THERMOMETER AND PYROMETER. 



of the thermometer to be graduated, 

 with those of the liquid in an accurately 

 formed cylinder of metal, two inches in 

 diameter and depth, and having cement- 

 ed to its top a glass pipe, just ^ of the 

 diameter of the cylinder:* measure off 

 two inches of the stem, above the cylin- 

 der of metal, and divide the space be- 

 tween them into 10 equal parts, so that 

 each division of the stem will = Tg ' 5 ^ of 

 the capacity of the cylinder. The ther- 

 mometer to be graduated has the com- 

 mencement of its scale, or 0, fixed by 

 marking the point at which the included 

 liquid stands in the stem, when the bulb 

 is plunged in distilled water just begin- 

 ing to freeze ; and the rest of the pro- 

 cess he details in these words. " Fill 

 this cylindrical vessel with the same 

 liquid wherewith the thermometers are 

 filled, then place both it and the ther- 

 mometer you are to graduate in water 

 that is ready to be frozen, and bring the 

 surface of the liquor in the thermo- 

 meter to the first mark, or ; then so 

 proportion the liquor in the cylindrical 

 vessel, that the surface of it may just be 

 at the lower end of the small glass 

 cylinder ; then very gently and gradu- 

 ally warm the water, in which both the 

 thermometer and the cylindrical vessel 

 stand, and as you perceive the tinged 

 liquor to rise in both stems, with the 

 point of a diamond give several marks 

 on the stem of the thermometer, at 

 those places which, by comparing the 

 expansion in both stems, are found to 

 correspond to the divisions of the cylin- 

 drical vessel ; and having by this means 

 marked some few of the divisions on 

 the stem, it will be very easy by these 

 to mark all the rest of the stem, and 

 accordingly to assign to every division 

 a proper character."-!* This ingenious 

 method is, however, more difficult in 

 execution than any one, unacquainted 

 with such operations, will readily sup- 

 pose ; and it presupposes, what is not 

 easy to accomplish, a very perfect ad- 

 justment of the metallic cylinder and 

 the glass stem in the standard instrument. 



Dr. Hooke appears invariably to have 

 used in his thermometers spirit of wine 

 " highly tinged with the lovely colour of 

 cochineal, which he deepened by pouring 

 in it some drops of common spirit of 

 urine. 1 ' 



The sagacity of our illustrious New- 

 ton saw the importance of improving 

 thermometers. He appears to have been 



* Alicrographia. 



| Micrographia, p. 3;). 



early aware of the inconvenience of 

 spirit as a thermometric fluid, and em- 

 ployed linseed oil to fill his thermo- 

 meter. It has the advantage of being 

 able to endure a very considerable tem- 

 perature, without endangering the burst- 

 ing of the tube, and therefore can be 

 applied to a higher range of tempera- 

 ture than a spirit thermometer. It has 

 the disadvantage, however, to be more 

 sluggish in its movements, and to ad- 

 here much to the inside of the tube, 

 while it differs greatly in its fluidity at 

 different temperatures. Newton per- 

 ceived the convenience of having two 

 fixed points in the construction of the 

 scale ; and he used the freezing and boil- 

 ing points of water as the most suitable 

 for this purpose.* His method of gra- 

 duating his oil thermometer is given in 

 the Principia. The oil, at the tempe- 

 rature of melting snow, was supposed to 

 consist of 10,000 equal parts, which, 

 when heated to the temperature of the 

 human body, expanded to 10,256 ; at 

 the temperature of water strongly boil- 

 ing to 10,725; and at that of tin be- 

 ginning to congeal, to 11,516 parts. In 

 the first instance the ratio of expansion 

 is as 40 to 39 ; in the second as 15 to 

 14 ; and in the third as 15 to 13 nearly. 

 Hence, by taking the temperature of the 

 oil in the ratio of the rarefaction and 

 assuming 12 as the heat of the human 

 body, the temperature of water briskly 

 boiling will be 34 degrees, and of con- 

 gealing tin 72 degrees.! 



Newton continued his scale of tem- 

 perature farther by observing the rate 

 of cooling of heated bodies, until he 

 could apply his thermometer to them, 

 on the principle that equal decrements 

 of temperature take place in equal 

 times. It was thus he estimated the 

 temperature of iron heated to the utmost 

 intensity of a small kitchen fire equal to 

 194 degrees, ar.d in a fire of wood'about 

 200 or 210 degrees of the same scale. 



It is perhaps unfortunate for the phi- 

 losophy of heat that more sublime 

 and dazzling objects drew Newton to 

 other pursuits. Though he led the way 

 to just views of the subject, neither he, 

 nor any of his predecessors, appear to 

 have been aware of the influence of the 

 varying atmospheric pressure on the 

 boiling points of liquids ; nor do any of 



* Phil. Trans. 



f " Ponendo caloris olei ipsius, rarefactione pro- 

 portionnlis, et pro calore corporis humani scribendo 

 12, prodest calor aqua? ubi vehementer ebullit par- 

 tium34, et calor stanni ubi liquescit prodest 

 72." Priucip. 



