142 



THE THERMOMETER. 



and partially filled with quicksilver, could be described, and have its properties 

 explained, in a much more limited space. It should, however, be remembered 

 that, trifling as this instrument may appear, its uses are, perhaps, more exten- 

 sive, and certainly not less important, than any other means of experimental 

 investigation by which we are enabled to scrutinise the laws of nature. There 

 is no department of natural science where experiment and observation are the 

 means of knowledge, in which the indications of this instrument are not abso- 

 lutely indispensable ; and this must be apparent, if it be considered how essen- 

 tially the states of all bodies, whether those contemplated in mechanical sci- 

 ence, in chemistry, nay, even in medicine and the natural sciences, are affected 

 both by the external application of heat and its internal development. Without 

 the thermometer, we should possess no means of determining those changes of 

 effects better than the very fallible and inaccurate perceptions of the senses ; 

 perceptions which, as it will hereafter appear, depend much more upon cir- 

 cumstances in our ever-changing states of body, than on the states of the bod- 

 ies around us. In physics, the thermometer is indispensable in almost every 

 experiment. In the laboratory, the chemist can scarcely conduct a process 

 with any degree of philosophical accuracy without an observation of tempera- 

 tures. In the observatory, the astronomer who is ignorant what effects chan- 

 ges of temperature produce on the indications of the large metallic instruments 

 which he uses instruments so highly susceptible of dilatation and contraction 

 would be surrounded with sources of error, of which it would be impossible 

 for him to estimate the amount, or even to detect the existence. Even the as- 

 pect of the heavens changes its appearance in obedience to the fluctuating tem- 

 peratures of air ; nor is there a single object in the firmament seen in the same 

 position for two successive hours, and never in the true position which it 

 would have independently of the effects of heat. The vicissitudes of heat and 

 cold, to which the atmosphere is subject, must, therefore, be appreciated before 

 the observer can pronounce on the position of any celestial object ; and to this 

 there is no guide but the thermometric tube. The naturalist, in investigating 

 the properties of the various classes of organized bodies, bases many of his 

 generalizations on their temperatures discovered by this instrument. In inves- 

 tigating the qualities of different parts of our planet, the variations of climate 

 corresponding with changes of latitude, the phenomena peculiar to land and 

 sea, the various meteorological facts essential to all knowledge of climate and 

 to all investigation in physical geography, depend on the indications of the 

 thermometer. The measurement of the heights of mountains, of the position 

 of balloons in the atmosphere, are estimated by combined observations on this 

 instrument and the barometer. When these and numerous other considerations 

 are called to mind, it will scarcely be deemed inappropriate, even in a work 

 of a popular nature, to enter into the details which have been here given re- 

 specting the construction and use of this instrument. For the same reasons, it 

 may not be uninteresting to the general reader shortly to trace the history of 

 the invention and improvement of thermometers before we conclude this lec- 

 ture. 



Like other inventions of very extensive utility and remote date, that of the 

 thermometer is disputed by many contending claimants ; and, like other inven- 

 tions, the merit is not to be ascribed to one person, but to be distributed among 

 many. The several arrangements which render the instrument useful and ac- 

 curate as a measure of a degree of temperature were suggested successively, 

 and adopted through a long period of time, and some of the latest of them have 

 not been of very remote date. 



The notion of using the expansion of a liquid contained in a bulb and tube 

 glass, as a means of indicating changes of temperature, is said by some to 



