We would call the attention of our readers bo the fact that there are only three weeks more before the successful competitors for our prize list of Fifteen Hundred Dollars will be rewarded for their exertions. Every one who has entered the field of competition cannot expect to reap the reward in cash, but fifteen of those who have best succeeded in obtaining subscribers for our publication will receive, in the aggregate, fifteen hundred dollars, in sums varying in amount to each, according to the number of mail subscribers they have sent in to us, and paid for, since this volume of the SCIENTIFIC AMERICAN was commenced. Those few who entered the course for competition when the announcement of the prizes was first made, but who have not, from their own inertness, or from the pressure of the times, continued in the contest, will, we hope, feel that they are well repaid for the exertion they at first put forth, in the consciousness that they have done-their neighbors and townsmen a favor by introducing the SCIENTIFIC AMERICAN to their notice, and inducing them to subscribe, and thus insnre the weekly visit of a paper in which no information of a pernicious or demoralizing character appears; we regret to say this can be asserted of but very few literary papers of the present day. The benefits and pleasures which these subscribers and their families derive from the weekly visit of the SCIENTIFIC AMERICAN, we leave for each patron to estimate for himself. A few days ago, we were led by curiosity to foot up the amounts remitted to us by a single competitor, and, from the names sent, to calculate whereabout in the list of prizes he would stand : we found to our astonishment that, if the awards had been made on that day, the competitor in question would not only have received back the full amount which he had remitted for all his subscribers, but would have received the additional sum of $160. We ceased our calculations when we arrived at this result, fearing lest we might arrive at still less satisfactory conclusions to ourselves by looking into the prospects of other competitors. This example shows what an opportunity still remains for some of the smaller competitors to augment their lists of names during the next three weeks, and then to receive a respectable sum for their exertions, over and above the total investment.