BURNING-GLASS. 



its weigSt 21 poTinds. Wlien the two ahove lenfcs are 

 compounded tojrcther, the leii'fth of tli« focus is 5 feet 

 3 Indies, and it-i diameter half an inch. C is a truncated 

 co:ie, conpof^d of 12 ribs of wood, at the larger end of 

 which is lix-jd the prreat hns A, and at :he fmallcr extremity 

 the lelTer k-ns B ; near the fmaller end is alfo fixed a rack, 

 D, padl.ig throUj^h the pilFar L, moveable bv a pinion 

 turning in the faid pillar, by means of the handle E, and 

 thus giving a verticil motion to the machine. F is a bar of 

 wood, fixed between the two lower ribs of the cone at G ; 

 having, within a chafed mortife in which it moves, an appa- 

 r.itn,;, H, with the iron plate 1, fixed to it ; and as this 

 part tnrns on a focket, K, a method is thus obtained of 

 placing the matter under experiment fo as that it may be 

 ailed upon by the focal rays in the nioH direft and powerful 

 manner. L L is a llrong mahogany frame, moving on 

 caftors, M M, and immediately under the table, N, are 

 three fri£lion wheels, by which the machine moves horizon- 

 tally. O is a ftrong iron bow, in which the lens and the 

 cone hang. 



hi Jig. 2. a fck^ion of the large Itns is feen : it, the large 

 lens, marked A in the former figure ; i // h the frame which 

 contains the lens ; c the finall lens, marked B in the pre- 

 ceding figure; (It/ is the frame wliich contains this lens; 

 f e \i the ti .■ cated cone, marked C in the otiicr figure ; 

 yi is the bar, on which the apparatus, marked F in the 

 former figure, moves ; ^ is the iron plate, denoted by 1 in 

 the faid figure ; h the cone of rays, formed by the refraftion 

 of the great lens a, and falling on the lens c ; and / is the 

 Gor.c of rays formed by the refraftion of the lens c. In 

 J:^r. ^. i exhibits the front view of the great lens;/ the 

 frame containing the large lens ; and m- the llrong iron bow 

 in which the lens hangs. 



It is difficult to form an accurate elliinate of the burning 

 power of this lens ; inafmuch as it is next to impoffible to 

 difcover what (liould be dedntted for the lofs of power, in 

 confequence of the impediments that the glafs of which 

 it was made mull occafion, as well as the four refleftions 

 (i. e. two reiletlions on the i;iimcrfion, and two more on the 

 emerfion of the rays of heat), by way of diminution ; but 

 ■vve v.ill endeavour to appreciate it after making a full 

 allowance for thefe dcduftious, which mud ueceffanly refult 

 from every means of concentrating the folar rsys, and 

 which mull be confidered to be as the friction of an engine, 

 of which nature they really partake. 



The folar lays received on a circular fnrface of 2 feet 8| 

 inches, when concentrated within the diameter uf one inch, will 

 be io56.25times its intenfity, or this number of times greater 

 than the heat of the fun as felt on the fnrface of the earth. 

 We will fuppofe that as the heat of the air, in ordinary 

 fummer, weather, is 63°, and in fultry weather is 75°, the 

 average of v/hich is 70°, and that we take this degree as the 

 average effeft, the accumulated power of the lens, on the 

 fuppofition of an equal effedl over the whole furface of the 

 focus, will be equal to 73938^. 



It muft be recollefled by thofe who have had an oppor- 

 tunity of examining the effttls of this lens, that the external 

 part of the focal light was lefs intenfe than that part which 

 was near the centre of it ; or, rather, that the effett was very 

 much accumulated in the centre : but as it is poffible that 

 the refraftion of the light and of the caloric fluid may not 

 take place in the fame angles, we think it fafeft to confider 

 it as ef an uniform effedl, and after dedufting one-fourth part 

 thereof as a compenfation, there remains 55454°, as the ex- 

 preffion of its power. 



As the application of the fecond lens reduced the diame- 

 ter of the focus to half an inch, the effed, without allowing 



for the rcduiflion of its power, would be equal to 221811')'', 

 but dedudlmg one-fourth for the fecond trai.fmiflion, lliere 

 remains \66y)z^ as the exprdTiin ol its power. 



Suppofing that the rays of the fun emanated from a point, 

 the focus of the lens would alfo be a point ; for the rays 

 traiifmitted through one edge of the lens would ipcet thofe 

 tranfmilted through the other edge, fo that the focus would 

 be without parts, or a mathematical point ; but as the fun 

 is known to meafure ,ji' 32" during the apogee, and 32' jS' 

 during the perig-e, the focus mull nectfTarily be 0.7105 partj 

 of an inch diameter at Midfummer-, and o.-j94at Chrillmas. 

 The length of the luminous axis at Midfummer muft, from 

 the fame caufe, be 3.428 inches, and, at the oppofite period, 

 3-557 ; becaufe the dimcnfion of the focus is arflual.y de- 

 pendent on the apparent diameter of the fun, the ftmi-dia- 

 meter being to be added to incrtafe the mtafure < f the con- 

 verging rays on each of ihc oppofite fides, the tangent of 

 which is the femi-diameter of the focus: and by the fame 

 means the length of the axis is difcovered, addiirg to and 

 fubtradling from the converging angle, by which means the 

 points at which they meet on the axis ai'e difcovered. Thefe 

 two points dilclofe the commencement of the luminous ap- 

 pearance thereon. 



The variation between the above calculated diameter of 

 the focus and what it really was, points out the errors in the 

 inllrument, which mull aiife from one of two caufes, or fr-on» 

 both, viz. that the axis of the two fides are not corncident, 

 or that they ar-e not oppofite to each other ; (hould the dif- . 

 ference have been occafioned by either, the focus would 

 have been eUiptical ; but as tliis was not afcerfiined, it can 

 only be faid that a very fm.nll deviation in each would have 

 occafioned tiiis enlai-gcment of the focns ; and pofilMy they 

 mighi: have adled in contrary ways, which would have made 

 it nearly circular. 



But as this lens was exported many years ago, we are not 

 able to determine whether the focus (Idted to have been 

 one inch diameter was afccrtained by its burniiio- lifecl, or 

 by the diameter of the focal rays : it by the diameter of the 

 rays, the enlargement thereof mufl have refulted from the 

 above caufes ; but if from its burning power, it tends rather 

 to elLnblilh the theory of Dr. Her-fchell, that the rays of 

 caloric arc lefs refrangible than thofe of light, and that 

 the true burning focus lies rather beyond the apparent focus. 

 We are far from thinking the above llatcment an extrava- 

 gant fuppofition, when it is recoUeaed, that all the experi- 

 ments on which we ground our opinion were made with a 

 total cxpofure of the fubllances to the acftion of the circum- 

 ambient air ; and ha<l the property of this fluid, as to its 

 coirdufting pov/er of heat, been then well underftood, fo as 

 to have prevented the diminution of the cfiedl, wc are well 

 fatisfied the lefult would havejullified our conclufion. 



The ingenious experiments of Mr. Wedgwood, and the ex- 

 ttnfive ufe of his pyrometer, fyr the admeafurcment ot heat re- 

 quired to melt metals, are now too well underftood to require 

 any eulogium in this place ; it is fufficient to (late that he 

 eftimates the heat required to melt the following fubftances 

 to be as follows : 



Fahr. or by Wedg. 

 Red heat vifible in day light '0/7° o 



Fine gold - . - 52J7 32 



Welding heat of iron greatcft '^34-7 OC 



Caft iron melts - - 17077 I^o 



Greateft heat of his fmall furnace 2Ii'77 ^60 



Extremityofthefcaleofhispyr'ometer 32277 240 



The power of the lens ellimated by this pyrometer will be 

 found to be 1096°, and fuppofing the hotteft furnace to be 

 240°, the focal heat thereof is 5.2975 times greater. 



In 



