very large silver sphere causing cloud emmisions slow and straight

object was veiwed by wife and husband in 1977 appeared to be very large silver sphere travelling relativly slowly over the sea viewed from helmsdale. moving from north to south, the sun was reflected on the south side and it seemed to generate what looked like thick cloud from the north side, no audible sound,  height from sea estimate 300 ft. traveled in straight line, no arcing effect. wife nurse,husband builder ages 25, 26. traveling to england, stopped to give dog a walk.
Because of the differences between the U. S. and Europed in how the date is expressed, we do not know whether the sighting occurred in July or August of 1977.  PD