S36 
ON THB EQUATORIAL TELESCOPE. 
is too great to be observed by the micrometer. Mr. Short’s 
instrument seems to have consisted of a complicated apparatus 
several parts of which are not essentially necessary to an equato- 
rial j and, therefore, must have been very expensive. 
Heliostati: B. Martin in his “ Gentleman and Lady’s Philosophy,” 
have^been^a when. describing the heliostata or planetary 
substitute to clock, a complicated and very expensive piece of machinery, 
^e^qua^tori^^. equatorial telescope will cost more than double 
uivented first, the price of this machine.” He alludes to it as consisting of 
hropk and S. ” ^ large and complicated system of wheel work j” which, 
Oravesande. however convenient in some respects, is certainly not essentially 
necessary for producing the requisite movements of the equato- 
rial circles. He also intimates, that the heliostata, which was 
intended to fix the images of the sun, moon, and planets in 
astronomical observations, was designed to succeed the equatorial 
telescope as its substitute. But I have not learned that the 
heliostata has ever come into general use ; for although it 
accomplishes one object which cannot be effected by the 
equatorial, viz., in fixing the sun and planets during an observa- 
tion, yet it cannot be applied to the same general purposes*. 
Short’s Equa- The difterent circles in Mr. Short’s instrument seem to have 
been about 6 or 7 inchesf in diameter, and were divided by 
nonius into 3 minutes of a degree, and one minute of time. 
The telescope adapted to the machinery was aGregorian reflector. 
Though the heliostata can sever .supersede the use of the equatorial, 
it would form an useful appendage to it, or to any telescope mounted 
on a polar axis j as it must be pleasant, and in many cases useful, to 
view the heavenly bodies in a quiescent state ; particularly, when we 
wish to delineate a map of the moon, or the spots and phases of the 
planets. I have not heard, however, of any movement of this kind 
I'.aving been introduced into any of the public observatories in the 
Itritish empire, except in that belonging to the Trinity college, Dublin,, 
which is furnkshed with a variety of excelletit instruments on a grand 
scale. 
t This estimate is formed from the proportion betw een the diameters 
of the circles and the length of the telescope, as represented in tiie 
C))gr;uing referred to in Martin’s Philosopliy. 
iMviflg 
