recorded fo little of the refults of his labours : 

 thisartift firlt availed himfelf of microfcopes for afcertaining the 

 terminal points of the fubjeft under examination by his pyro- 

 meter ; and the interval between one microfcope and the other 

 was rendered permanent by fixing them to arms proceeding 

 alright angles from a bar of iron, kept at 32° by furrounding 

 it with melting ice. Phil. Tranf. Vol. LXXV. 

 Methods" of Laftly, another error, p. 74.] Ibelieve thefmallfpring and fim- 



examining grid- pj s bar part, if any beneath the gridiron, are always taken into 

 iron pen u ums> t h e accoun t, Thefe pendulums have not unfrequently been 

 . fubjected to adual examination. An old friend of mine, Mr. 

 ftrument. J. H. de Magellan had an apparatus of tin, (Anno 1784) in 



which he inclofed the gridiron when to be examined. It was 

 hung by its fpring to an arm fixed in an upright plank of deal 

 wood, againft which plank were attached thermometers to (hew 

 its fteadinefs of temperature; and from the center of ofeilla- 

 tion proceeded a (temporary) rod, the end of which pre- 

 sented a dot as the object to be viewed through a microfcope 

 duly attached to the plank. Without dwelling minutely on 

 this apparatus, I need only obferve that neither the arm of fuC 

 penfion, nor the fupport of the microfcope, nor the rod from 

 the center of ofcillation had any parts but fuch as were paral- 

 lel to the horizon, while the pendulum itfelf was perpendicu- 

 lar to it, and confequently that no error could arife but from 

 expanfion in the plank, which r^emajned at the common tem- 

 perature; and that the tin apparatus which covered and enclofed 

 the pendulum had only three openings, one at the bottom, one 

 at top, and one through which the rod from the center of 

 ofcillation paffed, without touching the fides. In the experi- 

 ment, fteam from boiling water was admitted below, and when 

 the heat was fo raited, and the fupply kept up, that uncon. 

 denfed fteam efcaped above the pendulum was judged to be at 

 212°, and if the center of ofcillation continued before the 

 microfcope without either rifing or falling, the compenfation 

 vj ;as confidered as complete, 



Mr. 



