confiders the principal Obfervatrons, hitherto made of the Mo- 
tion of the Planets and the Fixed Stars , of their Magnitude, 
Figure , and other Accidents 5 drawing thence feveral Conclufi- 
ons, in which heeftabliflies his Hypothecs. The Seco^sd contains 
his A(lronomicd Tables , made according to the Hjfothefes of 
the Firft Part ^ together with InftrUiftions teaching the manner 
of ufing them. 
Thirdly That Aftronomers will find in this Book many very re-^ 
markable things , concerning the Affarent Diameter of the Sun 
and the other Stars , the Motion of the Libmion of the MooHp 
the Eclipfes , Parallaxes^ and Kefractions i And that this Author 
fliews, that there is a great difference hawteu optical mdjfirG^ 
mmical R^{ndlon,which Tycho and many others have confoundedi 
undertaking to prove^that^whereas thefe A(lronomers\mt believed^ 
that the remoter any Star is , the lefs is its Refradion , on the con« 
trary the Refradion is the greater , the more a Star isdiftant« 
Ana among many other things , he ingenioufly explicates the 
two contary Motions of the Sun , from Eaft to Weft 3 and 
vice verfa , by pne oaely Motion upon a Spiral, turning about 
a Cone.' 
Fourthly , That he reprefeats , How uneafie it is to eftablifii 
fure Principles of this Science, by reafon of the difficulties of ma- 
king exaft Obfervations, So, for example, in the Obfervation 
of the Equinox y every one ism.iftaken by kymmy Hours y as he 
i%o{ Minutes^ in the Elevation oi ihtFole ^ or the Diameter of 
theSun ^ or the Refradliion , or in any other drcumftance. In the 
Obfervation of the Sol/lice ^ the error of one only 5^'<:(?;^^caufeth 
a miftake of an Hour and an half: mean time 'tis almoft impoffible 
to avoid the moroi a Second t, and even the iTiarpeft fight will 
* not be able to perceive it., except it beaflifted with an Inftrument 
of a prodigious bignefs. For to mark Seconds , though Lines 
were drawn as fubtil as the lingle threds of a Silk-worms Clew , 
(which are the fraalleft fpaces to be difcerned by the fliarpeft 
Eye) by the Calculation made by this Author there would need 
an Inftrument of 48. feet i^Ww, fince Experience fliews , that 
there needs no more at moft , thaB 3600. threds of Silk to cover 
thefpace of an imh. Eut , fuppofe one could have a ^adranf 
of this bignefs ^ who can affure himfelf , that dividing it into 
Fff z 324OCO5 
