LET. X. OF THE LATITUDE AND LONGITUDE. 1 73

make nice telescopical observations with anydegree of accuracy. If we could fee the satel-lites of Jupiter with the naked eye, as well aswe can the moon, the method would be attend-ed with no difficulty ; it might be practised byany common sailor on board, and the longitudewould then be found with nearly as much easeand certainty as the latitude. We have several me-thods for finding the apparent time at any placeby celestial observations; and if a good pocketwatch be previously regulated by these means,the difference between the time when the phæ-nomenon happens, as shewn by the watch, andthe time as shewn by the tables, being convertedinto degrees, by allowing 15 to an hour, willgive the longitude of the place from London.

Several attempts have been made to adapttelescopes to the purposes of marine observations,but they have all hitherto failed of success ; andthere is reason to fear, that the obstacles whichstand in the way of this improvement are suchas cannot be removed. One of the methodswhich have been proposed for rendering theuse of the telescope easy on board a ship, is byfixing a chair, in such a manner that it shallnot be agitated by the motions of the vessel; andif this could be accomplished, it would then beas easy to observe an eclipse at sea as at land.The late ingenious Mr. Irwin, contrived a ma-chine of this kind, and laid claim to the par-liamentary