Sentence:  It is most important to notice that the experiment at Sperenberg shews that there is no regular, uninterrupted increase of temperature corresponding to the increase of depth, but that the increase of heat occurs more slowly at very great depths; in other words, that the degrees of the <WSD>geothermometer</WSD> grow longer as it descends. 
Meaning of the word marked with <WSD> tags:  A thermometer designed for measuring extremely high temperatures below ground level.