188 FERGUSON'S LECTURES. 



little ; so that the thermometers made in this manner 

 ( will agree with one another ; and the heat of several 

 bodies will be shewn by them, and expressed by the 

 numbers upon the scale, thus : 



Air, in severe cold weather, in our climate, from 15 

 to 25. Air in winter, from 26 to 42. Air in spring 

 and autumn, from 43 to 53. Air at midsummer, from 

 65 to 68. Extreme heat of the summer sun, from 86 

 to 100. Butter just melting, 95. Alcohol boils with 

 174 or 175. Brandy with 190. Water 212. Oil of 

 turpentine 250. Tin melts with 408, and lead with 540. 

 Milk freezes about 30, vinegar, 28, and blood 27." 



A body specifically lighter than a fluid will swim 

 upon its surface, in such a manner, that a quantity of 

 the fluid equal in bulk with the immersed part of the 

 body, will be as heavy as the whole body. Hence, the 

 lighter a fluid is, the deeper a body will sink in it ; 

 upon which depends the construction of the hydrometer 

 or water-poise.** 



Note 51. A very simple and yet delicate mode of constructing a 

 thermometer may now be noticed. If a glass bulb be furnished with 

 a tube, and inverted in a vessel of coloured water, the most minute 

 augmentation in the temperature of the atmosphere will be observable. 

 To effect this, it is merely necessary to expel a portion of the air by 

 external heat, and as the bulb afterwards cools, it will rise in the 

 tube ; to which a scale may be attached in the ordinary way. A still 

 more delicate mode of constructing the air thermometer, consists in 

 substituting hydrogen gas for atmospheric air, as the former is by far 

 the most dilatable. 



Note 52. As our Author has omitted to describe this instrument, 

 which is now so commonly en-ployed in the science of hydrostatics, it 

 may be advisable to examine the construction of it with sufficient mi- 

 nuteness to enable the general reader to understand it. The hydro- 

 meter consists of a hollow ball, furnished with a small hollow sphere, 

 screwed beneath, partly filled with mercury or small shot, iu order to 

 render it but little specifically lighter than water. The larger ball has 

 also a short neck, into which is screwed a graduated brass wire, 

 which, by a small weight, causes the body of the instrument to descend 

 in the fluid with part of the stem. 



When thfl instrument is swimming in any liquor, the part of the 



