l60 PHILOSOPHICAL TRANSACTIONS. [aNNO 1786. 



I made a short memorandum of the objects I chanced to see; and when a new 

 nebula or ckister of stars came in my way, I made a delineation of the stars in 

 the field of view, both of the finder and of the telescope, that it might serve to 

 find them again. This being done, the instrument was, by means of a fine 

 motion under my hands, either lowered or raised about 8 or 10 minutes, and 

 another oscillation was then performed like the first. Thus I continued gene- 

 rally for about 10, 20, or 30 oscillations, according as circumstances would per- 

 mit; and the whole of it was then called a sweep, and as such numbered and 

 registered in my journal. 



When I had completed 41 sweeps, the disadvantages of this method were too 

 evident to proceed any longer. By going into the light so often as was necessary 

 to write down my observations, the eye could never return soon enough to that 

 full dilatation of the iris which is absolutely required for delicate observations. 

 The difficulty also of keeping a proper memorandum of the parts of the heavens 

 which had been examined in so irregular a manner, intermixed with many short 

 and long stops while I was writing, as well as the fatigue attending the motion, 

 on a not very convenient gallery, with a telescope in my hands of no little weight, 

 especially at the extremes of the oscillations, where it made a considerable arch 

 upwards, were sufficient motives to induce me to look out for another method of 

 sweeping. And it is evident, that the places of nebulae hitherto determined, 

 which was till the 13th of December, 1783, must be liable to great inaccuracy. 

 I therefore began now to sweep with a vertical motion; and as this increased the 

 labour of continually elevating and depressing the telescope by hand, I called in 

 the assistance of a workman to do that part of the business, by which means I 

 could observe very commodiously, and for a much longer time than before. 

 Soon after I removed also the only then remaining obstacle to seeing well, by 

 having recourse to an assistant, whose care it was to write down, and at the same 

 time loudly to repeat after me, every thing I required to be written down. In 

 this manner all the descriptions of nebulae and other observations were recorded; 

 by which I obtained the singular advantage that the descriptions were actually 

 writing and repeating to me while I had the object before my eye, and could at 

 pleasure correct them, whenever they disagreed with the picture before me with- 

 out looking from it. 



In about half a dozen sweeps, done according to this new way, I found 

 that the stars of Flamsteed's catalogue entered nearly at the time when they 

 were expected; this suggested the possibility of converting my telescope into a 

 transit instrument. By way of trial, Dec. 18, 1783, I began to use a watch, 

 and noted the times of the transits of stars and nebulae to the nearest minute; 

 and, this succeeding, Dec. 24, a sidereal time-piece was introduced. 1 found 

 also that, by the turns of the handle which gave motion to the telescope, it was 



