210 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 bis workshop much longer than 
he anticipated when he wrote the letter from which the last extract 
was made. Five years afterwards he succeeded in panera 
an object at which he had long been aiming. 
In a letter. to Messrs. Sinan, 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 I imported one of your 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 firmness, 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.” 
