24^ Memoir of Rev. John Prince, LL. D. 



The ebullition and belching out of the matter, and the boiling in the 

 crater which is formed, and the appearance of the black scoria are 

 very striking." 



He had another method of exhibiting a volcanic eruption, which 

 was by a series of engravings, representing its successive stages, and 

 subjected to the magic lantern or camera obscura. 



Dr. Prince continued to resist the approaches of age and its in- 

 firmities, and to labor effectually in his workshop much longer than 

 he anticipated when he wrote the letter from which the last extract 

 was made. Five years afterwards he succeeded in accomplishing 

 an object at which he had long been aiming. 



In a letter to Messrs. Jones, July 10, 1800, he thus expresses 

 himself: " I have seen Mr. Dowse's large telescope which you sent 

 him. I think it an elegant and well executed instrument, and the 

 mounting makes it very convenient in management. But with the 

 greatest magnifying power there will be a tremor, when the hand has 

 hold of the adjusting screw, which makes it very difficult to define 

 the object ; and indeed all instruments so large, and supported in the 

 center, as telescopes generally are mounted, must be affected in this 

 "way in some degree, notwithstanding the bracing bar. A small mo- 

 tion in the atmosphere will also affect them. 1 tried this experiment 

 with a gentleman for whom 1 imported one of you/ three and a half 

 feet achromatic. Placing it on a firm table, in a room where the 

 motion of the air could not affect it, with a high power, we placed a 

 book at such a distance in the garden as that we could scarcely read 

 the words by the best adjustment we could make ; then, taking the 

 telescope from the stand, and laying it on the table, firmly support- 

 ing it at both ends, we could read at the same distance very distinct- 

 ly, and the book would bear moving farther off with distinct vision.'* 



Dr. Prince brought his philosophical career to a close, by contri- 

 ving a stand for a telescope by which the uncertainty always before 

 introduced into the observations of astronomers, by the tremor and 

 vibration here spoken of, is completely avoided. The telescope rests 

 in a solid bed with perfect Brmness, and at the same time is mova- 

 ble in every direction, and by the slightest touch of the finger. The 

 following is the conclusion of the description given by him of this 

 ingenious structure, as published by the American Academy of 

 Arts and Sciences. " I made the brass work myself, and finished 

 it on my birth day — eighty years old." 



