I got this from the Toronto Globe's Saturday, 14 September 1878 newspaper (page 8), where it is noted that it was raining steadily from late Tuesday well into Friday. Some 12 cm was reported to have fallen at Port Dover on Lake Erie. The emphasis was of course on Toronto where four lives were said to have been lost in the Don River. From my community (then village) of Weston it was reported that the iron bridge on the Grand Trunk Railway at Black Creek gave way and fell at 9 AM September 13, after fourteen hours of very heavy rain.
In many ways this was a forewarning of 1954's Hurricane Hazel and, indeed, this appears to have been the tail end of Hurricane #5 of 1878. This morning I adjusted the Wikipedia entry for that storm by replacing its September 13 extratropical placement in Virginia with one in Ontario. A remarkably modern storm path could be figured out even back then from the news of the day:
The storm originated in the Gulf of Mexico, where the barometer was low on Sept. 6th. During the latter part of that day there were high north-easterly winds and heavy rains in Florida. The disturbance hovered over Cuba and Southern Florida until the night of the 10th. It then began to travel in a northerly direction, and by the morning of the 12th it was over South Carolina, accompanied by heavy rain. During the 12th it moved at the rate of over 30 miles an hour, and by Friday morning was over the western end of Lake Ontario.