Dr. Pearson's Introduction to Practical Astronomy. 371 



There is no mechanical art in which so much delicacy and so 

 much ingenuity are required as in tlie construction of astronomi- 

 cal instruments. The mechanical turn of our author's mind pecu- 

 liarly qualifies him for treating of the proper management of this 

 extremely delicate species of mechanism. It seems to have long 

 been a source of peculiar delight to him, and of never-failing amuse- 

 ment. If we refer to Rees's Cyclopaedia, we shall there find large 

 contributions from the pen of our author, consisting of the descrip- 

 tion of various pieces of horological and planetary mechanism, as 

 well as of astronomical instruments ; but his mechanical genius is 

 more particularly developed in the description of some instruments 

 of his own contrivance. We refer to the construction of some ex- 

 tremely ingenious planetaria, in which the motions are conducted by 

 trains of mechanism which exhibit an epitome of the celestial motions 

 in the most striking and accurate manner : these, however, may per- 

 haps be considered as astronomical toys, but they are the prettiest 

 toys of the kind we have ever seen ; and for those who are but ch\l- 

 dren in the science, are well calculated to convey a correct notion 

 of the complicated celestial motions which they represent. 



But further than this, the author has long been habituated to the 

 use of instruments of a complicated construction, requiring the 

 nicest adjustments ; for we understand that most of the instruments 

 described in this work have been long in his own possession, and in 

 constant use; and this long practical experience qualifies him duly 

 to appreciate the value even of those instruments he has never seen. 

 His account of an instrument, and his suggestions for its improve- 

 ment are not those of a mere enthusiastic theorist, who overlooking 

 all intervening objections comes at once to his favourite conclu- 

 sion. Such an one, for example, might imagine, and many have 

 imagined, that in the construction of instruments the larger they 

 are made the better ; and that the increased power of the reading 

 microscopes, combined with the additional power of the telescope 

 and the enlarged divisions on the limb, would enable him to read ofi" 

 an angle with certainty to the tenth part of a second. But a person 

 who, like our author, has had much experience in the use of instru- 

 ments, will reason otherwise, and will come to a different conclu- 

 sion. He will take into his consideration the imperfection of the 

 materials necessary for constructing the instrument, the variable 

 temperature to which the different parts of a large vertical circle 

 (for example) are exposed : and he will be led to conclude that 

 errors may, and probably will, be induced by these various unavoid- 

 able circumstances, which will more than counterbalance the ad- 

 vantage derived from the increased dimensions of the circle. These 

 sources of error must of necessity limit the maximum size for an 

 instrument, and will in our opinion reduce this maximum much 

 below what it is generally imagined to be. It is our firm belief that 

 a ten-feet mural circle* would so far exceed the legitimate maxi- 



• Ramsden constructed the Dublin circle, often feet diiuncter, aud after- 

 wards reduced it successively to nine aud eight feet, the latter of which is 

 its present size. 



3 B2 



